the maintenance of species-richness in plant communities
the maintenance of species-richness in plant communities
the maintenance of species-richness in plant communities
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
Biol. Rev. (1977), 52, pp. 107-145<br />
BRC PAH 52-4<br />
THE MAINTENANCE OF SPECIES-RICHNESS<br />
IN PLANT COMMUNITIES: THE IMPORTANCE OF THE<br />
REGENERATION NICHE<br />
BY P. J. GRUBB<br />
Botany School, University <strong>of</strong> Cambridge<br />
(Received 7 September 1976)<br />
CONTENTS<br />
I. Introduction . .<br />
11. Mechanisms not <strong>in</strong>volv<strong>in</strong>g <strong>the</strong> process <strong>of</strong> regeneration<br />
(a) Variation <strong>in</strong> life-form . .<br />
(b) Phenological spread . .<br />
(c) Fluctuations <strong>in</strong> <strong>the</strong> environment .<br />
(d) Balancedmixturea .<br />
(e) Variation <strong>in</strong> competitive ability with physiological age<br />
111. Induction to <strong>the</strong> <strong>in</strong>volvement <strong>of</strong> regeneration .<br />
IV. Def<strong>in</strong>ition <strong>of</strong> a <strong>plant</strong>’s niche . . . . .<br />
V. Patterns <strong>of</strong> regeneration <strong>in</strong> different <strong>plant</strong> <strong>communities</strong><br />
VI. Examples <strong>of</strong> differentiation <strong>in</strong> <strong>the</strong> regeneration niche .<br />
(a) Production <strong>of</strong> viable seed .<br />
(a) Dispersal .<br />
(c) Germ<strong>in</strong>ation . . . . . .<br />
(d) Establishment <strong>of</strong> seedl<strong>in</strong>gs .<br />
(e) Fur<strong>the</strong>r development <strong>of</strong> <strong>the</strong> immature <strong>plant</strong><br />
.<br />
VII. Discussion .<br />
VIII. summary .<br />
IX. References . . .<br />
.<br />
I. INTRODUCTION<br />
. 107<br />
. 108<br />
. 108<br />
. 111<br />
. I11<br />
. 113<br />
. 114<br />
. 116<br />
. I19<br />
. 119<br />
. . . I21<br />
. I21<br />
. 125<br />
. 126<br />
. 128<br />
. 130<br />
. I31<br />
. . . I34<br />
. 136<br />
Accord<strong>in</strong>g to ‘ Gause’s hypo<strong>the</strong>sis ’ a corollary <strong>of</strong> <strong>the</strong> process <strong>of</strong> evolution by natural<br />
selection is that <strong>in</strong> a community at equilibrium every <strong>species</strong> must occupy a different<br />
niche. This idea is generally accepted by zoologists (Krebs, 1g72), but most botanists<br />
f<strong>in</strong>d difficulty <strong>in</strong> understand<strong>in</strong>g how all <strong>the</strong> <strong>species</strong> <strong>in</strong> a <strong>species</strong>-rich <strong>plant</strong> community<br />
can possibly occupy different niches. Although many different factors are <strong>in</strong>volved <strong>in</strong><br />
<strong>the</strong> full def<strong>in</strong>ition <strong>of</strong> an animal’s niche, one can fairly readily imag<strong>in</strong>e sufficient niches<br />
for all <strong>the</strong> animal <strong>species</strong> known, us<strong>in</strong>g food-requirements alone; <strong>the</strong> million or so<br />
animal <strong>species</strong> can easily be ‘expla<strong>in</strong>ed’ <strong>in</strong> terms <strong>of</strong> <strong>the</strong> 300000 <strong>species</strong> <strong>of</strong> <strong>plant</strong>s<br />
(so many <strong>of</strong> which have markedly different parts such as leaves, bark, wood, roots,<br />
etc.) and <strong>the</strong> existence <strong>of</strong> three to four tiers <strong>of</strong> carnivores (Hutch<strong>in</strong>son, 1959). There is<br />
no comparable ‘explanation’ for autotrophic <strong>plant</strong>s ; <strong>the</strong>y all need light, carbon dioxide,<br />
water and <strong>the</strong> same m<strong>in</strong>eral nutrients.<br />
Many ideas have been put forward to expla<strong>in</strong> <strong>the</strong> <strong>in</strong>def<strong>in</strong>ite co-existence<strong>of</strong> numerous<br />
<strong>species</strong> <strong>in</strong> particular <strong>communities</strong>, but it is <strong>the</strong> author’s contention that most <strong>of</strong> <strong>the</strong>se go
108 P. J. GRUBB<br />
only a very little way toward expla<strong>in</strong><strong>in</strong>g <strong>the</strong> persistence <strong>of</strong> numerous <strong>species</strong> <strong>in</strong> <strong>the</strong> more<br />
<strong>species</strong>-rich <strong>communities</strong>. One reason for <strong>the</strong> lack <strong>of</strong> understand<strong>in</strong>g on <strong>the</strong> part <strong>of</strong><br />
most botanists results from <strong>the</strong>ir failure to take <strong>in</strong>to account <strong>the</strong> phenomenon <strong>of</strong><br />
regeneration <strong>in</strong> <strong>plant</strong> <strong>communities</strong>, which was first discussed <strong>in</strong> general terms by<br />
A. S. Watt <strong>in</strong> 1947. Most <strong>communities</strong> that are not successional are longer-lived than<br />
<strong>the</strong>ir constituent <strong>in</strong>dividual <strong>plant</strong>s. When any one <strong>plant</strong> <strong>in</strong>dividual dies, a gap is<br />
created and a new <strong>in</strong>dividual ultimately takes its place. It may or may not be <strong>of</strong> <strong>the</strong><br />
same <strong>species</strong> and this is <strong>the</strong> crucial po<strong>in</strong>t. Heterogeneity <strong>of</strong> <strong>the</strong> environment (physicochemical<br />
and biotic) does <strong>in</strong> fact ensure that <strong>the</strong> replacement <strong>plant</strong> (or <strong>plant</strong>s) is<br />
sometimes <strong>of</strong> <strong>the</strong> same <strong>species</strong> as died and sometimes not. It is suggested <strong>in</strong> this review<br />
that this replacement stage is <strong>of</strong> great importance not only for understand<strong>in</strong>g <strong>species</strong><strong>richness</strong><br />
as such but also for understand<strong>in</strong>g <strong>the</strong> basic processes <strong>of</strong> evolutionary<br />
divergence <strong>in</strong> <strong>plant</strong>s and for <strong>the</strong> management <strong>of</strong> <strong>plant</strong> <strong>communities</strong> <strong>in</strong> conservation.<br />
In order to view <strong>the</strong> problem as a whole, ideas not <strong>in</strong>volv<strong>in</strong>g regeneration are<br />
considered briefly <strong>in</strong> <strong>the</strong> first part <strong>of</strong> <strong>the</strong> review (Section 11), and ideas that do <strong>in</strong>volve<br />
regeneration are covered <strong>in</strong> more detail <strong>in</strong> <strong>the</strong> second part (Sections 111-VII). The<br />
nomenclature follows that used <strong>in</strong> <strong>the</strong> papers quoted except where o<strong>the</strong>rwise <strong>in</strong>dicated.<br />
11. MECHANISMS NOT INVOLVING THE PROCESS OF REGENERATION<br />
(u) Vuriation <strong>in</strong> life-form<br />
It is useful to consider <strong>in</strong>itially <strong>the</strong> various dom<strong>in</strong>ance-structures found <strong>in</strong> <strong>plant</strong><br />
<strong>communities</strong>. The f<strong>in</strong>d<strong>in</strong>gs <strong>of</strong> many ecologists on <strong>the</strong> structures <strong>of</strong> natural <strong>communities</strong><br />
are reflected <strong>in</strong> <strong>the</strong> dom<strong>in</strong>ance-diversity ideas <strong>of</strong> Whittaker (1965), who concluded<br />
that <strong>plant</strong> <strong>communities</strong> could be represented by a spectrum <strong>of</strong> curves relat<strong>in</strong>g <strong>the</strong><br />
importance values <strong>of</strong> <strong>species</strong> and <strong>the</strong> number <strong>of</strong> <strong>species</strong> (Fig. I). In <strong>the</strong> <strong>communities</strong><br />
poorest <strong>in</strong> <strong>species</strong>, represented by type (u) curves, one <strong>species</strong> has a much greater<br />
productivity (or cover-abundance or stand<strong>in</strong>g crop) than any o<strong>the</strong>rs and <strong>the</strong>re is a<br />
short series <strong>of</strong> successively less productive <strong>species</strong>. At <strong>the</strong> o<strong>the</strong>r extreme (type (c)<br />
curves) <strong>the</strong>re is no clear s<strong>in</strong>gle dom<strong>in</strong>ant, but a group <strong>of</strong> most productive <strong>species</strong><br />
followed by a larger group <strong>of</strong> moderately productive <strong>species</strong> and a f<strong>in</strong>al group <strong>of</strong> rare<br />
<strong>species</strong> contribut<strong>in</strong>g little to <strong>the</strong> productivity <strong>of</strong> <strong>the</strong> community. Many <strong>communities</strong><br />
are <strong>in</strong>termediate <strong>in</strong> nature (type (b) curves).<br />
The co-existence <strong>of</strong> <strong>species</strong> <strong>in</strong> type (u) <strong>communities</strong> can largely be expla<strong>in</strong>ed <strong>in</strong><br />
terms <strong>of</strong> complementarity <strong>of</strong> life-form. Very few, if any, dom<strong>in</strong>ant <strong>species</strong> are able to<br />
utilize <strong>the</strong> resources <strong>of</strong> an area totally or block <strong>of</strong>f with complete success those <strong>the</strong>y do<br />
not use <strong>the</strong>mselves. Thus it is very likely that epiphytes (<strong>of</strong>ten cryptogamic) will be<br />
found on <strong>the</strong> mature dom<strong>in</strong>ant. Almost always one or a few <strong>species</strong> <strong>of</strong> shade-tolerant<br />
herbs or subshrubs can co-exist - <strong>the</strong>ir shoots receiv<strong>in</strong>g just enough light for persistence<br />
if not for flower<strong>in</strong>g and <strong>the</strong>ir roots enough m<strong>in</strong>eral nutrients and water.<br />
A tolerant <strong>species</strong> <strong>of</strong> climber may similarly co-exist with <strong>the</strong> dom<strong>in</strong>ant. The different<br />
productivities <strong>of</strong> <strong>the</strong> <strong>species</strong> <strong>in</strong> such systems are determ<strong>in</strong>ed more by <strong>in</strong>herent differences<br />
<strong>in</strong> potential than by simple competition from <strong>the</strong> dom<strong>in</strong>ant. If <strong>the</strong> latter is
100<br />
- 10<br />
al<br />
$ 1<br />
><br />
8<br />
Y rn<br />
b,<br />
0<br />
a<br />
- E<br />
0.1 0.01<br />
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> 109<br />
0.001 0 10 C 10 20<br />
Species numbers<br />
Fig. I. The three major types <strong>of</strong> dom<strong>in</strong>ance-diversity curves presented by Whittaker (1965):<br />
(a) <strong>species</strong>-poor <strong>communities</strong>, (b, c) <strong>species</strong>-rich.<br />
Fig. 2. A section <strong>of</strong> a model <strong>of</strong> a <strong>species</strong>-poor community with only one <strong>species</strong> <strong>in</strong> each major<br />
life-form; <strong>the</strong> resources are represented by a space <strong>of</strong> f<strong>in</strong>ite volume and <strong>the</strong> requirements <strong>of</strong> <strong>the</strong><br />
three <strong>species</strong> by <strong>in</strong>compressible spheres. The larger spheres are seen to be unable to exclude<br />
<strong>the</strong> smaller ones. The arrangement <strong>of</strong> <strong>the</strong> spheres would be different if <strong>the</strong>ir sizes were kept<br />
constant and <strong>the</strong> space represent<strong>in</strong>g <strong>the</strong> resources were made bigger or smaller. e.g. if it were<br />
made <strong>the</strong> size <strong>of</strong> one <strong>of</strong> <strong>the</strong> four unit-spaces drawn <strong>in</strong> <strong>the</strong> figure.
I I0<br />
P. J. GRUBB<br />
Fig. 3. A section <strong>of</strong> <strong>the</strong> central part <strong>of</strong> a model <strong>of</strong> a <strong>species</strong>-poor community with dependencerelationships<br />
as well as <strong>the</strong> exclusion-relationship; <strong>the</strong> cyl<strong>in</strong>ders and cones cannot fit <strong>in</strong>to each<br />
o<strong>the</strong>r’s holes and <strong>the</strong> larger <strong>of</strong> <strong>the</strong> small spheres are also excluded.<br />
removed, and no <strong>species</strong> is artificially <strong>in</strong>troduced, none <strong>of</strong> <strong>the</strong> rema<strong>in</strong><strong>in</strong>g <strong>species</strong> may<br />
be able to take over <strong>the</strong> role <strong>of</strong> dom<strong>in</strong>ant ei<strong>the</strong>r <strong>in</strong> terms <strong>of</strong> stand<strong>in</strong>g crop or <strong>in</strong><br />
dom<strong>in</strong>at<strong>in</strong>g <strong>the</strong> o<strong>the</strong>rs <strong>in</strong> a more general sense. The long-term co-existence <strong>of</strong> several<br />
<strong>species</strong> <strong>in</strong> <strong>the</strong>se simple <strong>communities</strong> can largely be ‘expla<strong>in</strong>ed’ <strong>in</strong> terms <strong>of</strong> a model <strong>in</strong><br />
which <strong>the</strong>y fit <strong>in</strong> <strong>the</strong> gaps between each o<strong>the</strong>r. Thus, if <strong>the</strong> available resources are<br />
represented by a cube <strong>of</strong> f<strong>in</strong>ite size and <strong>the</strong> <strong>plant</strong>s by <strong>in</strong>compressible spheres, <strong>the</strong> size<br />
<strong>of</strong> which is proportional to <strong>the</strong> resources utilized or blocked <strong>of</strong>f, <strong>the</strong>n <strong>the</strong> larger<br />
spheres are not able to occupy all <strong>the</strong> space that encloses <strong>the</strong>m (Fig. 2). Such a model<br />
represents only part <strong>of</strong> <strong>the</strong> truth because <strong>the</strong> <strong>plant</strong>s <strong>in</strong> such a system do not grow<br />
<strong>in</strong>dependently and <strong>the</strong> spheres ought to be conceived <strong>of</strong> as capable <strong>of</strong> dent<strong>in</strong>g each<br />
o<strong>the</strong>r to some extent, just as <strong>the</strong> circles can overlap <strong>in</strong> <strong>the</strong> model <strong>of</strong> Putwa<strong>in</strong> & Harper<br />
(1970). Thus <strong>the</strong> trees <strong>in</strong> forest systems <strong>of</strong> this type reduce <strong>the</strong> yield <strong>of</strong> <strong>the</strong> few <strong>species</strong><br />
<strong>of</strong> herb beneath <strong>the</strong>m (Watt & Fraser, 1933 ; Karpov, 1961) while <strong>the</strong> yield <strong>of</strong> <strong>the</strong> trees<br />
may be reduced by <strong>the</strong> herbs-after all, <strong>the</strong> grass sward <strong>in</strong> an orchard can reduce<br />
<strong>the</strong> fruit crop above it (Bedford & Picker<strong>in</strong>g, 1919).<br />
The model represented <strong>in</strong> Fig. z is also <strong>in</strong>adequate <strong>in</strong> that it fails to <strong>in</strong>clude <strong>the</strong><br />
dependence-relationship which, toge<strong>the</strong>r with competition and complementarity,<br />
characterizes <strong>the</strong> <strong>in</strong>tegrated <strong>plant</strong> community. Dependence is most clearly demonstrated<br />
by <strong>the</strong> epiphytes already mentioned, by <strong>the</strong> parasites and hemi-parasites and by
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong><br />
<strong>the</strong> epi-parasites (‘saprophytes’) <strong>of</strong>ten found grow<strong>in</strong>g <strong>in</strong> very dense shade where few<br />
autototrophic <strong>plant</strong>s can persist. The dependence-relationship can be <strong>in</strong>corporated, as<br />
<strong>in</strong> Fig. 3, by conceiv<strong>in</strong>g <strong>the</strong> requirements <strong>of</strong> <strong>the</strong> host <strong>species</strong> as represented by large<br />
spheres with hollows <strong>of</strong> different shapes and sizes on <strong>the</strong>ir outsides and by represent<strong>in</strong>g<br />
<strong>the</strong> dependent <strong>species</strong> as cyl<strong>in</strong>ders or cones that fit <strong>the</strong>se hollows exactly.<br />
I11<br />
(b) Phenological spread<br />
The second major way <strong>in</strong> which <strong>species</strong> are complementary to each o<strong>the</strong>r is <strong>in</strong><br />
<strong>the</strong>ir seasonal development. A useful simple example comes from <strong>the</strong> persistence <strong>of</strong><br />
certa<strong>in</strong> weeds <strong>in</strong> crops. Quite <strong>of</strong>ten <strong>the</strong> crop, dur<strong>in</strong>g its own active period <strong>of</strong> growth,<br />
very strongly suppresses a particular <strong>species</strong> <strong>of</strong> weed, as <strong>in</strong> <strong>the</strong> case <strong>of</strong> barley and white<br />
persicaria (Asp<strong>in</strong>all & Milthorpe, 1959; Asp<strong>in</strong>all, 1960), but <strong>the</strong> weed is not elim<strong>in</strong>ated<br />
from <strong>the</strong> field because it comb<strong>in</strong>es an ability to persist <strong>in</strong> a suppressed state with an<br />
ability to develop rapidly to <strong>the</strong> stage <strong>of</strong> ripe seed production before harvest, start<strong>in</strong>g<br />
as soon as <strong>the</strong> crop beg<strong>in</strong>s to divert its photosynthate <strong>in</strong>to production <strong>of</strong> its own seed<br />
ra<strong>the</strong>r than <strong>in</strong>to vegetative organs directly active <strong>in</strong> competition. At this time <strong>the</strong> crop<br />
may actually leak out nutrients <strong>of</strong> value to <strong>the</strong> weed, as potassium escapes from <strong>the</strong><br />
grass Phalaris tuberosa (Richardson, Trumble & Shapter, I 932). Opportunism,<br />
coupled with persistence while suppressed, may be especially important for <strong>species</strong><br />
at <strong>the</strong> bottom <strong>of</strong> <strong>the</strong> rank order <strong>in</strong> a particular natural community. Differences <strong>in</strong><br />
seasonal development or phenology are one <strong>of</strong> <strong>the</strong> ma<strong>in</strong> f<strong>in</strong>d<strong>in</strong>gs <strong>of</strong> those who have<br />
developed <strong>the</strong> analytical approach with natural <strong>communities</strong>. The vernal flora <strong>of</strong><br />
north temperate deciduous woodland is most <strong>of</strong>ten quoted but equally impressive are<br />
<strong>the</strong> successive flushes <strong>of</strong> activity <strong>in</strong> steppe (Walter, 1968) or semi-desert (Bykov, 1974)<br />
and <strong>the</strong> non-co<strong>in</strong>cidence <strong>of</strong> flush<strong>in</strong>g <strong>in</strong> trees <strong>of</strong> tropical ra<strong>in</strong>-forest (Medway, 1972;<br />
Frankie, Baker & Opler, 1974).<br />
(c) Fluctuations <strong>in</strong> <strong>the</strong> environment<br />
In most, if not all, <strong>plant</strong> <strong>communities</strong> <strong>the</strong> processes by which certa<strong>in</strong> <strong>species</strong> tend to<br />
oust o<strong>the</strong>rs as a result <strong>of</strong> competition <strong>in</strong> <strong>the</strong> vegetative stage are greatly h<strong>in</strong>dered by<br />
temporal fluctuations <strong>in</strong> <strong>the</strong> environment. The effects <strong>of</strong> short-term changes <strong>in</strong> <strong>the</strong><br />
environment on <strong>species</strong>-balance can be very considerable. If a mature <strong>plant</strong> community<br />
is ‘<strong>in</strong> equilibrium’, <strong>the</strong>n <strong>the</strong> equilibrium is highly dynamic. Thus, <strong>in</strong> certa<strong>in</strong> sem<strong>in</strong>atural<br />
meadows <strong>in</strong> <strong>the</strong> U.S.S.R. <strong>the</strong> grasses Agropyron repens and Bromus <strong>in</strong>ermis are<br />
particularly prom<strong>in</strong>ent <strong>in</strong> dry years while Alopecurus patensis tends to dom<strong>in</strong>ate <strong>in</strong><br />
wet years (Rabotnov, 1974). In <strong>the</strong> same general area temporary blockage <strong>of</strong> dra<strong>in</strong>age,<br />
which can occur naturally, leads to an <strong>in</strong>crease <strong>in</strong> Ranunculus repens and release <strong>of</strong> <strong>the</strong><br />
blockage returns <strong>the</strong> balance <strong>in</strong> favour <strong>of</strong> Alopecurus patensis (Rabotnov, 1974).<br />
Fluctuation <strong>in</strong> temperature is probably equally important, especially where <strong>species</strong><br />
with a nor<strong>the</strong>rn distribution and greater resistance to w<strong>in</strong>ter-kill<strong>in</strong>g are mixed naturally<br />
with those <strong>of</strong> sou<strong>the</strong>rn distribution and lesser hard<strong>in</strong>ess, e.g. Elymus canadensis and<br />
Sorghastrum (Andropogon) nutans <strong>in</strong> some North American prairies (Clements,<br />
Weaver & Hanson, 1929). Fluctuations with a period <strong>of</strong> 2-3 years grade <strong>in</strong>to those
I12<br />
P. J. GRUBB<br />
last<strong>in</strong>g a decade, e.g. <strong>the</strong> long drought <strong>of</strong> <strong>the</strong> 1930s <strong>in</strong> <strong>the</strong> prairies <strong>of</strong> <strong>the</strong> U.S.A. and<br />
<strong>the</strong> wetter than average 1950s (Coupland, 1974). S<strong>in</strong>ce <strong>in</strong>dividuals <strong>of</strong> some grasses and<br />
<strong>plant</strong>s <strong>of</strong> <strong>the</strong> woodland door possibly persist for a thousand years or more (Harberd,<br />
1961; Ohenen, 1967a, b), climatic fluctuations on <strong>the</strong> scale <strong>of</strong> centuries may be<br />
important <strong>in</strong> determ<strong>in</strong><strong>in</strong>g an <strong>in</strong>constant balance between <strong>the</strong> vegetative, <strong>plant</strong>s <strong>of</strong> some<br />
<strong>species</strong>.<br />
The biotic environment also fluctuates. Chilvers & Britta<strong>in</strong> (1972) have tried to<br />
expla<strong>in</strong> co-existence <strong>of</strong> similar <strong>plant</strong> <strong>species</strong> <strong>in</strong> terms <strong>of</strong> a negative feed-back <strong>in</strong>volv<strong>in</strong>g<br />
fluctuations <strong>in</strong> <strong>the</strong> numbers <strong>of</strong> host-specific parasites or herbivores. However, fluctuations<br />
<strong>in</strong> <strong>the</strong> severity <strong>of</strong> pests determ<strong>in</strong>ed by climatic <strong>in</strong>fluences or by predators ra<strong>the</strong>r<br />
than by <strong>the</strong> abundance <strong>of</strong> <strong>the</strong> host will also play a role. So also will fluctuations <strong>in</strong> pests<br />
that are not host-specific but still selective, e.g. lea<strong>the</strong>rjackets (TZpula spp.) eat<strong>in</strong>g<br />
roots <strong>of</strong> clover more than those <strong>of</strong> grass <strong>in</strong> a ley (White & French, 1968). Several<br />
observations suggest <strong>in</strong>directly that pests may be important <strong>in</strong> limit<strong>in</strong>g <strong>the</strong> abundance<br />
<strong>of</strong> <strong>plant</strong> <strong>species</strong> under natural conditions, e.g. <strong>the</strong> better growth <strong>of</strong> certa<strong>in</strong> Eucalyptus<br />
spp. outside Australia but <strong>in</strong> climates very similar to those <strong>of</strong> <strong>the</strong>ir natural range<br />
(Gillett, 1962; Harper, 1969). The effectiveness <strong>of</strong> specific <strong>in</strong>sects brought <strong>in</strong> to<br />
control particular weeds <strong>in</strong>troduced by man is also impressive, e.g. <strong>in</strong> <strong>the</strong> control <strong>of</strong><br />
St John’s Wort (Hypericum perforaturn) by <strong>the</strong> beetle Chrysol<strong>in</strong>u quadrigem<strong>in</strong>a <strong>in</strong><br />
California (Huffaker & Kennet, 1959). At one typical range-land site <strong>the</strong> cover <strong>of</strong> weed<br />
was reduced from cu. 75 yo to zero <strong>in</strong> four years. Similar effects <strong>in</strong> more nearly natural<br />
vegetation were found by Cantlon (1969), who studied <strong>the</strong> annual herb Melumpymm<br />
Z<strong>in</strong>eare <strong>in</strong> deciduous forest <strong>in</strong> Michigan. The <strong>plant</strong> is grazed by <strong>the</strong> katydid Atlanticus<br />
testaceus and <strong>in</strong> plots treated with a mixture <strong>of</strong> <strong>in</strong>secticides <strong>the</strong> number <strong>of</strong> seedl<strong>in</strong>gs<br />
was twice as great as <strong>in</strong> <strong>the</strong> control plots after I year, three times as great after 2 years<br />
and over three times as great after 4 years. Fluctuations <strong>in</strong> localized and rov<strong>in</strong>g<br />
populations <strong>of</strong> mammals, which are generally less selective than <strong>in</strong>sects (Summerhayes<br />
1941 ; Watt, 1957; Kydd, 1964; Stewart & Stewart, I ~ I ) must , also have been an<br />
important <strong>in</strong>fluence before man dom<strong>in</strong>ated <strong>the</strong> scene.<br />
Temporal fluctuations <strong>in</strong> microbial pests may also be important for <strong>the</strong> control <strong>of</strong><br />
<strong>the</strong> abundance <strong>of</strong> particular <strong>plant</strong>s <strong>in</strong> <strong>the</strong> wild but little is known about this. Theeffects<br />
<strong>of</strong> spatial fluctuation - seen <strong>in</strong> <strong>the</strong> fairy r<strong>in</strong>gs <strong>of</strong> grasslands <strong>in</strong> <strong>the</strong> North Temperate<br />
Zone - are highly suggestive. These r<strong>in</strong>gs have been a great source <strong>of</strong> mythology but<br />
are still poorly understood <strong>in</strong> scientific terms (Shantz & Piemeisel, 1917; Ramsbottom<br />
1953). Apparently a fungus spreads outwards slowly from its site <strong>of</strong> <strong>in</strong>fection, suppress<strong>in</strong>g<br />
<strong>the</strong> dom<strong>in</strong>ant grass to a variable degree (sometimes almost completely).<br />
Often, it seems, considerable quantities <strong>of</strong> major nutrients are liberated when <strong>the</strong><br />
fungus dies and <strong>the</strong>se stimulate <strong>the</strong> growth <strong>of</strong> <strong>the</strong> grass to a particularly high level at<br />
<strong>the</strong> marg<strong>in</strong> <strong>of</strong> <strong>the</strong> <strong>in</strong>fected area, thus form<strong>in</strong>g <strong>the</strong> ‘r<strong>in</strong>g’.<br />
It has been suggested (Gillett, 1962; Janzen, 1970; Connell, 1971) that host-specific<br />
parasites and herbivores may have a ra<strong>the</strong>r special role to play <strong>in</strong> ma<strong>in</strong>ta<strong>in</strong><strong>in</strong>g <strong>species</strong><strong>richness</strong><br />
through <strong>the</strong>ir activities be<strong>in</strong>g concentrated on adult <strong>plant</strong>s and <strong>in</strong>flict<strong>in</strong>g<br />
especially heavy mortality on any <strong>of</strong>fspr<strong>in</strong>g <strong>in</strong> <strong>the</strong> immediate neighbourhood <strong>of</strong> <strong>the</strong><br />
parents. This idea will be fur<strong>the</strong>r discussed on page 125.
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> 113<br />
0' I<br />
Time<br />
Fig. 4. The two major k<strong>in</strong>ds <strong>of</strong> result found when two <strong>species</strong> (x and y) are grown <strong>in</strong> mixture,<br />
start<strong>in</strong>g with contrasted proportions <strong>of</strong> x (90 % and 10 %) : (a) x always replaces y, (b) a balance<br />
between x and y is set up. The shape <strong>of</strong> <strong>the</strong> time course <strong>of</strong> <strong>the</strong> ratio may vary appreciably<br />
depend<strong>in</strong>g on <strong>the</strong> <strong>species</strong> tested, and <strong>the</strong>re are likely to be fluctuations about <strong>the</strong> mean l<strong>in</strong>e <strong>of</strong><br />
<strong>the</strong> time course depend<strong>in</strong>g on <strong>the</strong> season, as shown (discont<strong>in</strong>uous l<strong>in</strong>e) for one <strong>of</strong> <strong>the</strong> mixtures<br />
<strong>in</strong> (a).<br />
(d) Balanced mixtures<br />
Many experiments on competition have been made <strong>in</strong> recent years us<strong>in</strong>g even-aged<br />
mixtures <strong>of</strong> pairs <strong>of</strong> <strong>species</strong>, and with <strong>the</strong> two <strong>species</strong> sown <strong>in</strong> vary<strong>in</strong>g proportions as<br />
suggested by de Wit (1960, 1961). In most cases one <strong>species</strong> <strong>of</strong> <strong>the</strong> pair tends to oust<br />
<strong>the</strong> o<strong>the</strong>r, whatever <strong>the</strong> proportions at <strong>the</strong> start <strong>of</strong> <strong>the</strong> experiment (Fig. 4 a), but <strong>in</strong><br />
a few cases <strong>the</strong>re is evidence for a position <strong>of</strong> balance, which could persist <strong>in</strong>def<strong>in</strong>itely<br />
and toward which mixtures <strong>in</strong> all proportions tend to change (Fig. 4b). In <strong>the</strong> first case<br />
<strong>the</strong>re is eventually complete replacement <strong>of</strong> one <strong>species</strong> by <strong>the</strong> o<strong>the</strong>r. If <strong>the</strong> advantage<br />
<strong>of</strong> A over B is relatively small, as is <strong>of</strong>ten <strong>the</strong> case, replacement will take a very long<br />
time. However, such replacement can occur <strong>in</strong> quite short periods <strong>in</strong> annual <strong>species</strong><br />
as is shown by <strong>the</strong> experiment <strong>of</strong> Harlan & Mart<strong>in</strong>i (1938). They sowed I I varieties <strong>of</strong><br />
barley <strong>in</strong> equal proportions at several different sites, harvested gra<strong>in</strong> at random at <strong>the</strong><br />
end <strong>of</strong> each season for sow<strong>in</strong>g <strong>the</strong> next year, and found that less suitable varieties were<br />
completely elim<strong>in</strong>ated with<strong>in</strong> 4-1 I years at any given site.<br />
The <strong>ma<strong>in</strong>tenance</strong> <strong>of</strong> a balanced mixture, as <strong>in</strong> Fig. 4b, is dependent on both <strong>the</strong><br />
grow<strong>in</strong>g conditions (van der Bergh & de Wit, 1960) and <strong>the</strong> density <strong>of</strong> <strong>the</strong> <strong>plant</strong>s<br />
(Marshall & Ja<strong>in</strong>, 1969). It can be expla<strong>in</strong>ed <strong>in</strong> general terms <strong>of</strong> ei<strong>the</strong>r a balance <strong>of</strong><br />
<strong>in</strong>traspecific versus <strong>in</strong>terspecific competition (Harper, 1967) or limitation <strong>of</strong> <strong>the</strong> two<br />
<strong>species</strong> by different factors (Harper, Clatworthy, McNaughton & Sagar, 1961). The<br />
exact mechanisms <strong>in</strong>volved <strong>in</strong> <strong>the</strong> first explanation are obscure, but may possibly<br />
<strong>in</strong>volve <strong>the</strong> production <strong>of</strong> autotox<strong>in</strong>s, i.e. materials more toxic to <strong>the</strong> producer <strong>species</strong><br />
than to <strong>the</strong> o<strong>the</strong>r <strong>species</strong>. Newman & Rovira (1975) have recently obta<strong>in</strong>ed evidence<br />
that production <strong>of</strong> materials that are more autoxic than allotoxic may be characteristic<br />
<strong>of</strong> those <strong>species</strong> that occur as isolated <strong>in</strong>dividuals, whereas <strong>the</strong> reverse may be true <strong>of</strong><br />
<strong>species</strong> that naturally form long-last<strong>in</strong>g and cont<strong>in</strong>uous swards. Limitation by different<br />
factors can be illustrated tentatively with respect to m<strong>in</strong>eral requirements. For example,<br />
on some soils nitrogen is <strong>the</strong> primary limit<strong>in</strong>g m<strong>in</strong>eral for grasses, but phosphorus and<br />
potassium limit <strong>the</strong> growth <strong>of</strong> legumes (Thurston, 1969); on o<strong>the</strong>r soils phosphorus<br />
a BRE 52
1’4 P. J. GRUBB<br />
may be <strong>the</strong> primary limit<strong>in</strong>g m<strong>in</strong>eral for grasses and nitrogen for sedges (Willis, 1963).<br />
Similarly <strong>the</strong> root growth <strong>of</strong> one <strong>species</strong> may be best <strong>in</strong> soil pores <strong>of</strong> a certa<strong>in</strong> size and<br />
that <strong>of</strong> ano<strong>the</strong>r <strong>species</strong> best <strong>in</strong> adjacent pores <strong>of</strong> a different size (Sheikh & Rutter, 1969).<br />
It is <strong>of</strong>ten forgotten that <strong>the</strong> growth <strong>of</strong> most <strong>plant</strong>s is limited by several or many<br />
factors simultaneously. In <strong>the</strong> cases just cited climatic and biotic factors limit as well<br />
as soil factors. It has to be emphasized that <strong>the</strong> persistence <strong>of</strong> mixtures <strong>of</strong> <strong>the</strong> <strong>species</strong><br />
can only be accounted for <strong>in</strong> terms <strong>of</strong> different limit<strong>in</strong>g m<strong>in</strong>erals or pore sizes if <strong>the</strong>se<br />
are <strong>of</strong> overwhelm<strong>in</strong>g importance <strong>in</strong> <strong>the</strong> limitation <strong>of</strong> growth.<br />
The significance under natural conditions <strong>of</strong> <strong>the</strong> type <strong>of</strong> balance shown <strong>in</strong> Fig. 4b is<br />
not at all clear. The strong dependence on grow<strong>in</strong>g conditions and density must be<br />
emphasized. Thus two <strong>species</strong> <strong>of</strong> Avena were proven to have <strong>the</strong> appropriate comb<strong>in</strong><strong>in</strong>g<br />
ability <strong>in</strong> certa<strong>in</strong> artificial experiments, but, <strong>in</strong> <strong>the</strong> field, more than half <strong>the</strong> populations<br />
with<strong>in</strong> <strong>the</strong> geographical range <strong>of</strong> both <strong>species</strong> <strong>in</strong> California were found to conta<strong>in</strong> only<br />
one <strong>of</strong> <strong>the</strong> <strong>species</strong> (Marshall & Ja<strong>in</strong>, 1969). No experiments yield<strong>in</strong>g evidence <strong>of</strong><br />
stability <strong>in</strong> even-aged mixtures <strong>of</strong> three or more <strong>species</strong> have been reported. However,<br />
some highly relevant experimentson mixtures <strong>of</strong> four or more <strong>species</strong> have been made.<br />
In two sets <strong>of</strong> experiments on mixtures <strong>of</strong> four <strong>species</strong> <strong>the</strong> different <strong>species</strong> were all<br />
sown <strong>in</strong> <strong>the</strong> same proportion (Bornkamm, 1961a, b). The results show that a fairly<br />
clear rank order (‘peck<strong>in</strong>g order’) was established (Table I a). The same result emerged<br />
(Table I b) when five grass and four legume <strong>species</strong> were grown <strong>in</strong> all possible pairs<br />
(Caputa, 1948; Jacquard, 1968). Similar results were obta<strong>in</strong>ed when I I different<br />
weeds were tested aga<strong>in</strong>st two different crops (Welbank, 1963); experiments with only<br />
three <strong>species</strong> tested aga<strong>in</strong>st each o<strong>the</strong>r are clearly much less likely to yield a def<strong>in</strong>ite<br />
rank order (Haizel & Harper, 1973). The rank order was certa<strong>in</strong>ly not completely<br />
constant <strong>in</strong> Caputa’s experiment and one <strong>species</strong>, Dactylis glomerata, behaved very<br />
erratically (Table I b). It seems improbable that <strong>in</strong> mixtures <strong>in</strong>volv<strong>in</strong>g three or more<br />
<strong>species</strong> many <strong>of</strong> <strong>the</strong> encounters between one <strong>species</strong> and its neighbours will lead to<br />
a self-balanc<strong>in</strong>g equilibrium. However, <strong>the</strong>re may be very many occasions when a nearbalance<br />
is struck (<strong>the</strong> equivalent <strong>of</strong> a small shift <strong>in</strong> Fig. 4a) and this may be extremely<br />
important, as becomes clear later (see page I 18).<br />
(e) Variation <strong>in</strong> competitive ability with physiological age<br />
The relevance <strong>of</strong> all <strong>the</strong> experiments mentioned <strong>in</strong> subsection II(d) is limited by <strong>the</strong><br />
fact that most natural and semi-natural <strong>plant</strong> <strong>communities</strong> are <strong>of</strong> uneven age. The<br />
uneven age-structure is, <strong>in</strong> turn, important because success <strong>in</strong> competition depends on<br />
which developmental stage <strong>of</strong> one <strong>species</strong> is pitted aga<strong>in</strong>st which stage <strong>of</strong> <strong>the</strong> o<strong>the</strong>r.<br />
A. S. Watt (1955) first illustrated this idea with an analysis <strong>of</strong> a semi-natural mixture<br />
<strong>of</strong> hea<strong>the</strong>r (Calluna vulgaris) and <strong>the</strong> bracken fern (Pteridium aquil<strong>in</strong>um) on acid,<br />
nutrient-poor sands <strong>in</strong> eastern England. He divided <strong>the</strong> life-history <strong>of</strong> each <strong>species</strong><br />
<strong>in</strong>to four successive phases on <strong>the</strong> basis <strong>of</strong> morphology alone though <strong>the</strong>se phases<br />
pla<strong>in</strong>ly reflected physiological age (pioneer, build<strong>in</strong>g, mature, degenerate). He <strong>the</strong>n<br />
observed which <strong>species</strong> succeeded when one <strong>in</strong>vaded <strong>the</strong> o<strong>the</strong>r, and, <strong>in</strong> particular, how<br />
this depended on <strong>the</strong> growth phase <strong>of</strong> <strong>the</strong> <strong>species</strong> <strong>in</strong>vaded. The pioneer phase <strong>of</strong> one
Ma<strong>in</strong>twnce <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> 115<br />
Table I. The results <strong>of</strong> two experiments <strong>in</strong> which all possible pairs <strong>of</strong> <strong>species</strong> with<strong>in</strong><br />
certa<strong>in</strong> groups <strong>of</strong> <strong>species</strong> were grown toge<strong>the</strong>r and a rank order establkhed<br />
Species<br />
affected<br />
(a) From Bomkamm (1961 a)<br />
Decreas<strong>in</strong>gly strong<br />
competitors<br />
S S A>VaT<br />
A S A TaV<br />
V S ABV T<br />
T S A V T<br />
S = Simpis alba, A = Avena sativa, V = Vicia sativa, T = Triticum aestivum.<br />
Species affected<br />
L<br />
A<br />
D<br />
P<br />
F<br />
M<br />
X<br />
C<br />
H<br />
(b) From Caputa (1948) - results <strong>of</strong> 1944<br />
Species decreas<strong>in</strong>gly aggressive to test <strong>species</strong><br />
M L H X F P C A D<br />
L M A X F P C H D<br />
L M=X A=F D P C H<br />
L D F A X M P H C<br />
L M=A P X F C H D<br />
L X M C H A P D F<br />
L D A X P F M = C H<br />
L M X A D F H P C<br />
L M X A D P F H C<br />
strongly aggressive (<strong>in</strong> top two places for 719 <strong>species</strong>) :<br />
L = Lolium italicurn<br />
M = Medicago sativa<br />
Moderately aggressive (<strong>in</strong> third, fourth or fifth places for 7/9 <strong>species</strong>):<br />
A = Arrhena<strong>the</strong>rum elatius<br />
X = Trifolium pratense<br />
Slightly aggressive (<strong>in</strong> fifth, sixth or seventh places for 719 <strong>species</strong>):<br />
F = Festucapratensis<br />
P = Phleumpratense<br />
Not aggressive (<strong>in</strong> seventh, eighth or n<strong>in</strong>th places for 719 <strong>species</strong>) :<br />
C = Lotus corniculatus<br />
H = T7Z~ObII hybridurn<br />
Very variable (from second to n<strong>in</strong>th place) :<br />
D = Dactyl& glomerata<br />
<strong>species</strong> only succeeded if it <strong>in</strong>vaded <strong>the</strong> pioneer or degenerate phase <strong>of</strong> <strong>the</strong> o<strong>the</strong>r. Such<br />
a relationship provides a possible basis for <strong>the</strong> <strong>in</strong>def<strong>in</strong>ite persistence <strong>of</strong> two such<br />
<strong>species</strong> <strong>in</strong> mixture.<br />
These various approaches to <strong>the</strong> analysis <strong>of</strong> <strong>plant</strong> <strong>communities</strong> that do not <strong>in</strong>volve<br />
a consideration <strong>of</strong> regeneration - or only a m<strong>in</strong>imal study <strong>of</strong> it - provide several<br />
dist<strong>in</strong>ct ways <strong>of</strong> expla<strong>in</strong><strong>in</strong>g long-term co-existence <strong>of</strong> some <strong>plant</strong> <strong>species</strong>, but <strong>the</strong>y do<br />
not conv<strong>in</strong>c<strong>in</strong>gly ‘expla<strong>in</strong>’ <strong>the</strong> long-term co-existence <strong>of</strong> <strong>the</strong> numerous <strong>species</strong> with<br />
essentially <strong>the</strong> same life-form, adult phenology and habitat-range, that are characteristic<br />
<strong>of</strong> so many <strong>species</strong>-rich <strong>communities</strong>.
I 16<br />
P. J. GRUBB<br />
111. INTRODUCTION TO THE INVOLVEMENT OF REGENERATION<br />
The idea <strong>of</strong> two <strong>species</strong> with much <strong>the</strong> same life-form, phenology and habitat-range<br />
differ<strong>in</strong>g <strong>in</strong> requirements for regeneration is most easily grasped for tree <strong>species</strong>.<br />
Gaps aris<strong>in</strong>g <strong>in</strong> a forest are <strong>of</strong> different sizes. When a whole large tree is blown over,<br />
<strong>the</strong> gap produced is large; when an <strong>in</strong>dividual limb falls <strong>of</strong>f a diseased tree, <strong>the</strong> gap<br />
formed is small. If <strong>the</strong> gap is large, an <strong>in</strong>dividual tree <strong>of</strong> some light-demand<strong>in</strong>g <strong>species</strong><br />
is most likely to be <strong>the</strong> next dom<strong>in</strong>ant on <strong>the</strong> spot. If <strong>the</strong> gap is small, a tree <strong>of</strong> some<br />
shade-tolerant <strong>species</strong> is likely to grow through and dom<strong>in</strong>ate <strong>the</strong> spot. Thus <strong>the</strong><br />
light-demand<strong>in</strong>g Betula pendula can persist with shade-tolerant Fagus sylvatica <strong>in</strong><br />
semi-natural woodland on acid soils <strong>in</strong> Brita<strong>in</strong> (Watt, 1934; 1947). One can easily<br />
envisage o<strong>the</strong>r factors that might sw<strong>in</strong>g <strong>the</strong> balance between coloniz<strong>in</strong>g <strong>species</strong> <strong>in</strong> <strong>the</strong><br />
gap or <strong>in</strong> different parts <strong>of</strong> it. Such factors <strong>in</strong>clude <strong>the</strong> particular <strong>species</strong> which happen<br />
to be present at <strong>the</strong> right time as seed, seedl<strong>in</strong>gs, sapl<strong>in</strong>gs or poles; <strong>the</strong> presence <strong>of</strong> <strong>the</strong><br />
crown <strong>of</strong> <strong>the</strong> fallen tree (and its smo<strong>the</strong>r<strong>in</strong>g effect) or just <strong>the</strong> tall bole; presence or<br />
absence <strong>of</strong> leaves on <strong>the</strong> fallen tree (and <strong>the</strong> green-manur<strong>in</strong>g effect); <strong>the</strong> degree <strong>of</strong><br />
disturbance to <strong>the</strong> litter and topsoil: <strong>the</strong> presence or absence <strong>of</strong> sunflecks <strong>in</strong> sunny<br />
wea<strong>the</strong>r depend<strong>in</strong>g on <strong>the</strong> side <strong>of</strong> <strong>the</strong> gap concerned (north or south) <strong>in</strong> a forest far<br />
away from <strong>the</strong> equator (Coombe, 1966); <strong>the</strong> adult trees left stand<strong>in</strong>g and <strong>the</strong> ground<br />
flora - both compete for light, water and m<strong>in</strong>eral nutrients, and may exert allelopathic<br />
effects or harbour particular pests and diseases.<br />
Table 2. Processes <strong>in</strong>volved <strong>in</strong> <strong>the</strong> successful <strong>in</strong>vasion <strong>of</strong> a gap by a given<br />
<strong>plant</strong> <strong>species</strong> and characters <strong>of</strong> <strong>the</strong> gap that may be important<br />
Processes<br />
Production <strong>of</strong> viable seed<br />
Flower<strong>in</strong>g<br />
Poll<strong>in</strong>ation<br />
Sett<strong>in</strong>g <strong>of</strong> seed<br />
Dispersal <strong>of</strong> seed<br />
Through space<br />
Through time<br />
Germ<strong>in</strong>ation<br />
Establishment<br />
Onward growth<br />
Characters<br />
Time <strong>of</strong> formation<br />
Size and shape<br />
Orientation<br />
Nature <strong>of</strong> soil surface<br />
Litter present<br />
O<strong>the</strong>r <strong>plant</strong>s present<br />
Animals present<br />
Fungi, bacteria and viruses present<br />
The possibilities seem almost limitless. The processes <strong>in</strong>volved <strong>in</strong> <strong>the</strong> successful<br />
<strong>in</strong>vasion <strong>of</strong> a gap by a given <strong>plant</strong> <strong>species</strong> and characters <strong>of</strong> <strong>the</strong> gap that may be important<br />
are summarized <strong>in</strong> Table 2. Similar considerations apply when <strong>in</strong>vasion <strong>of</strong> a gap<br />
is by vegetative means ra<strong>the</strong>r than by seed.<br />
It becomes apparent that <strong>species</strong>-<strong>richness</strong> may be ma<strong>in</strong>ta<strong>in</strong>ed by a heterogeneous<br />
environment act<strong>in</strong>g on all <strong>of</strong> aseries <strong>of</strong> stages <strong>in</strong> <strong>the</strong> reproduction <strong>of</strong> <strong>the</strong> <strong>plant</strong>s present.<br />
The variable environment determ<strong>in</strong>es whe<strong>the</strong>r <strong>the</strong>re is or is not at a particular place<br />
and time for a particular <strong>species</strong> an abundance <strong>of</strong> flower-formation, poll<strong>in</strong>ation, good<br />
seed-set, dispersal, germ<strong>in</strong>ation, establishment <strong>of</strong> <strong>the</strong> juvenile <strong>plant</strong> and passage <strong>of</strong> <strong>the</strong><br />
juvenile to <strong>the</strong> adult. The idea that all <strong>the</strong>se stages are important <strong>in</strong> <strong>the</strong> <strong>ma<strong>in</strong>tenance</strong> <strong>of</strong><br />
<strong>species</strong>-<strong>richness</strong> has been all too <strong>of</strong>ten ignored. It was not a matter <strong>of</strong> chance that <strong>the</strong>
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> 117<br />
example given above to illustrate <strong>the</strong> reality <strong>of</strong> differences <strong>in</strong> requirements for regeneration<br />
concerned trees. The examples that we now f<strong>in</strong>d easiest to comprehend and most<br />
conv<strong>in</strong>c<strong>in</strong>g were also historically <strong>the</strong> first to be appreciated, It was <strong>the</strong> foresters who<br />
first realized <strong>the</strong> importance <strong>of</strong> differences <strong>in</strong> regeneration-requirements. Heyer (1852)<br />
seems to have been <strong>the</strong> first to publish an explicit account <strong>of</strong> differences <strong>in</strong> ‘tolerance’<br />
<strong>of</strong> tree seedl<strong>in</strong>gs; as Tourney & Korstian (1937) po<strong>in</strong>ted out, <strong>the</strong> orig<strong>in</strong>al emphasis<br />
was on degrees <strong>of</strong> tolerance <strong>of</strong> shade, but <strong>in</strong> <strong>the</strong> present century differences <strong>in</strong> <strong>the</strong><br />
ability to tolerate root competition have been <strong>in</strong>creas<strong>in</strong>gly emphasized. In <strong>the</strong> 1950s<br />
<strong>the</strong> importance <strong>of</strong> differences between <strong>species</strong> at all <strong>the</strong> stages <strong>in</strong> <strong>the</strong> regeneration<br />
cycle came to be appreciated particularly <strong>in</strong> relation to tropical ra<strong>in</strong>-forest and tropical<br />
deciduous forest (Jones, 1955-6; Hewetson, 1956; Steenis, 1956). More recently <strong>the</strong><br />
question has been discussed and illustrated for tropical ra<strong>in</strong>-forest by Budowski (1965),<br />
Poore (1967,1968), Ashton (1969), Richards (1969), Rollet (1969), Janzen (1970) and<br />
Whitmore (1974, 1975) and for a subtropical ra<strong>in</strong>-forest by Webb,Tracey & Williams<br />
(1972). A particular idea that attracted much attention at <strong>the</strong> time was that <strong>of</strong> Aubreville<br />
(1938), who suggested that any one tree <strong>species</strong> never regenerated under itself;<br />
later workers, however, have shown that this generalization was not true (Jones, 1955-6;<br />
Foggie, 1960; Schulz, 1960; Rollet, 1969). Most recently emphasis among writers<br />
concerned with <strong>species</strong>-<strong>richness</strong> <strong>in</strong> tropical forests has been concentrated on differences<br />
<strong>in</strong> flower<strong>in</strong>g phenology (Gentry, 1974, 1976; Stiles, 1975). There seems to have been<br />
much less explicit discussion <strong>of</strong> <strong>ma<strong>in</strong>tenance</strong> <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> temperate forests,<br />
although more <strong>in</strong>terest has been shown <strong>in</strong> <strong>the</strong> last two years - see, for example, <strong>the</strong><br />
papers <strong>of</strong> Forcier (1975), Horn (1975), Pigott (1975) and Ash & Barkham (1976).<br />
Many earlier studies <strong>of</strong> regeneration showed implicitly that <strong>the</strong> same pr<strong>in</strong>ciples<br />
apply as <strong>in</strong> tropical forests, e.g. those <strong>of</strong> Watt (1934, 1947), Graham (1941), Jones<br />
(1945), Bray (1956), Curtis (1959), Franklyn & Dyrness (1969), Auclair & Cottam<br />
(1971), Williamson (1975), Sprugel (1976) and <strong>of</strong> various Cont<strong>in</strong>ental authors as<br />
summarized by Ellenberg (1963). Whittaker (1969) mentioned <strong>the</strong> ‘successional<br />
niche’ and appeared to refer to <strong>the</strong> parts played by different <strong>species</strong> <strong>in</strong> <strong>the</strong> natural<br />
regeneration <strong>of</strong> eastern deciduous forest <strong>in</strong> <strong>the</strong> U.S.A. after sheet destruction by fire;<br />
<strong>in</strong> a later paper (Whittaker, 1972, pp. 239-40) he seemed to broaden his concept <strong>of</strong> <strong>the</strong><br />
successional niche but he considered it only briefly. Loucks (1970), <strong>in</strong> a general treatment<br />
<strong>of</strong> diversity, also started from a forestry background; he emphasized <strong>the</strong> importance<br />
<strong>of</strong> <strong>species</strong> react<strong>in</strong>g differently to periodic perturbations, notably fires and cyclones.<br />
In treatments generally detached from <strong>the</strong> discussion <strong>of</strong> particular <strong>communities</strong><br />
Skellam (1951) has shown <strong>the</strong> importance <strong>of</strong> differences <strong>in</strong> dispersability, and Harper<br />
and colleagues have repeatedly emphasized <strong>the</strong> potential importance <strong>of</strong> different<br />
requirements for germ<strong>in</strong>ation and establishment, and <strong>the</strong> nature <strong>of</strong> ‘safe sites’ for<br />
particular <strong>species</strong> (Harper, 1961, 1965 ; Harper et al. 1961 ; Harper, Williams & Sagar,<br />
1965; Cavers & Harper, 1967; Sheldon, 1974).<br />
Most recent discussions on diversity <strong>in</strong> grasslands have been notable for a failure to<br />
appreciate <strong>the</strong> relevance <strong>of</strong> regeneration processes - see, for example, <strong>the</strong> articles <strong>of</strong><br />
McNaughton (1967, 1968b), Grime (1937a, b) and Newman (1973). They have<br />
concentrated on <strong>the</strong> negative correlation <strong>of</strong>ten found between <strong>species</strong>-<strong>richness</strong> and
I I8<br />
P. J. GRUBB<br />
grassland productivity, i.e. <strong>the</strong> yield <strong>of</strong> new <strong>plant</strong> matter per unit area and time. This<br />
situation illustrates a po<strong>in</strong>t <strong>of</strong> great importance for <strong>the</strong> present discussion. Differences<br />
<strong>in</strong> requirements for regeneration can only be effective <strong>in</strong> ma<strong>in</strong>ta<strong>in</strong><strong>in</strong>g <strong>species</strong>-<strong>richness</strong><br />
if <strong>the</strong> adult <strong>plant</strong>s <strong>of</strong> <strong>the</strong> many <strong>species</strong> are reasonably evenly balanced <strong>in</strong> competition.<br />
In some k<strong>in</strong>ds <strong>of</strong> vegetation this is less likely to be <strong>the</strong> case if <strong>the</strong> grow<strong>in</strong>g conditions are<br />
generally very favourable to <strong>plant</strong> growth; under such conditions a few potentially<br />
vigorous and tall-grow<strong>in</strong>g <strong>species</strong> generally elim<strong>in</strong>ate many <strong>species</strong> which could<br />
persist <strong>in</strong> <strong>the</strong> absence <strong>of</strong> competition. The grassland vegetation <strong>of</strong> N.W. Europe is<br />
a useful example; here <strong>the</strong>re is a strikiig contrast between <strong>the</strong> <strong>species</strong>-<strong>richness</strong> <strong>of</strong> <strong>the</strong><br />
grasslands on shallow calcareous soils (short <strong>of</strong> nitrogen, phosphorus and o<strong>the</strong>r<br />
m<strong>in</strong>eral nutrients) and <strong>the</strong> <strong>species</strong>-paucity <strong>of</strong> much grassland on deeper soils at pH 5-6<br />
(with a better supply <strong>of</strong> N and P). The potential for cont<strong>in</strong>ued co-existence by differentiation<br />
<strong>in</strong> requirements for regeneration is very great <strong>in</strong> <strong>the</strong> calcareous grassland but<br />
much reduced <strong>in</strong> <strong>the</strong> grassland at pH 5-6. When <strong>species</strong>-rich, nutrient-poor grasslands<br />
are fertilized, <strong>the</strong> stand<strong>in</strong>g crop is <strong>in</strong>creased and <strong>the</strong> <strong>species</strong>-<strong>richness</strong> sooner or later<br />
decl<strong>in</strong>es. This has been shown for calcareous fixed dunes by Willis (1963), for wet<br />
heaths by Sougnez (1966), and for montane calcareous pasture by Jeffrey & Pigott<br />
(1973). Whe<strong>the</strong>r <strong>the</strong>re is a similar effect to be found generally <strong>in</strong> forests seems doubtful.<br />
Thus Watt (1934) recorded only 47 <strong>species</strong> <strong>in</strong> <strong>the</strong> herb-layer <strong>in</strong> 23 samples <strong>of</strong> escarpment<br />
beechwoods on calcareous soils <strong>in</strong> <strong>the</strong> Chiltern Hills compared with 64 <strong>species</strong> <strong>in</strong><br />
10 samples <strong>of</strong> plateau woods on clay-derived soils <strong>of</strong> pH 4-7-6.0 <strong>in</strong> <strong>the</strong> same area; ash,<br />
beech and oak all grow taller on <strong>the</strong> clay-soils than on <strong>the</strong> chalk soils (e.g. 25-32 m versus<br />
20-23 m tall for beech) and <strong>the</strong> fertility for most <strong>plant</strong>s is greater on <strong>the</strong> clay-soils. The<br />
data from Watt’s two samples are not strictly comparable because <strong>the</strong> list for <strong>the</strong> escarpment<br />
beechwoods is <strong>in</strong>complete. However, this fact is counter-balanced by <strong>the</strong> <strong>in</strong>clusion<br />
<strong>in</strong> <strong>the</strong> lists <strong>of</strong> many <strong>species</strong> not typical <strong>of</strong> mature woods and suggest<strong>in</strong>g a more recent<br />
orig<strong>in</strong> for at least parts <strong>of</strong> <strong>the</strong> woodlands on <strong>the</strong> scarp. Watt’s data cannot be taken to<br />
prove that <strong>the</strong> field-layer is more <strong>species</strong>-rich on <strong>the</strong> more fertile soil, but <strong>the</strong>y illustrate<br />
a general f<strong>in</strong>d<strong>in</strong>g <strong>in</strong> central and nor<strong>the</strong>rn Europe that beechwoods on deeper<br />
soils <strong>of</strong> pH 5-6 (Asperulo-Fagion) are generally more <strong>species</strong>-rich than those on<br />
shallow calcareous soils (Cephalan<strong>the</strong>ro-Fagion), as documented by Ellenberg (I 963).<br />
The contrast with grasslands <strong>in</strong> <strong>the</strong> same region <strong>in</strong> <strong>the</strong> trend <strong>in</strong> <strong>species</strong>-<strong>richness</strong><br />
reflects <strong>the</strong> much smaller number <strong>of</strong> strictly calcicolous <strong>species</strong> found <strong>in</strong> woodlands,<br />
i.e. <strong>species</strong> strictly conf<strong>in</strong>ed to calcareous soils (Fenner, 1975). The fact that many<br />
<strong>species</strong> able to grow on soils <strong>of</strong> pH < 5 can also grow at pH 5-6, but not on calcareous<br />
soils, is also important.<br />
Despite <strong>the</strong> considerable history <strong>of</strong> discussion <strong>of</strong> regeneration processes, it is not<br />
only <strong>the</strong> students <strong>of</strong> grassland diversity who have ignored <strong>the</strong>m. Several dist<strong>in</strong>guished<br />
authors, who have written recently about co-existence <strong>of</strong> <strong>plant</strong> <strong>species</strong> <strong>in</strong> general, have<br />
failed even to mention <strong>the</strong> possibilities <strong>of</strong> differentiation <strong>in</strong> requirements for successful<br />
regeneration, e.g. Goodall (I970), Walter (1971) and Whittaker (1975).
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> 119<br />
IV. DEFINITION OF A PLANT’S NICHE<br />
The niche <strong>of</strong> a <strong>plant</strong> is taken here to be <strong>the</strong> def<strong>in</strong>ition <strong>of</strong> its total relationship with<br />
its environment, both physico-chemical and biotic; such a def<strong>in</strong>ition necessarily<br />
<strong>in</strong>cludes a statement <strong>of</strong> <strong>the</strong> role played by <strong>the</strong> <strong>plant</strong> as well as a statement <strong>of</strong> its tolerance.<br />
The present author sees no advantage <strong>in</strong> replac<strong>in</strong>g <strong>the</strong> term ‘niche’, used <strong>in</strong> this sense<br />
by ‘ecotope’, as suggested elsewhere (Whittaker, Lev<strong>in</strong> & Root, 1973). My usage <strong>of</strong><br />
‘niche’ is essentially <strong>in</strong> agreement with that <strong>of</strong> Richards (1969) and Wuenscher<br />
(1969, 1974). Four component niches have to be recognized <strong>in</strong> a complete def<strong>in</strong>ition<br />
<strong>of</strong> a <strong>plant</strong>’s niche.<br />
(i) The habitat niche, i.e. <strong>the</strong> physical and chemical limits tolerated by <strong>the</strong> mature<br />
<strong>plant</strong> <strong>in</strong> nature. The def<strong>in</strong>ition should <strong>in</strong>clude an expression <strong>of</strong> <strong>the</strong> k<strong>in</strong>ds <strong>of</strong> fluctuations<br />
from <strong>the</strong> mean climatic conditions which favour <strong>the</strong> <strong>plant</strong>’s vegetative development.<br />
(ii) The life-form niche, <strong>in</strong>clud<strong>in</strong>g an expression <strong>of</strong> size and annual productivity as<br />
well as three-dimensional pattern.<br />
(iii) The phenological niche, i.e. <strong>the</strong> pattern <strong>of</strong> seasonal development.<br />
(iv) The regeneration niche, i.e. an expression <strong>of</strong> <strong>the</strong> requirements for a high chance<br />
<strong>of</strong> success <strong>in</strong> <strong>the</strong> replacement <strong>of</strong> one mature <strong>in</strong>dividual by a new mature <strong>in</strong>dividual <strong>of</strong><br />
<strong>the</strong> next generation, concern<strong>in</strong>g all <strong>the</strong> processes and characters <strong>in</strong>dicated <strong>in</strong> Table 2.<br />
The habitat niche is <strong>the</strong> <strong>plant</strong>’s address. The life-form and phenological niches are<br />
<strong>the</strong> <strong>plant</strong>’s pr<strong>of</strong>ession. The regeneration niche <strong>in</strong>cludes elements <strong>of</strong> both address and<br />
pr<strong>of</strong>ession.<br />
Hutch<strong>in</strong>son’s (1957) idea <strong>of</strong> recogniz<strong>in</strong>g a ‘potential niche’ (that effective <strong>in</strong> <strong>the</strong><br />
absence <strong>of</strong> competitors and predators) and a ‘realized niche’ (that effective <strong>in</strong> <strong>the</strong><br />
presence <strong>of</strong> competitors and predators) can be applied to all four aspects <strong>of</strong> <strong>the</strong> niche<br />
but is particularly important for <strong>the</strong> habitat niche.<br />
Before consider<strong>in</strong>g examples <strong>of</strong> differentiation <strong>in</strong> <strong>the</strong> regeneration niche, it is<br />
necessary to recapitulate briefly what is known <strong>of</strong> <strong>the</strong> process <strong>of</strong> regeneration <strong>in</strong> <strong>the</strong><br />
different major types <strong>of</strong> <strong>plant</strong> community.<br />
V. PATTERNS OF REGENERATION IN DIFFERENT PLANT COMMUNITIES<br />
Four major types <strong>of</strong> vegetation may be recognized <strong>in</strong> this context, determ<strong>in</strong>ed by <strong>the</strong><br />
dom<strong>in</strong>ant life-forms present: forests and woodlands, shrub-dom<strong>in</strong>ated <strong>communities</strong>,<br />
grasslands and associations <strong>of</strong> ephemeral herbs.<br />
In most natural forests regeneration <strong>in</strong>volves clear<strong>in</strong>gs <strong>of</strong> some k<strong>in</strong>d, but <strong>the</strong> size <strong>of</strong><br />
<strong>the</strong> clear<strong>in</strong>gs varies enormously. The two extremes are represented by forests suffer<strong>in</strong>g<br />
occasional sheet-destruction by fire and those with gaps created by <strong>the</strong> fall <strong>of</strong> s<strong>in</strong>gle<br />
trees or branches. Intermediate types <strong>in</strong>clude those <strong>in</strong> which gaps <strong>of</strong> 0.1-1-0 ha are<br />
caused by cyclone damage (Webb, 1958; Whitmore, 1974) and those with strips <strong>of</strong><br />
dead and dy<strong>in</strong>g trees somehow related to strong and constant w<strong>in</strong>ds (Oshima, Kimura,<br />
Iwaki & Kuroiwa, 1958; Iwaki & Totsuka, 1959; Sprugel, 1976). In many areas<br />
occasional regeneration <strong>in</strong> sheets after fire alternates with a patchwork regeneration
I20<br />
P. J. GRUBB<br />
pattern dur<strong>in</strong>g fire-free periods, e.g. <strong>in</strong> <strong>the</strong> Noth<strong>of</strong>qus forests <strong>of</strong> Tasmania where<br />
Eucalyptus regnans colonizes after fire (Gilbert, 1959; Jackson, 1968), or <strong>in</strong> <strong>the</strong> Piceu-<br />
Tmgu forests <strong>of</strong> western North America where Pseudotsuga mxiesii <strong>in</strong>vades after<br />
fire (Franklyn & Dyrness, 1969). In most forests some trees die ‘on <strong>the</strong>ir feet’ and new<br />
trees grow up <strong>in</strong> <strong>the</strong>ir place without <strong>the</strong> formation <strong>of</strong> an obvious gap <strong>in</strong> <strong>the</strong> canopy;<br />
this pattern is probably very widespread <strong>in</strong> low-stature forests (e.g. many subalp<strong>in</strong>e<br />
and upper montane tropical forests), <strong>in</strong> which <strong>the</strong> <strong>in</strong>dividual trees rarely develop large<br />
trunks and appear to collapse after death without mak<strong>in</strong>g gaps (Grubb & Stevens, 1976;<br />
Tanner, 1977). Regeneration <strong>of</strong> trees <strong>in</strong> all <strong>the</strong>se forest-types is primarily by seed, but<br />
regeneration from burnt stumps may be important <strong>in</strong> mediterranean climates;<br />
a valuable summary <strong>of</strong> <strong>the</strong> relevant f<strong>in</strong>d<strong>in</strong>gs <strong>in</strong> California is given by Mooney & DUM<br />
(1970) and some basic <strong>in</strong>formation for <strong>the</strong> European Mediterranean zone by Trabaud<br />
(1970). For shrubs and herbs, vegetative spread <strong>in</strong>to clear<strong>in</strong>gs is relatively more<br />
important (Kujala, 1926; Whitford, 1949; Flower-Ellis, 1971 ; Korchag<strong>in</strong> & Karpov,<br />
1974), but <strong>the</strong>re are certa<strong>in</strong>ly many forest herbs for which regeneration is ma<strong>in</strong>ly by<br />
seed (Curtis, 1959; Wilson, 1959; Knight, 1964). Regeneration <strong>of</strong> climbers, epiphytes<br />
and parasites appears to occur most <strong>of</strong>ten by seed <strong>in</strong> a patchwork pattern, broadly<br />
analogous to that for most trees, though <strong>in</strong>vasion <strong>of</strong> new gaps by vegetative spread is<br />
no doubt important for some climbers and scramblers, e.g. many bamboos.<br />
In shrub-dom<strong>in</strong>ated <strong>communities</strong> regeneration is sometimes chiefly by seed as <strong>in</strong><br />
<strong>the</strong> <strong>species</strong>-poor Callunetum <strong>of</strong> eastern England (Watt, 1947, 1955) and <strong>in</strong> semideserts<br />
(Bykov, 1974; Davies, 1976), but <strong>in</strong> many wetter <strong>communities</strong> with creep<strong>in</strong>g<br />
Ericaceae or Epacridaceae (e.g. many subalp<strong>in</strong>e heaths <strong>of</strong> both hemispheres) <strong>in</strong>vasion<br />
<strong>of</strong> gaps by vegetative spread is more common (Burges, 1951; Chambers, 1959;<br />
Barrow, Cost<strong>in</strong> & Lake, 1968; Keat<strong>in</strong>ge, 1975).<br />
In closed grassland, i.e. grassland with a cont<strong>in</strong>uous cover, <strong>the</strong> extent <strong>of</strong> regeneration<br />
by seed is not clear. Vegetative regeneration <strong>in</strong> a kaleidoscopic pattern is certa<strong>in</strong>ly<br />
important <strong>in</strong> a variety <strong>of</strong> grasslands, e.g. <strong>in</strong> meadows <strong>in</strong> Sweden (Tamm, 1956,1972u,<br />
b) and Russia (Rabotnov 1969b), <strong>in</strong> hill pasture <strong>in</strong> Scotland and Wales (Harberd, 1961 ;<br />
Chadwick, 1961), and <strong>in</strong> lowland grassland on chalk <strong>in</strong> England (Aust<strong>in</strong>, 1968). The<br />
rates <strong>of</strong> vegetative spread vary greatly but have been estimated at 2-6 cm per year for<br />
certa<strong>in</strong> dom<strong>in</strong>ant grasses (Harberd, 1961 ; Chadwick, 1961 ; Aust<strong>in</strong>, 1968). However,<br />
<strong>the</strong> half-lives <strong>of</strong> adult <strong>plant</strong>s <strong>of</strong> <strong>the</strong> perennial <strong>species</strong> commonly vary from 1-2 years up<br />
to more than 50 years (Harper, 1967; Tamm, 1972u, b; Sarukhhn & Harper, 1973),<br />
and for some <strong>species</strong> may be <strong>of</strong> <strong>the</strong> order <strong>of</strong> several hundred years (Harberd, 1961 ;<br />
O<strong>in</strong>enen, 1967u, b). The short-lived perennials as well as <strong>the</strong> annuals and biennials,<br />
found <strong>in</strong> cont<strong>in</strong>uous turf with gaps only 1-5 cm across must regenerate by seed<br />
(Sarukhhn & Harper, 1973 ; Grubb, 1976), but for many grassland <strong>species</strong> creation <strong>of</strong><br />
larger gaps by animals (e.g. soil-heaps <strong>of</strong> moles or gophers, or scratch<strong>in</strong>gs by rabbits<br />
or badgers) may be necessary for regeneration by seed (Watt, 1971, 1974; Miles, 1973 ;<br />
Platt, 1975). The pattern <strong>of</strong> regeneration is particularly obscure <strong>in</strong> closed grassland<br />
dom<strong>in</strong>ated by large tussocks such as those <strong>of</strong> Mol<strong>in</strong>iu <strong>in</strong> western Europe (Godw<strong>in</strong>,<br />
1941 ; Rutter, 1955), Dunthonia and Poa <strong>in</strong> New Zealand (Cockayne, 1928) or Deyeuxiu<br />
(Culumugrostis) <strong>in</strong> <strong>the</strong> high Andes (Cuatrecasas, 1934; Diels, 1937).
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong><br />
In open grassland, developed <strong>in</strong> situations deficient <strong>in</strong> m<strong>in</strong>eral nutrients and/or<br />
water and consist<strong>in</strong>g <strong>of</strong> isolated <strong>plant</strong>s, <strong>the</strong> latter may have quite short half-lives <strong>of</strong><br />
5 7 years (SarukhQn & Harper, 1973) and probably regenerate chiefly by seed.<br />
However, <strong>the</strong> huge tussocks <strong>of</strong> Triodia and Plectrachne (‘sp<strong>in</strong>ifex’) <strong>in</strong> <strong>the</strong> semi-deserts<br />
<strong>of</strong> central Australia (W<strong>in</strong>kworth, 1967; Beard, 1969) appear to be much longer-lived.<br />
Natural associations <strong>of</strong> ephemeral herbs (<strong>plant</strong>s represented only by bulbs, corms or<br />
seeds <strong>in</strong> <strong>the</strong> unfavourable season) usually exist as sub-systems <strong>of</strong> more extensive<br />
<strong>communities</strong>, e.g. <strong>in</strong> semi-deserts, on cliffs or <strong>in</strong> successional stages on dunes. Little<br />
is known about <strong>the</strong> regeneration <strong>of</strong> <strong>plant</strong>s with bulbs and corms, but <strong>the</strong> o<strong>the</strong>rs, by<br />
def<strong>in</strong>ition, regenerate only by seed.<br />
In summary, it is apparent that very various patterns are <strong>in</strong>volved <strong>in</strong> regeneration<br />
and that much rema<strong>in</strong>s to be discovered about <strong>the</strong> process. However, it is clear that<br />
regeneration <strong>in</strong>volves both seed and vegetative organs and that <strong>the</strong> relative importance<br />
<strong>of</strong> <strong>the</strong>se two depends on <strong>the</strong> community concerned.<br />
It is necessary to comment here on <strong>the</strong> relation between regeneration and succession,<br />
two dist<strong>in</strong>ct processes which have <strong>of</strong>ten been confused, especially <strong>in</strong> <strong>the</strong> U.S.A., where<br />
much attention has been paid to <strong>the</strong> successions <strong>of</strong> <strong>species</strong> found after fire. Succession,<br />
as a term, is best reserved for non-cyclic series <strong>of</strong> vegetation types, ei<strong>the</strong>r <strong>in</strong> primary<br />
seres (e.g. on mora<strong>in</strong>es left by glaciers, <strong>in</strong> lakes or on sand dunes) or <strong>in</strong> secondary seres<br />
(e.g. on abandoned farm land). Confusion is bound to arise where man-made secondary<br />
successions merge with natural regeneration processes, as <strong>in</strong> <strong>the</strong> post-fire situation <strong>in</strong><br />
much <strong>of</strong> <strong>the</strong> U.S.A. but it is still worth while to dist<strong>in</strong>guish <strong>the</strong> processes <strong>in</strong> general.<br />
For most <strong>species</strong> <strong>the</strong> ‘successional niche’ is best treated as part <strong>of</strong> <strong>the</strong> def<strong>in</strong>ition <strong>of</strong> <strong>the</strong><br />
habitat niche, but <strong>in</strong> <strong>the</strong> case <strong>of</strong> <strong>species</strong> <strong>in</strong> certa<strong>in</strong> secondary seres (notably post-fire<br />
sere) it may be more useful to <strong>in</strong>clude it <strong>in</strong> <strong>the</strong> def<strong>in</strong>ition <strong>of</strong> <strong>the</strong> regeneration niche.<br />
Regeneration <strong>in</strong> a gap <strong>of</strong> any k<strong>in</strong>d usually <strong>in</strong>volves a succession <strong>of</strong> <strong>species</strong> - <strong>the</strong><br />
serule <strong>of</strong> Daubenmire (1968~). An obvious k<strong>in</strong>d <strong>of</strong> differentiation <strong>in</strong> <strong>the</strong> regeneration<br />
niche concerns time <strong>of</strong> appearance <strong>in</strong> this serule (G6mez-Pompa, 1971). However,<br />
s<strong>in</strong>ce several or many <strong>species</strong> are usually present at each stage, niche differentiation<br />
must concern gap quality as well.<br />
I21<br />
VI. EXAMPLES OF DIFFERENTIATION IN THE REGENERATION NICHE<br />
(a) Production <strong>of</strong> viable seed<br />
It is a commonplace that <strong>in</strong> a particular region certa<strong>in</strong> years are ‘good years’ for<br />
some crops andnot for o<strong>the</strong>rs and variation <strong>of</strong> this type for <strong>species</strong> cultivated <strong>in</strong> fields and<br />
orchards has been documented <strong>the</strong> world over. There is plenty <strong>of</strong> <strong>in</strong>formation too for<br />
forest trees <strong>in</strong> many parts <strong>of</strong> <strong>the</strong> world: Central Europe (Schwappach, 1895; Seeger,<br />
1913), North America (Morris, 1951; Boe, 1954; Fowells & Schubert, 1956;<br />
Daubenmire, 1960; Baron, 1969; Clark, 1970), <strong>the</strong> U.S.S.R. (MolEanoq, 1967)~<br />
Japan (Tagawa, 1969,1971) and New Zealand (Beveridge, 1964,1973; Wardle, 1970).<br />
However, <strong>the</strong>re is only a little <strong>in</strong>formation <strong>of</strong> this type for shrubs, e.g. that <strong>of</strong> Davies<br />
(1976) on Australian semi-desert <strong>species</strong>, or for perennial grassland herbs, e.g. that<br />
<strong>of</strong> Rabotnov (1950) and Golubeva (1968) for selected <strong>plant</strong>s <strong>in</strong> meadows and pastures
I22<br />
P. J. GRUBB<br />
74 76 78 ao a2 a4 a6 aa 90 92<br />
Years<br />
Fig. 5. Numbers <strong>of</strong> seed produced by three tree <strong>species</strong> <strong>in</strong> Prussia <strong>in</strong> <strong>the</strong> years 1874-93 (after<br />
Schwappach, 1895) : (a) P<strong>in</strong>ur sylvestris, (b) Quercus robur (open columns) and Fagus sylvatica<br />
(filled columns).<br />
r (a) 3442<br />
1000 -<br />
1<br />
n<br />
TI x<br />
L al<br />
n<br />
el<br />
0,<br />
V c<br />
3<br />
v) 0<br />
800<br />
600<br />
400<br />
200<br />
-<br />
-<br />
-<br />
-<br />
-<br />
-<br />
-<br />
-<br />
-<br />
-<br />
O U<br />
Fig. 6. Numbers <strong>of</strong> sound seed per unit area produced <strong>in</strong> <strong>the</strong> years 1961-7 by marked trees <strong>of</strong><br />
five <strong>species</strong> <strong>of</strong> Podocarpaceae <strong>in</strong> New Zealand: (a) Dacrydium cupress<strong>in</strong>unz, (b) Podocarpus<br />
ferrug<strong>in</strong>eus, (c) P. spicatus, (d) P. totara and (e) P. dacrydioides (after Beveridge, 1973).
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> 123<br />
1960 1961 1964 1965 1966 1967 1960 1969 1970 1971<br />
Scaevo/a sp<strong>in</strong>escens 0. ..OOO..<br />
Acacia aneura 0 ~ O . ~ . , .<br />
Acacia pru<strong>in</strong>ocarpa<br />
0<br />
O....0.<br />
Acacia tetragonophylla 0 0 . 0 . O 0.<br />
Cassia desolata 0 0 . 0 0.0 0.<br />
Cassia helrnsii 0 . 0 0 . ~ ~ .<br />
Erernophila fraseri ..........<br />
Acacia victoriae O ~ O O ~ O O<br />
Hakea lorea 0 O o....o<br />
Santalum spicatum o OOO..OO<br />
0 0<br />
0 1-25 26-50<br />
51-75 76-100<br />
Fig. 7. The crops <strong>of</strong> fruit produced by ten <strong>species</strong> <strong>of</strong> trees and shrubs over 10 years <strong>in</strong> an arid<br />
region <strong>of</strong> Western Australia (from Davies, 1976). The results are expressed as percentages <strong>of</strong> <strong>the</strong><br />
best year’s production. Blanks <strong>in</strong>dicate that nu recoras were kept <strong>in</strong> those years. Reproduced<br />
from <strong>the</strong> Journal <strong>of</strong> Ecology with permission.<br />
<strong>in</strong> <strong>the</strong> U.S.S.R. and <strong>of</strong> Sarukhh & Harper (1973) for <strong>plant</strong>s <strong>in</strong> a Welsh pasture.<br />
There are also very few data for annuals, e.g. those <strong>of</strong> Newman (1964) for certa<strong>in</strong><br />
w<strong>in</strong>ter annuals <strong>in</strong> eastern England.<br />
In each <strong>of</strong> <strong>the</strong> forest <strong>communities</strong> that have been studied, three ma<strong>in</strong> patterns <strong>of</strong><br />
seed production have emerged: (a) moderate production <strong>in</strong> most years, (b) fruit<strong>in</strong>g<br />
ra<strong>the</strong>r irregular, and (c) abundant fruit<strong>in</strong>g strongly periodic (see Figs. 5 and 6). The<br />
same patterns have been found <strong>in</strong> a sample <strong>of</strong> <strong>species</strong> <strong>of</strong> semi-desert scrub (Fig. 7).<br />
Different <strong>species</strong> <strong>in</strong> group (b) are favoured by different years. It is important to<br />
realize that <strong>the</strong> variability <strong>in</strong> <strong>the</strong> supply <strong>of</strong> viable seed to different gaps will be greater<br />
than that suggested by <strong>the</strong> mean differences between <strong>species</strong>, not only because <strong>of</strong><br />
differences <strong>in</strong> dispersal, etc., but also because <strong>of</strong> differences between <strong>in</strong>dividuals <strong>of</strong><br />
each <strong>species</strong> - at least those <strong>in</strong> groups (a) and (b). In any one year some <strong>in</strong>dividuals<br />
fruit abundantly and o<strong>the</strong>rs less SO.<br />
It is virtually certa<strong>in</strong> that <strong>the</strong> three ma<strong>in</strong> patterns <strong>of</strong> seed production reflect three<br />
correspond<strong>in</strong>g patterns <strong>of</strong> flower<strong>in</strong>g, although <strong>the</strong>re is little published evidence to<br />
prove <strong>the</strong> po<strong>in</strong>t. The three patterns have been shown clearly <strong>in</strong> herbs <strong>in</strong> Europe<br />
(Fig. 8) and trees <strong>in</strong> Malaya (Medway, 1972). However, no exact correspondence<br />
between <strong>the</strong> numbers <strong>of</strong> flowers and <strong>of</strong> viable seeds can be expected because <strong>of</strong> wide<br />
variations <strong>in</strong> <strong>the</strong> success <strong>of</strong> poll<strong>in</strong>ation and <strong>the</strong> ‘sett<strong>in</strong>g’ or ripen<strong>in</strong>g <strong>of</strong> <strong>the</strong> seed.
c<br />
46 47 4% 49 50 51 52 53 54 55 56 57 50 59 60 61 62 63<br />
124 P. J. GRUBB<br />
Years<br />
Fig. 8. (a) The flower<strong>in</strong>g <strong>in</strong> 1946-56 <strong>of</strong> mapped <strong>in</strong>dividuals <strong>of</strong> Annnone hepatica (-) and<br />
Sanicula europaea (---) that flowered <strong>in</strong> 1946 <strong>in</strong> certa<strong>in</strong> quadrats <strong>in</strong> Sweden and were still<br />
alive <strong>in</strong> 1956 (data from Tamm, 1956). (b) Total numbers <strong>of</strong> <strong>plant</strong>s flower<strong>in</strong>g <strong>in</strong> a small colony<br />
<strong>of</strong> Senecio <strong>in</strong>tegrifolius <strong>in</strong> England (data from Morgan, 1965).<br />
Unfortunately little is known about year-to-year variation <strong>in</strong> ei<strong>the</strong>r <strong>of</strong> <strong>the</strong>se processes.<br />
Circumstantial evidence exists for massive failure <strong>of</strong> poll<strong>in</strong>ation <strong>in</strong> some <strong>species</strong> <strong>in</strong><br />
some years <strong>in</strong> <strong>communities</strong> as widely separated as <strong>the</strong> lowland tropics (Medway, 1972)<br />
and <strong>the</strong> Arctic (Kevan, 1972) and it seems virtually certa<strong>in</strong> that less-dramatic fluctuations<br />
<strong>in</strong> <strong>the</strong> success <strong>of</strong> poll<strong>in</strong>ation also occur, e.g. <strong>in</strong> relation to <strong>the</strong> <strong>in</strong>cidence <strong>of</strong> high<br />
w<strong>in</strong>ds or ra<strong>in</strong>. As for <strong>the</strong> ripen<strong>in</strong>g <strong>of</strong> fruit <strong>the</strong> optimum conditions are known for only<br />
a very few wild <strong>species</strong>. Examples are <strong>the</strong> Douglas fir, Pseudotsuga menziesii (Lowry,<br />
1966), <strong>the</strong> perennial thistle, Cirsium acaulon (Pigott, 1968), and <strong>the</strong> w<strong>in</strong>ter annual<br />
Teesdalia nudicaulis (Newman, 1964, 1965). For crop <strong>plant</strong>s it is known that both<br />
optimum conditions and susceptibility to damag<strong>in</strong>g conditions vary with <strong>the</strong> stage <strong>of</strong><br />
development. For example, <strong>the</strong> yield <strong>of</strong> maize seeds was shown to suffer a 50%<br />
reduction as a result <strong>of</strong> a standardized drought treatment last<strong>in</strong>g 6-8 days at <strong>the</strong> stage<br />
when <strong>the</strong> primordia were grow<strong>in</strong>g most actively, but negligible reduction if <strong>the</strong> same<br />
stress was applied a little later <strong>in</strong> development (Robb<strong>in</strong>s & Dom<strong>in</strong>go, 1953). Similar<br />
results were obta<strong>in</strong>ed for barley by Asp<strong>in</strong>all, Nicholls & May (1964). Very high daytime<br />
temperatures (32-35 "C) cause flowers <strong>of</strong> <strong>the</strong> bean to abort if <strong>the</strong>y occur on <strong>the</strong><br />
day <strong>of</strong> open<strong>in</strong>g or one day before or after that, but <strong>the</strong>y cause much less damage if <strong>the</strong>y<br />
occur at a later stage <strong>in</strong> development (Smith & Pryor, 1962). High night temperatures<br />
are especially damag<strong>in</strong>g to seeds <strong>of</strong> peas about 5 days after flower<strong>in</strong>g (Lambert &<br />
L<strong>in</strong>ck, 1958; Karr, L<strong>in</strong>ck & Swanson, 1959) and similar results have been obta<strong>in</strong>ed<br />
for cherries (Tukey, 1952), apples (Tukey, 1955) and grapes (Tukey, 1958). The<br />
<strong>in</strong>cidence <strong>of</strong> short periods <strong>of</strong> drought and high temperatures <strong>in</strong> relation to <strong>the</strong> flower<strong>in</strong>g<br />
periods <strong>of</strong> wild <strong>plant</strong>s certa<strong>in</strong>ly varies from year to year and such periods must<br />
account for some <strong>of</strong> <strong>the</strong> year-to-year variation <strong>in</strong> seed-set. Much more <strong>in</strong>vestigation<br />
<strong>of</strong> this topic is needed. The losses due to seed-eat<strong>in</strong>g <strong>in</strong>sects and birds, and to fungal<br />
and bacterial pathogens, also need to be quantified, for very little <strong>in</strong>formation is at<br />
present available (Rabotnov, 1969a; Janzen, 1971 ; Bohart & Koeber, 1972).<br />
Great <strong>in</strong>terest attaches to <strong>the</strong> physiological processes <strong>in</strong>volved <strong>in</strong> <strong>the</strong> production
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> 125<br />
<strong>of</strong> viable seed and to <strong>the</strong> evolution <strong>of</strong> <strong>the</strong> variety <strong>of</strong> flower<strong>in</strong>g and fruit<strong>in</strong>g 'strategies'<br />
(not least <strong>the</strong> variation <strong>in</strong> phenology), but attention is concentrated here on <strong>the</strong> fact<br />
that <strong>in</strong> any one <strong>species</strong>-rich community <strong>the</strong> proportions <strong>of</strong> viable seeds <strong>of</strong> different<br />
<strong>species</strong> fall<strong>in</strong>g on any one area are certa<strong>in</strong> to vary appreciably from year to year.<br />
(b) Dispersal<br />
First, we are concerned with dispersal <strong>in</strong> space. In every <strong>species</strong>-rich community<br />
that has been studied a great variety <strong>of</strong> dispersal mechanisms has been found (van der<br />
Pijl, 1971). There can be little doubt that seeds <strong>of</strong> some <strong>species</strong> are, <strong>in</strong> general,<br />
dispersed fur<strong>the</strong>r afield than those <strong>of</strong> o<strong>the</strong>rs and it has long been appreciated that wide<br />
dispersal is important <strong>in</strong> <strong>species</strong> less fit <strong>in</strong> a competitive struggle between vegetative<br />
<strong>plant</strong>s (Skellam, 1951). In addition <strong>the</strong> relative effectiveness <strong>of</strong> different types <strong>of</strong><br />
dispersal <strong>in</strong> space will vary from year to year <strong>in</strong> response to variations <strong>in</strong> w<strong>in</strong>d speed,<br />
w<strong>in</strong>d direction, abundance <strong>of</strong> particular animal vectors, <strong>in</strong>cidence <strong>of</strong> flood<strong>in</strong>g, etc.<br />
The size <strong>of</strong> an <strong>in</strong>dividual gap <strong>in</strong> <strong>the</strong> community can also be important, e.g. <strong>in</strong> a large<br />
forest-clear<strong>in</strong>g few seeds <strong>of</strong> a heavy-fruited tree like Fagus reach <strong>the</strong> centre <strong>of</strong> <strong>the</strong> gap,<br />
but large numbers <strong>of</strong> light seeds such as those <strong>of</strong> Bet& may land <strong>the</strong>re; so Bet& is<br />
helped to persist <strong>in</strong> a forest dom<strong>in</strong>ated by Fagus (Watt, 1934).<br />
Secondly, we are concerned with dispersal <strong>in</strong> time. It is well established that <strong>the</strong> seeds<br />
<strong>of</strong> <strong>the</strong> various <strong>species</strong> <strong>in</strong> particular <strong>communities</strong> vary <strong>in</strong> <strong>the</strong> length <strong>of</strong> time for which<br />
<strong>the</strong>y rema<strong>in</strong> viable and that this variability is reflected <strong>in</strong> <strong>the</strong> seed bank <strong>of</strong> <strong>the</strong> soil.<br />
Information on which <strong>species</strong> <strong>in</strong> particular <strong>communities</strong> are most abundant <strong>in</strong> <strong>the</strong> seed<br />
bank is available for temperate grasslands (Chipp<strong>in</strong>dale & Milton, 1934; Champness<br />
& Morris, 1948; Major & Pyott, 1966), and for temperate and tropical forests (Guyot,<br />
1960; Guevara & G6mez-Pompa, 1972; Marks, 1974). It is probably extremely<br />
important for many subsidiary <strong>species</strong> that <strong>the</strong>y can regenerate from seed that has been<br />
<strong>in</strong> <strong>the</strong> soil for decades, whenever and wherever a gap arises <strong>in</strong> <strong>the</strong> community.<br />
Examples are <strong>species</strong> <strong>of</strong> Agrostis, H o h and Poa <strong>in</strong> grasslands <strong>in</strong> western Europe able<br />
to benefit from gaps made by animals, and <strong>species</strong> <strong>of</strong> Hypen'cum, Juncus and Verbascum<br />
<strong>in</strong> forests <strong>in</strong> <strong>the</strong> same area able to benefit from gaps made by tree-falls.<br />
It is certa<strong>in</strong> that seeds <strong>of</strong> different <strong>species</strong> vary <strong>in</strong>herently <strong>in</strong> longevity but differences<br />
<strong>in</strong> survival under natural conditions may be more closely related to palatability,<br />
digestibility and <strong>the</strong> <strong>in</strong>cidence <strong>of</strong> predators (animals and microbes) than to <strong>in</strong>herent<br />
differences <strong>in</strong> longevity. Seeds certa<strong>in</strong>ly differ <strong>in</strong> palatability (Janzen, 1969) and <strong>the</strong><br />
fact that <strong>the</strong> loss to predators is tremendous has been documented not only for forest<br />
trees with relatively large seeds (Watt, 1919, 1923; Curtis, 1959; Janzen, 1970;<br />
Medway, 1972) but also for herbs with quite small seeds (Tevis, 1958; Bohart &<br />
Koeber, 1972; Sarukhhn, 1974). However, <strong>the</strong>re are still very few data available on <strong>the</strong><br />
absolute losses to microbes as opposed to animals (Sarukhhn, 1974).<br />
Variability <strong>in</strong> predation <strong>in</strong> time and space is bound to contribute to <strong>the</strong> variation <strong>in</strong><br />
<strong>the</strong> numbers <strong>of</strong> viable seeds present at <strong>the</strong> time <strong>of</strong> formation <strong>of</strong> a particular gap.<br />
Janzen (1970) has placed great stress on losses to animals at this stage as critical to <strong>the</strong><br />
<strong>ma<strong>in</strong>tenance</strong> <strong>of</strong> diversity <strong>in</strong> tropical ra<strong>in</strong>-forests, argu<strong>in</strong>g that more or less host-specific<br />
predators, accumulat<strong>in</strong>g on <strong>the</strong> parent tree, can destroy most seeds fall<strong>in</strong>g under it,
I 26<br />
P. J. GRUBB<br />
so that any one tree will rarely be replaced on that site by ano<strong>the</strong>r <strong>of</strong> <strong>the</strong> same <strong>species</strong>.<br />
However, <strong>the</strong> observations <strong>of</strong> Connell (1971) suggest that <strong>the</strong> effects <strong>of</strong> animals with<br />
respect to <strong>the</strong> <strong>ma<strong>in</strong>tenance</strong> <strong>of</strong> <strong>species</strong>-<strong>richness</strong> are generally more important at a later<br />
(seedl<strong>in</strong>g) stage.<br />
(c) Germ<strong>in</strong>ation<br />
There is a vast literature on this topic. The chief po<strong>in</strong>t to be made here is that very<br />
many <strong>of</strong> <strong>the</strong> differences between <strong>species</strong> are effective <strong>in</strong> ensur<strong>in</strong>g that <strong>the</strong>ir seeds<br />
germ<strong>in</strong>ate <strong>in</strong> sites that differ <strong>in</strong> time and space. Some seeds germ<strong>in</strong>ate <strong>in</strong> shade but<br />
o<strong>the</strong>rs only <strong>in</strong> <strong>the</strong> light (Koller, 1972; K<strong>in</strong>g, 1975). Most seeds germ<strong>in</strong>ate most rapidly<br />
<strong>in</strong> protected micro-sites, e.g. <strong>in</strong> small gaps, but some benefit from <strong>the</strong> alternat<strong>in</strong>g cycles<br />
<strong>of</strong> dry<strong>in</strong>g and rewett<strong>in</strong>g suffered <strong>in</strong> more exposed sites, e.g. large gaps (Miles, 1974).<br />
Many seeds are destroyed by <strong>the</strong> high temperatures suffered <strong>in</strong> <strong>the</strong> litter and topsoil<br />
dur<strong>in</strong>g a fire, but o<strong>the</strong>rs are stimulated to germ<strong>in</strong>ate by such an event (Beadle, 1940;<br />
Gratkowski, I 961). Seed-form varies appreciably with<strong>in</strong> most <strong>communities</strong> and it<br />
has been shown that <strong>the</strong> shape and size (Harper, Love11 & Moore, 1970) as well as <strong>the</strong><br />
behaviour <strong>of</strong> hygroscopic pappus hairs (Sheldon, 1974) or awns (Kerner von Marilaun,<br />
1902), are important <strong>in</strong> determ<strong>in</strong><strong>in</strong>g <strong>the</strong> degree <strong>of</strong> burial <strong>of</strong> a seed on a particular type<br />
<strong>of</strong> soil surface and <strong>the</strong> chance <strong>of</strong> its be<strong>in</strong>g able to take up enough water for enough time<br />
to germ<strong>in</strong>ate. Differences <strong>in</strong> soil chemistry also affect germ<strong>in</strong>ation, for example <strong>the</strong><br />
levels <strong>of</strong> nitrate (Williams & Harper, 1965; Williams, 1969), calcium (Parham, 1970),<br />
and ethylene (Olatoye & Hall, 1973). A special case is that <strong>of</strong> allelopathic <strong>in</strong>hibition,<br />
as seen <strong>in</strong> certa<strong>in</strong> scrub <strong>communities</strong> (Deleuil, 1951; McPherson & Muller, 1969);<br />
particularly <strong>in</strong>terest<strong>in</strong>g for <strong>the</strong> <strong>ma<strong>in</strong>tenance</strong> <strong>of</strong> diversity is <strong>the</strong> <strong>in</strong>hibition <strong>of</strong> germ<strong>in</strong>ation<br />
by <strong>the</strong> parents <strong>of</strong> <strong>the</strong> same <strong>species</strong> (Webb, Tracey & Haydock, 1967; McNaughton,<br />
1968a; Friedman & Orshan, 1975). There can be little doubt that <strong>in</strong> most <strong>communities</strong><br />
gaps <strong>of</strong> differ<strong>in</strong>g quality arise and that <strong>the</strong> proportions <strong>of</strong> <strong>the</strong> different <strong>species</strong> able to<br />
germ<strong>in</strong>ate differ from gap to gap.<br />
Gaps differ not only <strong>in</strong> <strong>the</strong>ir quality at any one time but also <strong>in</strong> <strong>the</strong>ir tim<strong>in</strong>g. That<br />
this is important for <strong>ma<strong>in</strong>tenance</strong> <strong>of</strong> <strong>species</strong> mixtures can be shown by two examples.<br />
First, it is known that gaps on river-banks <strong>in</strong> eastern England caused by scour <strong>in</strong><br />
autumn usually become occupied by Epilobium hirsutum and those formed <strong>in</strong> spr<strong>in</strong>g<br />
by Lythrum sulkaria (Whitehead, 1971). The Epilobium excludes <strong>the</strong> Lythrum from<br />
autumn gaps by its earlier development. The Epilobium is unable to become established<br />
as late as <strong>the</strong> spr<strong>in</strong>g so that Lythrum succeeds at that time. The physiological<br />
basis <strong>of</strong> <strong>the</strong> dist<strong>in</strong>ction is fairly well understood (Shamsi & Whitehead, 1974).<br />
Secondly, it is known that Avena ludoviciana, which germ<strong>in</strong>ates <strong>in</strong> <strong>the</strong> w<strong>in</strong>ter,<br />
is a serious weed <strong>of</strong> w<strong>in</strong>ter cereals but it is almost totally destroyed by plough<strong>in</strong>g <strong>in</strong><br />
early spr<strong>in</strong>g. A. fatua, which germ<strong>in</strong>ates <strong>in</strong> <strong>the</strong> spr<strong>in</strong>g, is a severe pest <strong>of</strong> spr<strong>in</strong>g-sown<br />
cereals (Thurston, 1951). In more general terms <strong>the</strong> periods <strong>of</strong> germ<strong>in</strong>ation <strong>of</strong><br />
different <strong>species</strong> <strong>in</strong> a particular community are spread throughout <strong>the</strong> year <strong>in</strong> such<br />
a way that <strong>species</strong> A has <strong>the</strong> maximum chance <strong>of</strong> success <strong>in</strong> a gap if it is formed by<br />
such-and-such a date and no catastrophe follows, but <strong>species</strong> B has <strong>the</strong> best chance if<br />
<strong>the</strong>re is a catastrophe and it germ<strong>in</strong>ates afterward. Thus an autumn-germ<strong>in</strong>at<strong>in</strong>g herb
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> 127<br />
<strong>in</strong> temperate grassland may succeed <strong>in</strong> regenerat<strong>in</strong>g an adult <strong>in</strong> a certa<strong>in</strong> gap if <strong>the</strong>re<br />
is no heavy frost-heav<strong>in</strong>g (and subsequent drought risk) and plenty <strong>of</strong> ra<strong>in</strong> <strong>in</strong> w<strong>in</strong>ter,<br />
but if <strong>the</strong>re is plenty <strong>of</strong> frost-heav<strong>in</strong>g and/or no ra<strong>in</strong>, <strong>the</strong>n a spr<strong>in</strong>g germ<strong>in</strong>at<strong>in</strong>g<br />
<strong>species</strong> is at an advantage. An example was observed by Salisbury (1929, p. 222);<br />
all <strong>the</strong> seedl<strong>in</strong>gs <strong>of</strong> <strong>the</strong> autumn-germ<strong>in</strong>at<strong>in</strong>g Ranumulus parviJeonCs were destroyed<br />
dur<strong>in</strong>g severe and prolonged w<strong>in</strong>ter frosts whereas seedl<strong>in</strong>gs <strong>of</strong> ‘ Helian<strong>the</strong>mum brewmi’<br />
(Tuberaria guttata (L.) Fourr.) germ<strong>in</strong>at<strong>in</strong>g <strong>in</strong> spr<strong>in</strong>g escaped <strong>the</strong> hazard. In fact <strong>the</strong><br />
year can be partitioned by <strong>the</strong> germ<strong>in</strong>ation periods <strong>of</strong> different <strong>species</strong> <strong>in</strong>to shorter<br />
spells than autumn or spr<strong>in</strong>g, as shown for weed <strong>species</strong> <strong>in</strong> Central Europe <strong>in</strong> Fig. 9.<br />
Some <strong>species</strong> spread <strong>the</strong> risk, germ<strong>in</strong>at<strong>in</strong>g <strong>in</strong> autumn and spr<strong>in</strong>g or discont<strong>in</strong>uously<br />
throughout <strong>the</strong> year. Certa<strong>in</strong> <strong>species</strong> with this ‘strategy’ show an obvious seed<br />
dimorphism related to <strong>the</strong> <strong>ma<strong>in</strong>tenance</strong> <strong>of</strong> at least two germ<strong>in</strong>ation periods (Harper<br />
et al. 1970).<br />
Much is known about <strong>the</strong> physiological control <strong>of</strong> <strong>the</strong> tim<strong>in</strong>g <strong>of</strong> germ<strong>in</strong>ation <strong>in</strong> <strong>the</strong><br />
k<strong>in</strong>d <strong>of</strong> succession shown <strong>in</strong> Fig. 9, based on temperature-responses (Went, 1949;<br />
Lauer, 1953 ; Newman, 1963 ; Mott, 1972), breakdown <strong>of</strong> hard coats (Salisbury, 1929),<br />
leach<strong>in</strong>g <strong>of</strong> <strong>in</strong>hibitors (Went, 1957; Koller, 1969), differences <strong>in</strong> <strong>the</strong> degree <strong>of</strong><br />
hydration necessary for germ<strong>in</strong>ation (McWilliam, Clements & Dowl<strong>in</strong>g, 1970;<br />
Mott, 1974) and <strong>the</strong> changes that occur dur<strong>in</strong>g after-ripen<strong>in</strong>g (Newman, 1963;<br />
Villiers, 1972) or as a result <strong>of</strong> burial (Wesson & Ware<strong>in</strong>g, 1969). It is particularly<br />
significant that appreciable differences <strong>in</strong> <strong>the</strong> temperature-responses <strong>of</strong> germ<strong>in</strong>ation<br />
have been found <strong>in</strong> groups <strong>of</strong> <strong>species</strong> with very similar habitat-tolerances and reproductive<br />
‘strategies’, e.g. among <strong>the</strong> w<strong>in</strong>ter annuals found on cliffs and sand-dunes <strong>in</strong><br />
Europe and North America (Ratcliffe, 1961; Newman, 1963; Bask<strong>in</strong> & Bask<strong>in</strong>,<br />
19714 b).<br />
A fur<strong>the</strong>r potentially important variable is <strong>the</strong> speed <strong>of</strong> germ<strong>in</strong>ation under a given<br />
set <strong>of</strong> conditions. Species <strong>in</strong> chalk grassland <strong>in</strong> England, for example, vary greatly <strong>in</strong><br />
this respect (J. B. Dickie, unpublished). Rapid germhators ga<strong>in</strong> a competitive advantage<br />
if subsequent conditions rema<strong>in</strong> favourable, but slow germ<strong>in</strong>ators, which may fail<br />
to germ<strong>in</strong>ate <strong>in</strong> short favourable periods, could benefit <strong>in</strong> <strong>the</strong> long run, provided <strong>the</strong>y<br />
rema<strong>in</strong> viable while <strong>the</strong> rapid germ<strong>in</strong>ators are killed <strong>in</strong> a subsequent catastrophe.<br />
Whereas <strong>the</strong>re is now available an impressive body <strong>of</strong> knowledge concern<strong>in</strong>g seed<br />
behaviour <strong>in</strong> <strong>the</strong> laboratory, <strong>the</strong>re is extremely little <strong>in</strong>formation available on germ<strong>in</strong>ation<br />
as such <strong>in</strong> <strong>the</strong> field - nearly all <strong>the</strong> apparently relevant reports concern <strong>the</strong> establishment<br />
<strong>of</strong> seedl<strong>in</strong>gs and <strong>the</strong> optimum conditions for this stage <strong>of</strong> <strong>the</strong> life-cycle can be<br />
quite different from those for germ<strong>in</strong>ation. Thus <strong>in</strong> temperate grasslands with dist<strong>in</strong>ct<br />
tussocks many seeds that come to rest beneath or with<strong>in</strong> <strong>the</strong> tussocks germ<strong>in</strong>ate freely<br />
<strong>in</strong> <strong>the</strong> autumn and spr<strong>in</strong>g but <strong>the</strong>ir chances <strong>of</strong> establishment <strong>in</strong> <strong>the</strong> dense shade are<br />
negligible. Similarly those that germ<strong>in</strong>ate <strong>in</strong> more open microsites on deep litter are<br />
generally killed by desiccation before <strong>the</strong>ir roots reach <strong>the</strong> m<strong>in</strong>eral soil below.<br />
Differences <strong>in</strong> requirements for germ<strong>in</strong>ation and establishment have also been reported<br />
for <strong>plant</strong>s <strong>in</strong> simple experimental systems (Harper et al. 1965 ; Sheldon, 1974).
I 28<br />
P. J. GRUBB<br />
Bromus secal<strong>in</strong>us<br />
Arenaria serpyllifolia<br />
Delph<strong>in</strong>ium consolida<br />
100 r<br />
Sonchus oleraceus<br />
100<br />
F 50.<br />
100 r<br />
u<br />
t 50 o =<br />
1 00<br />
L<br />
Galium apar<strong>in</strong>e<br />
Capsella bursa-pastoris<br />
Myosotis arvensis<br />
5oE 0<br />
100<br />
5oE 0<br />
100<br />
L<br />
Odontites rubra<br />
Sag<strong>in</strong>a procumbens<br />
5oE 0<br />
'N'D' J ' F'M'A'M' J '<br />
Solanum nigrurn<br />
Fig. 9. Emergence <strong>of</strong> seedl<strong>in</strong>gs <strong>of</strong> various weeds from November to June <strong>in</strong> central Europe<br />
(data <strong>of</strong> Salzmann, quoted by Ellenberg (1963), p. 797).<br />
(d) Establishment <strong>of</strong> seedl<strong>in</strong>gs<br />
Most <strong>of</strong> <strong>the</strong> <strong>in</strong>formation on <strong>the</strong> establishment <strong>of</strong> seedl<strong>in</strong>gs concerns <strong>the</strong> surfacetypes<br />
favoured by certa<strong>in</strong> <strong>species</strong>, but some <strong>of</strong> it concerns variation from year to year<br />
at particular sites. Many <strong>species</strong> are known to have specific requirements for establishment,<br />
e.g. Picea sitchensis is found ma<strong>in</strong>ly on fallen trunks whereas <strong>the</strong> accompany<strong>in</strong>g<br />
Thuja plicata and Tsuga heterophylla <strong>in</strong> <strong>the</strong> cool temperate ra<strong>in</strong>-forest <strong>of</strong> <strong>the</strong> Olympic
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> 129<br />
1000 r n<br />
I \<br />
I<br />
800 t<br />
I<br />
T<br />
I<br />
L<br />
I<br />
I<br />
! 600 I<br />
n<br />
I<br />
I<br />
f<br />
I<br />
v<br />
I<br />
0 400<br />
I<br />
C<br />
I<br />
.-<br />
-0<br />
a8<br />
v)<br />
200<br />
0<br />
Fig. 10. The numbers <strong>of</strong> emerged seedl<strong>in</strong>gs <strong>of</strong> Artemisia terrae-albae (-) and Alyssum<br />
desertorurn (---) <strong>in</strong> quadrats <strong>in</strong> <strong>the</strong> Turanian Desert <strong>in</strong> 1965-9 (data from Bykov, 1974).<br />
Pen<strong>in</strong>sula <strong>in</strong> <strong>the</strong> U.S.A. commonly become established on <strong>the</strong> ground (Franklyn &<br />
Dyrness, I 969). Seedl<strong>in</strong>gs <strong>of</strong> <strong>the</strong> dicotyledonous tree We<strong>in</strong>mannia racemosa most <strong>of</strong>ten<br />
become established on <strong>the</strong> trunks <strong>of</strong> tree-ferns <strong>in</strong> <strong>the</strong> warm temperate ra<strong>in</strong>-forest <strong>of</strong><br />
New Zealand (Beveridge, 1973), but this is not true <strong>of</strong> <strong>the</strong> o<strong>the</strong>r <strong>species</strong> <strong>of</strong> tree present.<br />
Published records show that some years favour establishment <strong>of</strong> certa<strong>in</strong> seedl<strong>in</strong>gs<br />
and o<strong>the</strong>r years o<strong>the</strong>r <strong>species</strong> (Daubenmire, 1968a; Bykov, 1974). Thus, <strong>in</strong> one part<br />
<strong>of</strong> <strong>the</strong> Turanian semi-desert, 1952 was good for Artemisia paucyora, Kochia prostrata<br />
and Agropyron pect<strong>in</strong>ifone, 1956 for Tanacetum achillaefolium, Festuca sulcata and<br />
Medicago romanica, 1958 for Astragalus virgatus and Tr<strong>in</strong>ia hispida (Bykov, 1974).<br />
Quantitative records for Artemisia terrae-albae and Alyssum desertmum <strong>in</strong> ano<strong>the</strong>r area<br />
for <strong>the</strong> years 1965-69 are shown <strong>in</strong> Fig. 10. Large differences between years have been<br />
found for heathland <strong>in</strong> Scotland (Miles, 1974) and for forests and meadows <strong>in</strong> F<strong>in</strong>land<br />
(Pertulla, 1941).<br />
The passage <strong>of</strong> <strong>the</strong> young seedl<strong>in</strong>g to <strong>the</strong> sapl<strong>in</strong>g or immature vegetative phase is<br />
only arbitrarily separated from <strong>the</strong> early seedl<strong>in</strong>g phase but most <strong>of</strong> <strong>the</strong> <strong>in</strong>formation<br />
concern<strong>in</strong>g <strong>the</strong> effects <strong>of</strong> <strong>the</strong> physical environment concern <strong>the</strong> later stage. Experiments<br />
have shown that <strong>the</strong> effeets <strong>of</strong> irradiance (NichoIson, 1960; Grime, 1966; Hutch<strong>in</strong>son,<br />
1967) and water supply (Jarvis, 1963; Fenner, 1975) certa<strong>in</strong>ly differentiate <strong>species</strong> <strong>of</strong><br />
<strong>the</strong> same community at this stage <strong>of</strong> <strong>the</strong> life-cycle. The ability to tolerate shade is <strong>of</strong>ten<br />
correlated with large seed-size, at any rate <strong>in</strong> <strong>species</strong> that do not depend on mycorrhiza<br />
for <strong>the</strong>ir organic nutrition as seedl<strong>in</strong>gs (Salisbury, 1941, 1974). However, <strong>the</strong>re<br />
are many exceptions to <strong>the</strong> rule and <strong>the</strong> largest-seeded trees <strong>in</strong> European deciduous<br />
forests (Castanea, Quercm) are all light-demand<strong>in</strong>g (Ellenberg, I 963). Possibly <strong>the</strong>ir<br />
large seeds evolved <strong>in</strong> relation to <strong>the</strong> occupation <strong>of</strong> dry sites and <strong>the</strong> advantage <strong>of</strong><br />
9 BRE 52
P. J. GRUBB<br />
rapidly develop<strong>in</strong>g a deep tap root; this seems to be <strong>the</strong> case <strong>in</strong> some o<strong>the</strong>r <strong>plant</strong>s <strong>of</strong><br />
open habitats, e.g. <strong>the</strong> strand-l<strong>in</strong>e <strong>species</strong> Cakile maritima. There is little doubt that<br />
differentiation <strong>in</strong> <strong>the</strong> regeneration niche has occurred through <strong>the</strong> function <strong>of</strong> seed<br />
size and its effects dur<strong>in</strong>g establishment.<br />
Not only physical environmental factors differentiate <strong>species</strong> at <strong>the</strong> establishment<br />
stage, but also biotic factors, notably graz<strong>in</strong>g, even when <strong>the</strong> animals concerned are<br />
not monophagous e.g. snails (Grime, MacPherson-Stewart & Dearman, 1968) or deer<br />
(Knapp, 1974). Probably <strong>the</strong> selective effects <strong>of</strong> microbial pests and <strong>of</strong> allelopathic<br />
relations with <strong>plant</strong>s <strong>of</strong> <strong>the</strong> same or o<strong>the</strong>r <strong>species</strong> are important <strong>in</strong> <strong>the</strong> field but <strong>the</strong>re<br />
is very little pro<strong>of</strong> (Webb et al., 1967). Particular <strong>in</strong>terest arises from <strong>the</strong> idea that <strong>the</strong><br />
risks <strong>of</strong> pests and allelopathic effects may be significantly greater near <strong>the</strong> parent<br />
<strong>plant</strong> so that <strong>in</strong>vad<strong>in</strong>g <strong>species</strong> are at an advantage. Connell (1971) has provided<br />
evidence that seedl<strong>in</strong>gs <strong>of</strong> certa<strong>in</strong> ra<strong>in</strong>-forest trees <strong>in</strong> eastern Australia may suffer a<br />
greater mortality risk (at a given density) if under a tree <strong>of</strong> <strong>the</strong> parent <strong>species</strong> as<br />
opposed to ano<strong>the</strong>r <strong>species</strong> and it is highly desirable that tests should be made on <strong>the</strong><br />
applicability <strong>of</strong> this idea to o<strong>the</strong>r <strong>communities</strong>.<br />
(e) Fur<strong>the</strong>r development <strong>of</strong> <strong>the</strong> immature <strong>plant</strong><br />
Least <strong>of</strong> all, perhaps, is known about <strong>the</strong> conditions favour<strong>in</strong>g <strong>the</strong> development <strong>of</strong> an<br />
estabIished seedl<strong>in</strong>g or sapl<strong>in</strong>g <strong>of</strong> a given <strong>species</strong> <strong>in</strong>to a mature adult. Although <strong>the</strong><br />
process is only arbitrarily separable from <strong>the</strong> establishment <strong>of</strong> <strong>the</strong> seedl<strong>in</strong>g or sapl<strong>in</strong>g,<br />
it is well known that <strong>the</strong> optimum conditions for several forest trees are different <strong>in</strong> <strong>the</strong><br />
two stages. Thus <strong>the</strong> Kauri, Agathis australis, needs a light cover (e.g. <strong>of</strong> Leptospermum)<br />
for survival as a seedl<strong>in</strong>g but requires a lack <strong>of</strong> overhead cover for development <strong>in</strong> <strong>the</strong><br />
sapl<strong>in</strong>g and pole stages (Beveridge, 1973). Similar differences are recorded for certa<strong>in</strong><br />
<strong>species</strong> <strong>in</strong> <strong>the</strong> deciduous and coniferous forests <strong>of</strong> <strong>the</strong> U.S.A. (Curtis, 1959; Franklyn<br />
& Dyrness, 1969) and <strong>in</strong> <strong>the</strong> ra<strong>in</strong>-forests <strong>of</strong> <strong>the</strong> Solomon Islands (Whitmore, 1974).<br />
Such considerations also apply to grasslands and heaths. It has been found that small<br />
clear<strong>in</strong>gs (5 x 5 cm) <strong>in</strong> heathland are most favourable to <strong>the</strong> early establishment <strong>of</strong><br />
<strong>of</strong> most seedl<strong>in</strong>gs, but that after two years larger clear<strong>in</strong>gs (50 x 50 cm, less overgrown<br />
than <strong>the</strong> smaller) have many more surviv<strong>in</strong>g <strong>plant</strong>s per unit area (Miles, 1974).<br />
In many studies on successions it has been found that certa<strong>in</strong> <strong>species</strong> effectively<br />
make possible <strong>the</strong> <strong>in</strong>vasion <strong>of</strong> <strong>species</strong> <strong>of</strong> a later stage, e.g. Jun$erus spp. act<strong>in</strong>g as<br />
nurses for Taxus (Watt, 1926) or Fugus (Watt, 1934) dur<strong>in</strong>g <strong>in</strong>vasion <strong>of</strong> ungrazed<br />
grassland <strong>in</strong> Brita<strong>in</strong> or for Quercus spp. on sand dunes <strong>in</strong> <strong>the</strong> Lake States (Yarranton &<br />
Morrison, 1974). This sort <strong>of</strong> relationship is also found <strong>in</strong> <strong>the</strong> regeneration serule<br />
(Daubenmire, 1968 a). The presence <strong>of</strong> certa<strong>in</strong> gap-<strong>in</strong>vaders cantip <strong>the</strong> balance between<br />
two <strong>species</strong> that might succeed on <strong>the</strong> site. One commercially important illustration<br />
<strong>of</strong> this po<strong>in</strong>t is <strong>the</strong> suppression <strong>of</strong> young Picea sitchensis, but not Larix leptolepis, by<br />
Calhnu vulgaris (Wea<strong>the</strong>rell, 1957). If <strong>the</strong> Picea and Larix are <strong>plant</strong>ed <strong>in</strong> mixture, <strong>the</strong><br />
Larix may suppress <strong>the</strong> Calluna and, <strong>in</strong> due course, <strong>the</strong> Picea <strong>the</strong> Larix. Such relationships<br />
are <strong>of</strong>ten dependent on <strong>the</strong> wea<strong>the</strong>r at a particular time. Thus <strong>in</strong> most years<br />
a carpet <strong>of</strong> <strong>the</strong> herb Mercurialisperennis will <strong>in</strong>hibit growth <strong>of</strong> <strong>in</strong>vad<strong>in</strong>g seedl<strong>in</strong>gs <strong>of</strong> <strong>the</strong>
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> 131<br />
tree Frax<strong>in</strong>us excelsior <strong>in</strong> woodlands on boulder clay <strong>in</strong> eastern England (Wade, 1g59),<br />
but <strong>in</strong> an unusually wet spr<strong>in</strong>g <strong>the</strong> Mercurialis dies back on marg<strong>in</strong>al sites (Mart<strong>in</strong>,<br />
1968) and Fram‘nw, which is tolerant <strong>of</strong> waterlogg<strong>in</strong>g, can <strong>the</strong>n grow through. The<br />
many relationships <strong>of</strong> this k<strong>in</strong>d, which must contribute to <strong>the</strong> <strong>ma<strong>in</strong>tenance</strong> <strong>of</strong> diversity,<br />
have hardly begun to be explored.<br />
VII. DISCUSSION<br />
There can be no serious doubt that <strong>the</strong> k<strong>in</strong>ds <strong>of</strong> differences between <strong>species</strong> <strong>in</strong><br />
regeneration requirements considered <strong>in</strong> this review contribute a f<strong>in</strong>ite amount to <strong>the</strong><br />
<strong>ma<strong>in</strong>tenance</strong> <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong>. The examples given all concern<br />
<strong>the</strong> regeneration <strong>of</strong> seed <strong>plant</strong>s, but a parallel case could be made out for <strong>the</strong> pteridophytes,<br />
bryophytes, attached algae or fungi and <strong>in</strong>deed for corals and o<strong>the</strong>r attached<br />
animals. The really difficult problem is to establish that any two <strong>species</strong> may co-exist<br />
<strong>in</strong>def<strong>in</strong>itely on <strong>the</strong> basis <strong>of</strong> differences <strong>in</strong> <strong>the</strong> regeneration niche. If <strong>species</strong> A always<br />
tends to oust <strong>species</strong> B over a period <strong>of</strong> years <strong>in</strong> a particular part <strong>of</strong> <strong>the</strong> world, when<br />
<strong>in</strong>dividuals <strong>of</strong> <strong>the</strong> same physiological age are matched aga<strong>in</strong>st each o<strong>the</strong>r, what are <strong>the</strong><br />
conditions necessary for differentiation <strong>in</strong> <strong>the</strong> regeneration niche to be a basis <strong>of</strong><br />
<strong>in</strong>def<strong>in</strong>ite co-existence? There are two major conditions. The first is that B must be<br />
able to persist <strong>in</strong> <strong>the</strong> community as long as <strong>the</strong> mean time between those events which<br />
cause ei<strong>the</strong>r <strong>the</strong> creation <strong>of</strong> a gap <strong>of</strong> a k<strong>in</strong>d which favours <strong>the</strong> establishment <strong>of</strong> B more<br />
than A or <strong>the</strong> creation <strong>of</strong> a gap at a place where B has propagules and A has not. The<br />
second condition is that sufficient ‘B gaps’ must cont<strong>in</strong>ue to arise as well as ‘A gaps’<br />
<strong>in</strong> <strong>the</strong> area concerned; <strong>the</strong> exact proportion will be a major determ<strong>in</strong>ant <strong>of</strong> <strong>the</strong><br />
relative abundance <strong>of</strong> <strong>species</strong> B and A <strong>in</strong> <strong>the</strong> community. Intuitively it seems reasonable<br />
that, <strong>in</strong> a temperate forest, large gaps should occur <strong>of</strong>ten enough to make possible <strong>the</strong><br />
persistence <strong>of</strong> birch as well as beech. It is less clear that <strong>the</strong> ho<strong>of</strong>-marks <strong>of</strong> sheep or deer<br />
<strong>in</strong> a grassland should occur sufficiently <strong>of</strong>ten <strong>in</strong> places where <strong>species</strong> B has seed present<br />
and <strong>species</strong> A has not. Clearly <strong>the</strong> question can be settled for any one <strong>plant</strong> community<br />
only on <strong>the</strong> basis <strong>of</strong> detailed long-term studies <strong>of</strong> <strong>the</strong> gaps that arise, <strong>the</strong> differences<br />
<strong>the</strong>re are between <strong>species</strong> <strong>in</strong> <strong>the</strong>ir responses to gaps <strong>of</strong> different k<strong>in</strong>ds and <strong>of</strong> <strong>the</strong><br />
differences <strong>the</strong>re are <strong>in</strong> practice <strong>in</strong> terms <strong>of</strong> propagules <strong>of</strong> different <strong>species</strong> be<strong>in</strong>g<br />
present on <strong>the</strong> site. The results <strong>of</strong> any such study are bound to be so complex as to<br />
necessitate <strong>the</strong> build<strong>in</strong>g <strong>of</strong> a computer model <strong>of</strong> <strong>the</strong> situation, broadly speak<strong>in</strong>g <strong>of</strong> <strong>the</strong><br />
k<strong>in</strong>d developed for temperate deciduous forest by Botk<strong>in</strong>, Janak & Wallis (1972).<br />
Persistence <strong>of</strong> <strong>species</strong> B <strong>in</strong> <strong>the</strong> presence <strong>of</strong> <strong>species</strong> A between <strong>the</strong> times <strong>of</strong> gap formation<br />
may not necessarily be <strong>in</strong> <strong>the</strong> form <strong>of</strong> seed-produc<strong>in</strong>g adults; it may be as dormant seed<br />
<strong>in</strong> <strong>the</strong> soil or as <strong>plant</strong>s <strong>in</strong> an adjacent community, e.g. on river gravels or rock outcrops<br />
next to <strong>the</strong> forest or grassland <strong>in</strong> question.<br />
The idea that <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> is ma<strong>in</strong>ta<strong>in</strong>ed <strong>in</strong> part as a<br />
result <strong>of</strong> recurrent perturbations by outside forces is shared with <strong>the</strong> <strong>the</strong>ory <strong>of</strong> Loucks<br />
(1970). However, <strong>in</strong> <strong>the</strong> present account emphasis is also placed on <strong>the</strong> importance <strong>of</strong><br />
gaps <strong>in</strong> <strong>the</strong> community that arise through <strong>the</strong> senescence and death <strong>of</strong> <strong>in</strong>dividuals more<br />
or less irrespective <strong>of</strong> outside forces and on <strong>the</strong> importance <strong>of</strong> a wide range <strong>of</strong> responses<br />
9-2
132 P. J. GRUBB<br />
to external factors <strong>in</strong> <strong>the</strong> <strong>plant</strong>s <strong>of</strong> a <strong>species</strong>-rich community. Fur<strong>the</strong>rmore, external<br />
factors are seen as very important not only <strong>in</strong> creat<strong>in</strong>g some <strong>of</strong> <strong>the</strong> gaps <strong>in</strong> <strong>the</strong> community<br />
but also <strong>in</strong> determ<strong>in</strong><strong>in</strong>g which <strong>species</strong> are represented <strong>in</strong> any particular gap by<br />
propagules (a function <strong>of</strong> recent seed-set, dispersal, etc.).<br />
If differences <strong>in</strong> <strong>the</strong> regeneration niche can account for <strong>the</strong> long-term co-existence<br />
<strong>of</strong> different <strong>species</strong>, just how different do <strong>species</strong> need to be? Particular <strong>in</strong>terest<br />
attaches to those genera where <strong>the</strong> most obvious differences are <strong>in</strong> phenology, e.g.<br />
Miconia with n<strong>in</strong>eteen <strong>species</strong> <strong>in</strong> Tr<strong>in</strong>idad (Snow, 1965, 1968) or less remarkably<br />
Guarea with four <strong>species</strong> <strong>in</strong> <strong>the</strong> ra<strong>in</strong>-forest <strong>of</strong> part <strong>of</strong> Costa Rica (Frankie et al., 1974)<br />
or Neolitsea with two <strong>species</strong> <strong>in</strong> <strong>the</strong> ra<strong>in</strong>-forest <strong>of</strong> Japan (Ohwi, 1965). It is widely<br />
accepted that <strong>the</strong>re is competition between <strong>plant</strong>s for poll<strong>in</strong>ators (Free, 1968 ; Hock<strong>in</strong>g,<br />
1968; He<strong>in</strong>rich & Raven, 1972; Kevan, 1972; Gentry, 1974) and animal vectors<br />
<strong>of</strong> <strong>the</strong> seed (Snow, 1965, 1968) and <strong>the</strong> evolution <strong>of</strong> phenological spread is generally<br />
seen to be a result <strong>of</strong> this competition (Richards, 1969; Mosqu<strong>in</strong>, 1971 ; Stiles, 1975).<br />
It is less <strong>of</strong>ten po<strong>in</strong>ted out that such spread is also seen <strong>in</strong> <strong>species</strong> poll<strong>in</strong>ated and<br />
dispersed by w<strong>in</strong>d, e.g. <strong>the</strong> annual grasses studied by Pemadasa & Love11 (1974).<br />
The groups <strong>of</strong> <strong>species</strong> concerned may well differ <strong>in</strong> <strong>the</strong>ir exact requirements for<br />
seed-set, as appears to be <strong>the</strong> case with six New Zealand <strong>species</strong> <strong>of</strong> Podocarpus (Fig. 6).<br />
However, even if <strong>the</strong>y have <strong>the</strong> same essential requirements for seed-set, <strong>the</strong> fact that<br />
<strong>the</strong> wea<strong>the</strong>r fluctuates through <strong>the</strong> year, coupled with <strong>the</strong>ir different times <strong>of</strong> flower<strong>in</strong>g<br />
and fruit<strong>in</strong>g, ensures (unless <strong>the</strong> <strong>plant</strong>s are very tolerant) that no two years are exactly<br />
similar <strong>in</strong> terms <strong>of</strong> <strong>the</strong> relative numbers <strong>of</strong> seeds produced per <strong>plant</strong> by different<br />
<strong>species</strong>. O<strong>the</strong>r th<strong>in</strong>gs be<strong>in</strong>g equal, this must tend to perpetuate ail <strong>the</strong> <strong>species</strong><br />
<strong>in</strong>def<strong>in</strong>itely.<br />
For most <strong>communities</strong> it seems unlikely that similar <strong>species</strong> will be found<br />
ultimately to differ <strong>in</strong> only a s<strong>in</strong>gle characteristic <strong>in</strong> <strong>the</strong> regeneration niche but it is<br />
possible that this is <strong>the</strong> case <strong>in</strong> <strong>the</strong> astound<strong>in</strong>gly long series <strong>of</strong> <strong>species</strong> <strong>in</strong> certa<strong>in</strong> genera<br />
<strong>in</strong> some ra<strong>in</strong>-forests, e.g. Eugenia and Shorea <strong>in</strong> south-east Asia or Diospyros and<br />
R<strong>in</strong>orea <strong>in</strong> Africa (Richards, 1969).<br />
It is fashionable to emphasize <strong>the</strong> possible roles <strong>of</strong> predation by animals and<br />
<strong>in</strong>traspecific allelopathy <strong>in</strong> <strong>the</strong> context <strong>of</strong> <strong>the</strong> most <strong>species</strong>-rich <strong>communities</strong> and<br />
especially <strong>in</strong> <strong>the</strong> case <strong>of</strong> <strong>the</strong> long series <strong>of</strong> <strong>species</strong> <strong>in</strong> certa<strong>in</strong> genera, but <strong>the</strong> general<br />
importance <strong>of</strong> <strong>the</strong>se two mechanisms has yet to be proven. Critical evidence is needed<br />
to show that <strong>species</strong> are <strong>in</strong>deed unable to regenerate beneath <strong>the</strong>ir parents (though <strong>in</strong><br />
a comparable degree <strong>of</strong> shade under o<strong>the</strong>r <strong>species</strong>) and that <strong>the</strong> lack <strong>of</strong> regeneration is<br />
specifically caused by predation or allelopathy.<br />
The emphasis on differentiation <strong>in</strong> <strong>the</strong> regeneration niche is highly relevant to <strong>the</strong><br />
contentious issue <strong>of</strong> cont<strong>in</strong>ua <strong>in</strong> vegetation. Whittaker (1965, 1967, 1969, 1972) has<br />
repeatedly emphasized that gradient analysis leads to <strong>the</strong> conclusion that, while <strong>species</strong><br />
are clumped <strong>in</strong> <strong>the</strong>ir habitat-tolerances, no two <strong>species</strong> are quite <strong>the</strong> same <strong>in</strong> this<br />
respect. He has suggested (Whittaker, 1969) that diversity <strong>in</strong> <strong>plant</strong> <strong>communities</strong> is<br />
made possible by a partition<strong>in</strong>g <strong>of</strong> <strong>the</strong> habitat, and <strong>in</strong>deed his emphasis is almost<br />
wholly on diversification <strong>in</strong> <strong>the</strong> habitat niche. In a popular text (Whittaker, 1975) he<br />
gives <strong>the</strong> student <strong>the</strong> impression that groups <strong>of</strong> <strong>species</strong> with strongly similar habitat
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong><br />
I33<br />
niches are not a general feature <strong>of</strong> vegetation. In fact most ecologists f<strong>in</strong>d that <strong>the</strong>y are,<br />
whe<strong>the</strong>r <strong>the</strong>y are work<strong>in</strong>g <strong>in</strong> <strong>the</strong> deciduous forests, grasslands or weed <strong>communities</strong> <strong>of</strong><br />
Europe (Ellenberg, 1954, 1963 ; Duchaufour, 1960), <strong>the</strong> conifer forests and steppes <strong>of</strong><br />
<strong>the</strong> western United States (Daubenmire, 1968 b, 1970), <strong>the</strong> forests <strong>of</strong> eastern Australia<br />
(Webb, Tracey, Williams & Lance, 1970) or those on tropical mounta<strong>in</strong>s (Wade &<br />
McVean, 1969; Grubb & Stevens, 1976; Grubb & Tanner, 1976). This group<strong>in</strong>g <strong>of</strong><br />
<strong>species</strong> <strong>in</strong> habitat niches is very <strong>of</strong>ten accompanied by a strik<strong>in</strong>g conformation <strong>in</strong><br />
life-form, leaf-form, etc., and clearly many physiological characteristics <strong>in</strong> <strong>the</strong><br />
vegetative adult are similar <strong>in</strong> all <strong>the</strong> <strong>species</strong> with<strong>in</strong> each group. Just why <strong>the</strong> <strong>species</strong><br />
<strong>in</strong>volved should have ‘partitioned’ <strong>the</strong> physical environment <strong>in</strong>to just so many<br />
sections and not more is a very deep question, e.g. why are <strong>the</strong>re four ma<strong>in</strong> forest<br />
formations on most high tropical mounta<strong>in</strong>s and not two or six or twenty? Presumably<br />
patterns <strong>of</strong> vegetational history are partly <strong>in</strong>volved here. With<strong>in</strong> most habitat-groups<br />
<strong>of</strong> <strong>species</strong> differentiation <strong>in</strong> <strong>the</strong> regeneration niche is almost certa<strong>in</strong>ly considerable.<br />
Emphasis on <strong>the</strong> regeneration niche is also relevant to <strong>the</strong> many ord<strong>in</strong>ation studies<br />
made on vegetation <strong>in</strong> <strong>the</strong> last decade. Some <strong>of</strong> <strong>the</strong> axes <strong>of</strong> variation detected are<br />
bound to reflect regeneration characteristics ra<strong>the</strong>r than <strong>the</strong> features <strong>of</strong> <strong>the</strong> physical<br />
environment that those who ord<strong>in</strong>ate usually seek to identify; <strong>in</strong> <strong>the</strong> case <strong>of</strong> <strong>the</strong><br />
tropical ra<strong>in</strong>-forest on one <strong>of</strong> <strong>the</strong> Solomon Islands some <strong>of</strong> <strong>the</strong> variation first found by<br />
an ord<strong>in</strong>ation analysis (Greig-Smith, Aust<strong>in</strong> & Whitmore, 1967) is now known to<br />
reflect vegetation history (Whitmore, 1974).<br />
The conclusion that <strong>species</strong> diversity has more to do with requirements for regeneration<br />
than with partition<strong>in</strong>g <strong>of</strong> <strong>the</strong> habitat niche <strong>of</strong> <strong>the</strong> adult fits well with <strong>the</strong> grow<strong>in</strong>g<br />
conviction <strong>of</strong> some evolutionists, e.g. Grant (1949), Stebb<strong>in</strong>s (1951, 1970, 1g71),<br />
Baker (1959, 1963, 1970) and van der Pijl(1960, 1961), that speciation and ultimately<br />
<strong>the</strong> differentiation <strong>of</strong> genera and families <strong>of</strong> <strong>plant</strong>s have primarily to do with adaptive<br />
changes <strong>in</strong> reproductive characteristics, even though <strong>the</strong>re are still many characters <strong>of</strong><br />
taxonomic importance that are hard to expla<strong>in</strong> functionally (Ehrendorfer, I 973). It<br />
seems that at all taxonomic levels evolutionary divergence may concern any aspect <strong>of</strong> <strong>the</strong><br />
niche. Thus several families have evolved <strong>in</strong> certa<strong>in</strong> narrow and well-def<strong>in</strong>ed habitat<br />
niches, while o<strong>the</strong>rs have tremendously wide ranges. An <strong>in</strong>spection <strong>of</strong> any modern<br />
nor<strong>the</strong>rn European flora will show that <strong>the</strong> ecological separation <strong>of</strong> <strong>species</strong> <strong>in</strong><br />
most genera <strong>in</strong> that region is ma<strong>in</strong>ly through <strong>the</strong> habitat niche and <strong>the</strong> ecological<br />
picture is primarily one <strong>of</strong> each habitat niche be<strong>in</strong>g filled by a s<strong>in</strong>gle <strong>species</strong> from<br />
several to many genera. Each genus is almost certa<strong>in</strong> to be different from <strong>the</strong> next <strong>in</strong><br />
several aspects <strong>of</strong> <strong>the</strong> regeneration niche. There are, <strong>of</strong> course, cases <strong>of</strong> <strong>species</strong> with<br />
virtually <strong>the</strong> same or very strongly overlapp<strong>in</strong>g habitat niches <strong>in</strong> certa<strong>in</strong> genera, e.g.<br />
Carex, Cephalan<strong>the</strong>ra, Orchis, Ophrys, Rosa, Trifolium. Such cases are particularly<br />
common <strong>in</strong> man-made <strong>communities</strong> and differences <strong>in</strong> <strong>the</strong> regeneration niche are<br />
presumably very important <strong>in</strong> <strong>the</strong> <strong>ma<strong>in</strong>tenance</strong> <strong>of</strong> co-existence <strong>the</strong>re, e.g. <strong>of</strong> Plantago<br />
media and P. lanceolata <strong>in</strong> basic grassland (Sagar & Harper, 1964) or Ranuncuhs acris,<br />
R. bulbosus and R. repens <strong>in</strong> certa<strong>in</strong> pastures (Sarukhhn & Harper, 1973).<br />
In regions <strong>of</strong> greater floristic <strong>richness</strong> than nor<strong>the</strong>rn Europe differentiation <strong>in</strong> <strong>the</strong><br />
regeneration niche separat<strong>in</strong>g <strong>species</strong> <strong>in</strong> a genus appears to be generally more impor-<br />
9-3
‘34 P. J. GRUBB<br />
tant. This is true not only <strong>of</strong> tropical regions but also <strong>of</strong> certa<strong>in</strong> temperate regions,<br />
e.g. parts <strong>of</strong> nor<strong>the</strong>rn Ch<strong>in</strong>a where <strong>the</strong> primary forests conta<strong>in</strong>ed eight <strong>species</strong> <strong>of</strong><br />
Acer, n<strong>in</strong>e <strong>of</strong> Betula, five <strong>of</strong> Tilia and five <strong>of</strong> Ulmus (Wang, 1961).<br />
The evolution <strong>of</strong> diversity is here envisaged as an <strong>in</strong>evitable process whereby <strong>the</strong><br />
possible regeneration niches <strong>in</strong> <strong>the</strong> community are partitioned. There is no need to<br />
<strong>in</strong>volve <strong>the</strong> idea that <strong>the</strong>re are selective advantages <strong>in</strong> evolution aris<strong>in</strong>g from greater<br />
stability <strong>in</strong> systems <strong>of</strong> greater diversity, as suggested by MacArthur (1955) and<br />
Hutch<strong>in</strong>son (1959). This supposed <strong>in</strong>creased stability is, <strong>in</strong> any case, very much<br />
questioned at <strong>the</strong> present time (May, 1975).<br />
One <strong>of</strong> <strong>the</strong> central aims <strong>of</strong> much conservation is <strong>the</strong> preservation <strong>of</strong> diversity<br />
(Westh<strong>of</strong>f, 1971 ; van der Maarel, 1971) and <strong>the</strong> <strong>the</strong>me <strong>of</strong> this review is <strong>the</strong>refore<br />
highly relevant. It places an emphasis on <strong>the</strong> dynamic relations <strong>of</strong> <strong>plant</strong> <strong>communities</strong>,<br />
relations which have been repeatedly overlooked <strong>in</strong> an age when classical phytosociology,<br />
numerical analysis <strong>of</strong> vegetation and productivity studies have sapped <strong>the</strong><br />
energies <strong>of</strong> those who might o<strong>the</strong>rwise have provided a sounder basis for positive<br />
management <strong>in</strong> <strong>the</strong> conservation field. The need now is for proper attention to be paid<br />
to each stage <strong>in</strong> <strong>the</strong> regeneration cycle. The most sophisticated studies will require<br />
teams <strong>of</strong> research workers, long-term programmes and expertise <strong>in</strong> realistic modell<strong>in</strong>g;<br />
however, <strong>in</strong> <strong>the</strong> record<strong>in</strong>g <strong>of</strong> many <strong>of</strong> <strong>the</strong> processes <strong>in</strong>volved, e.g. annual flower<strong>in</strong>g and<br />
fruit<strong>in</strong>g, amateur naturalists could play a vital role <strong>in</strong> <strong>the</strong> accumulation <strong>of</strong> valuable<br />
<strong>in</strong>formation.<br />
VIII. SUMMARY<br />
I. Accord<strong>in</strong>g to ‘Gause’s hypo<strong>the</strong>sis’ a corollary <strong>of</strong> <strong>the</strong> process <strong>of</strong> evolution by<br />
natural selection is that <strong>in</strong> a community at equilibrium every <strong>species</strong> must occupy a<br />
different niche. Many botanists have found this idea improbable because <strong>the</strong>y have<br />
ignored <strong>the</strong> processes <strong>of</strong> regeneration <strong>in</strong> <strong>plant</strong> <strong>communities</strong>.<br />
2. Most <strong>plant</strong> <strong>communities</strong> are longer-lived than <strong>the</strong>ir constituent <strong>in</strong>dividual<br />
<strong>plant</strong>s. When an <strong>in</strong>dividual dies, it may or may not be replaced by an <strong>in</strong>dividual <strong>of</strong> <strong>the</strong><br />
same <strong>species</strong>. It is this replacement stage which is all-important to <strong>the</strong> argument<br />
presented.<br />
3. Several mechanisms not <strong>in</strong>volv<strong>in</strong>g regeneration also contribute to <strong>the</strong> <strong>ma<strong>in</strong>tenance</strong><br />
<strong>of</strong> <strong>species</strong>-<strong>richness</strong> :<br />
(a) differences <strong>in</strong> life-form coupled with <strong>the</strong> <strong>in</strong>ability <strong>of</strong> larger <strong>plant</strong>s to exhaust or<br />
cut <strong>of</strong>f all resources, also <strong>the</strong> development <strong>of</strong> dependence-relationships,<br />
(b) differences <strong>in</strong> phenology coupled with tolerance <strong>of</strong> suppression,<br />
(c) fluctuations <strong>in</strong> <strong>the</strong> environment coupled with relatively small differences <strong>in</strong><br />
competitive ability between many <strong>species</strong>,<br />
(d) <strong>the</strong> ability <strong>of</strong> certa<strong>in</strong> <strong>species</strong>-pairs to form stable mixtures because <strong>of</strong> a balance<br />
<strong>of</strong> <strong>in</strong>traspecific competition aga<strong>in</strong>st <strong>in</strong>terspecific competition,<br />
(e) <strong>the</strong> production <strong>of</strong> substances more toxic to <strong>the</strong> producer-<strong>species</strong> than to <strong>the</strong><br />
o<strong>the</strong>r <strong>species</strong>,<br />
(f) differences <strong>in</strong> <strong>the</strong> primary limit<strong>in</strong>g m<strong>in</strong>eral nutrients or pore-sizes <strong>in</strong> <strong>the</strong> soil for<br />
neighbour<strong>in</strong>g <strong>plant</strong>s <strong>of</strong> different soecies. and
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> I3 5<br />
(g) differences <strong>in</strong> <strong>the</strong> competitive abilities <strong>of</strong> <strong>species</strong> dependent on <strong>the</strong>ir physiological<br />
age coupled with <strong>the</strong> uneven-age structure <strong>of</strong> many populations.<br />
4. The mechanisms listed above do not go far to expla<strong>in</strong> <strong>the</strong> <strong>in</strong>def<strong>in</strong>ite persistence<br />
<strong>in</strong> mixture <strong>of</strong> <strong>the</strong> many <strong>species</strong> <strong>in</strong> <strong>the</strong> most <strong>species</strong>-rich <strong>communities</strong> known.<br />
5. In contrast <strong>the</strong>re seem to be almost limitless possibilities for differences between<br />
<strong>species</strong> <strong>in</strong> <strong>the</strong>ir requirements for regeneration, i.e. <strong>the</strong> replacement <strong>of</strong> <strong>the</strong> <strong>in</strong>dividual<br />
<strong>plant</strong>s <strong>of</strong> one generation by those <strong>of</strong> <strong>the</strong> next. This idea is illustrated for tree <strong>species</strong><br />
and it is emphasized that foresters were <strong>the</strong> first by a wide marg<strong>in</strong> to appreciate its<br />
importance.<br />
6. The processes <strong>in</strong>volved <strong>in</strong> <strong>the</strong> successful <strong>in</strong>vasion <strong>of</strong> a gap by a given <strong>plant</strong><br />
<strong>species</strong> and some characters <strong>of</strong> <strong>the</strong> gap that may be important are summarized <strong>in</strong><br />
Table 2.<br />
7. The def<strong>in</strong>ition <strong>of</strong> a <strong>plant</strong>’s niche requires recognition <strong>of</strong> four components:<br />
(a) <strong>the</strong> habitat niche,<br />
(b) <strong>the</strong> life-form niche,<br />
(c) <strong>the</strong> phenological niche, and<br />
(d) <strong>the</strong> regeneration niche.<br />
8. A brief account is given <strong>of</strong> <strong>the</strong> patterns <strong>of</strong> regeneration <strong>in</strong> different k<strong>in</strong>ds <strong>of</strong><br />
<strong>plant</strong> community to provide a background for studies <strong>of</strong> differentiation <strong>in</strong> <strong>the</strong> regeneration<br />
niche.<br />
9. All stages <strong>in</strong> <strong>the</strong> regeneration-cycle are potentially important and examples <strong>of</strong><br />
differentiation between <strong>species</strong> are given for each <strong>of</strong> <strong>the</strong> follow<strong>in</strong>g stages:<br />
(a) Production <strong>of</strong> viable seed (<strong>in</strong>clud<strong>in</strong>g <strong>the</strong> sub-stages <strong>of</strong> flower<strong>in</strong>g, poll<strong>in</strong>ation and<br />
seed-set),<br />
(b) dispersal, <strong>in</strong> space and time,<br />
(c) germ<strong>in</strong>ation,<br />
(d) establishment, and<br />
(e) fur<strong>the</strong>r development <strong>of</strong> <strong>the</strong> immature <strong>plant</strong>.<br />
10. In <strong>the</strong> conclud<strong>in</strong>g discussion emphasis is placed on <strong>the</strong> follow<strong>in</strong>g <strong>the</strong>mes:<br />
(a) <strong>the</strong> k<strong>in</strong>ds <strong>of</strong> work needed <strong>in</strong> future to prove or disprove that differentiation <strong>in</strong><br />
<strong>the</strong> regeneration niche is <strong>the</strong> major explanation <strong>of</strong> <strong>the</strong> <strong>ma<strong>in</strong>tenance</strong> <strong>of</strong> <strong>species</strong>-<strong>richness</strong><br />
<strong>in</strong> <strong>plant</strong> <strong>communities</strong>,<br />
(b) <strong>the</strong> relation <strong>of</strong> <strong>the</strong> present <strong>the</strong>sis to published ideas on <strong>the</strong> orig<strong>in</strong> <strong>of</strong> phenological<br />
spread,<br />
(c) <strong>the</strong> relevance <strong>of</strong> <strong>the</strong> present <strong>the</strong>sis to <strong>the</strong> discussion on <strong>the</strong> presence <strong>of</strong> cont<strong>in</strong>ua<br />
<strong>in</strong> vegetation,<br />
(d) <strong>the</strong> co-<strong>in</strong>cidence <strong>of</strong> <strong>the</strong> present <strong>the</strong>sis and <strong>the</strong> emerg<strong>in</strong>g ideas <strong>of</strong> evolutionists<br />
about differentiation <strong>of</strong> angiosperm taxa, and<br />
(e) <strong>the</strong> importance <strong>of</strong> regeneration-studies for conservation.<br />
I am grateful to Dr A. S. Watt for <strong>in</strong>spir<strong>in</strong>g my <strong>in</strong>terest <strong>in</strong> <strong>the</strong> regeneration <strong>of</strong> vegetation, to Dr E. W.<br />
Jones for references on <strong>the</strong> fruit<strong>in</strong>g <strong>of</strong> forest trees, and to Drs R. S. Clymo, E. I. Newman and R. W.<br />
Snaydon for criticiz<strong>in</strong>g a first draft <strong>of</strong> this paper. Miss S. M. Bishop drew Figs. 1-6 and 8-10.
P. J. GRUBB<br />
IX. REFERENCES<br />
ASH, J. E. & BARKHAM, J. P. (1976). Changes and variability <strong>in</strong> <strong>the</strong> field layer <strong>of</strong> a coppiced woodland <strong>in</strong><br />
Norfolk, England. Journal <strong>of</strong> Ecology 64, 697-712.<br />
ASHTON, P. S. (1969). Speciation among tropical forest trees: some deductions <strong>in</strong> <strong>the</strong> light <strong>of</strong> recent<br />
evidence. Biological Journal <strong>of</strong> <strong>the</strong> L<strong>in</strong>nean Society I, 155-96.<br />
ASPINALL, D. (1960). An analysis <strong>of</strong> competition between barley and white persicaria. 11. Factors<br />
determ<strong>in</strong><strong>in</strong>g <strong>the</strong> course <strong>of</strong> competition. Annals <strong>of</strong> Applied Biology 48, 637-54.<br />
ASPINALL, D. & MILTHORPE, F. L. (1959). An analysis <strong>of</strong> <strong>the</strong> competition between barley and white<br />
persicaria. I. The effects <strong>of</strong> growth. Annals <strong>of</strong> Applied Biology 47, 156-72.<br />
ASPINALL, D., NICHOLLS, P. B. & MAY, L. H. (1964). The effects <strong>of</strong> soil moisture stress on <strong>the</strong> growth<br />
<strong>of</strong> barley. I. Vegetative development and gra<strong>in</strong> yield. AustralianJournal <strong>of</strong> Agricultural Research IS,<br />
729-45.<br />
AUBREVILLE, A. (1938). La for& coloniale: les for&ts de l'Afrique occidentale fraqaise. Annales<br />
Acadkmie des sciences coloniales, Paris, 9, 1-245.<br />
AUCLAIR, A. N. & COTTAM, G. (1971). Dynamics <strong>of</strong> Black Cherry, Prunus serot<strong>in</strong>a Erhr., <strong>in</strong> sou<strong>the</strong>rn<br />
Wiscons<strong>in</strong> oak forests. Ecological Monographs 41, I 53-77.<br />
AUSTIN, M. P. (1968). Pattern <strong>in</strong> a Zerna erecta dom<strong>in</strong>ated community.Journa1 <strong>of</strong> Ecology 56, 197-218.<br />
BAKER, H. G. (1959). Reproductive methods as factors <strong>in</strong> speciation <strong>in</strong> flower<strong>in</strong>g <strong>plant</strong>s. Cold Spr<strong>in</strong>g<br />
Harbour Symposia on Quantitative Biology 24, 177-91.<br />
BAKER, H. G. (1963). Evolutionary mechanisms <strong>in</strong> poll<strong>in</strong>ation biology. Science 139, 877-83.<br />
BAKER, H. G. (1970). Evolution <strong>in</strong> <strong>the</strong> tropics. Biotropica 2, 101-111.<br />
BARON, J. F. (1969). Ten years <strong>of</strong> forest seed crops <strong>in</strong> California. Journal <strong>of</strong> Forestry 67, 490-2.<br />
BARROW, M. D., COSTIN, A. B. & LAKE, P. (1968). Cyclical changes <strong>in</strong> an Australian fjaeldmark<br />
community. Journal <strong>of</strong> Ecology 56, 89-96.<br />
BASKIN, J. M. & BASKIN, C. C. (1971 a). Germ<strong>in</strong>ation ecology and adaptation to habitat <strong>in</strong> Leavenworthia<br />
spp. (Cruciferae). American Midland Naturalist 85, 22-35.<br />
BASKIN, J. M. & BASKIN, C. C. (1971 b). Germ<strong>in</strong>ation ecology <strong>of</strong> Phacelia dubia var. dubia <strong>in</strong> Tennessee<br />
glades. American Journal <strong>of</strong> Botany 58, 98-104.<br />
BEADLE, N. C. W. (I 940). Soil temperaturesdur<strong>in</strong>g forest fires and <strong>the</strong>ir effect on <strong>the</strong> survival <strong>of</strong> vegetation.<br />
Journal <strong>of</strong> Ecology 28, 18-92.<br />
BEARD, J. S. (1969). The natural regions <strong>of</strong> <strong>the</strong> deserts <strong>of</strong> Western Australia. Journal <strong>of</strong> Ecology 57,<br />
677-7 I I.<br />
BEDFORD, THE DUKE OF, & PICKERING, S. U. (1919). Science and Fruit Grow<strong>in</strong>g. Macmillan, London.<br />
BERGH, J. P. VAN DER & WIT, C. G. DE (1960). Concurrentie tussen Timo<strong>the</strong>n en Reukgras. Mededel<strong>in</strong>gen<br />
Instituut voor biologisch en scheikundig onderzoek van landbouwgewassen IZI, I 55-65.<br />
BEVERIDGE, A. E. (1964). Dispersal and destruction <strong>of</strong> seed <strong>in</strong> central North Island podocarp forests.<br />
Proceed<strong>in</strong>gs <strong>of</strong> <strong>the</strong> New Zealand Ecological Society 11, 48-55.<br />
BEVERIDGE, A. E. (1973). Regeneration <strong>of</strong> podocarps <strong>in</strong> central North Island forest. New ZeaZandJournal<br />
<strong>of</strong> Forestry 18, 23-35.<br />
BOE, K. N. (1954). Periodicity <strong>of</strong> cone crops for five Montana conifers. Proceed<strong>in</strong>gs <strong>of</strong> <strong>the</strong> Montana<br />
Academy <strong>of</strong> Sciences 14, 5-9.<br />
BOHART, G. E. & KOEBER, T. W. (1972). Insects and seed production. In Seed Biology, vol. 3 (ed. T. T.<br />
Kozlowski), pp. 1-53. Academic Press, New York.<br />
BORNKAMM, R. (I 96 I a). Zur quantitativen Bestimmungvon Konkurrenzkraft und Wettbewerbsspannung.<br />
Berichte der deutschen Botanischen Geselkchaft 74, 75-83.<br />
BORNKAMM, R. (1961 b). Zur lichtkonkurrenz von Ackerunkrautern. Flora, Jena N.F. 151, 126-43.<br />
BOTKIN, D. B., JANAK, J. F. &WALLIS, J. R. (1972). Some ecological consequences<strong>of</strong> acomputer model<br />
<strong>of</strong> forest growth. Journal <strong>of</strong> Ecology 60, 849-72.<br />
BRAY, J. R. (1956). Gap phase replacement <strong>in</strong> a maple-basswood forest. Ecology 37, 598-600.<br />
BUDOWSI~I, G. (1965). The distribution <strong>of</strong> tropical American ra<strong>in</strong> forest <strong>species</strong> <strong>in</strong> <strong>the</strong> light <strong>of</strong> successional<br />
processes. Turrialba 15, 40-2.<br />
BURGES, A. (1951). The ecology <strong>of</strong> <strong>the</strong> Cairngorms. 111. The Empetrum-Vacc<strong>in</strong>ium zone. Journal <strong>of</strong><br />
Ecology 39, 271-84.<br />
BYKOV, B. A. (1974). Fluctuations <strong>in</strong> <strong>the</strong> semidesert and desert vegetation <strong>of</strong> <strong>the</strong> Turanian pla<strong>in</strong>. In<br />
Vegetation Dynamics (ed. R. Knapp), Part 8 <strong>of</strong> Handbook <strong>of</strong> Vegetation Science, pp. 243-51. Junk,<br />
The Hague.<br />
CANTLON, J. E. (1969). The stability <strong>of</strong> natural populations and <strong>the</strong>ir sensitivity to technolog). Diversity<br />
and Stability <strong>in</strong> Ecological Systems. Brookhaven Symposia <strong>in</strong> Biology 22, 197-203.<br />
CAPUTA, J. (1948). Untersuchungen uber die Entwicklung e<strong>in</strong>iger Grsser und Kleearten <strong>in</strong> Re<strong>in</strong>saat und<br />
Mischung. Landwirtschaftliches Jahrbuch der Schweiz TO (849), 1-127.
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong><br />
CAVE=, P. B. & HARPER, J. L. (1967). Studies <strong>in</strong> <strong>the</strong> dynamics <strong>of</strong> <strong>plant</strong> populations. I. The fate <strong>of</strong> seed<br />
and trans<strong>plant</strong>s <strong>in</strong>troduced <strong>in</strong>to various habitats. Journal <strong>of</strong> Ecology 55, 59-71,<br />
CHADWICK, M. J. (1961). Nardus stricta, a weed <strong>of</strong> hill graz<strong>in</strong>gs. In The biology <strong>of</strong> Weeds (ed. J. L.<br />
Harper). Symposia <strong>of</strong> <strong>the</strong> British Ecological Society I, 246-56.<br />
CHAMBERS, T. C. (1959). Pattern and process <strong>in</strong> a New Zealand subalp<strong>in</strong>e <strong>plant</strong> community. Vegetatio<br />
8, 209-14.<br />
CHAMPNESS, S. S. & MORRIS, K. (1948). The population <strong>of</strong> buried viable seeds <strong>in</strong> relation to contrast<strong>in</strong>g<br />
pasture and soil types. Journal <strong>of</strong> Ecology 36, 1 4 ~ 3 .<br />
CHILVERS, G. A. & BRITTAIN, E. G. (1972). Plant competition mediated by host-specific parasites - a<br />
simple model. Australian Journal <strong>of</strong> Biological Sciences 25, 749-56.<br />
CHIPPINDALE, H. G. & MILTON, W. E. J. (1934). On <strong>the</strong> viable seeds present <strong>in</strong> <strong>the</strong> soil beneath<br />
pastures. Journal <strong>of</strong> Ecology 22, 508-3 I.<br />
CLARK, M. B. (1970). Seed production <strong>of</strong> hemlock and cedar <strong>in</strong> <strong>the</strong> Interior Wet Belt Region <strong>of</strong> British<br />
Columbia related to dispersal and regeneration. Research Notes British Columbia Forest Service (5 I),<br />
1-11.<br />
CLEMENTS, F.E., WEAVER, J. E. & HANSON, H. C. (1929). Plant Competition. Publications <strong>of</strong> <strong>the</strong> Carnegie<br />
Institution, Wash<strong>in</strong>gton (398), i-xvi, 1340.<br />
COCKAYNE, L. (1928). The Vegetation <strong>of</strong> Nee0 Zealand, 2nd ed. Engelmann, Leipzig.<br />
CONNELL, J. H. (1971). On <strong>the</strong> role <strong>of</strong> natural enemies <strong>in</strong> prevent<strong>in</strong>g competitive exclusion <strong>in</strong> some<br />
mar<strong>in</strong>e animals and <strong>in</strong> ra<strong>in</strong> forest trees. In Proceed<strong>in</strong>gs <strong>of</strong> <strong>the</strong> Advanced Study Institute on Dynamics <strong>of</strong><br />
Numbers <strong>in</strong> Populations, Oosterbeek, 1970 (ed. P.J. den Boer and G. R. Gradwell), pp. 298-312.<br />
Centre for Agricultural Publish<strong>in</strong>g and Documentation, Wagen<strong>in</strong>gen.<br />
COOMBE, D. E. (1966). The seasonal light climate and <strong>plant</strong> growth <strong>in</strong> a Cambridgeshire wood. In Light<br />
as an Ecological Factor (ed. R. Ba<strong>in</strong>bridge, G. C. Evans and 0. Rackham). Symposia <strong>of</strong> <strong>the</strong> British<br />
Ecological Society 6, 14846.<br />
COUPLAND, R. T. (1974). Fluctuations <strong>in</strong> North American grassland vegetation. In Vegetation Dynamics<br />
(ed. R. Knapp, Part 8 <strong>of</strong> Handbook <strong>of</strong> Vegetational Science, pp. 233-41. Junk, The Hague.<br />
CUATRECASAS, J. (1934). Observaciones geobotanicas en Colombia. Trabajos del Muse0 nacional de<br />
ciencias naturales, Madrid, serie botanica (27), 1-14.<br />
CURTIS, J. T. (1959). The Vegetation <strong>of</strong> Wiscons<strong>in</strong>. University <strong>of</strong> Wiscons<strong>in</strong> Press, Madison.<br />
DAUBENMIRE, R. (1960). A seven-year study <strong>of</strong> cone production as related to xylem layers and temperature<br />
<strong>in</strong> P<strong>in</strong>us ponderosa. American Midland Naturalist 64, 187-93.<br />
DAUBENMIRE, R. (1968~). Plant Communities. Harper and Row, New York.<br />
DAUBENMIRE, R. (1968 b). Forest vegetation <strong>of</strong> eastern Wash<strong>in</strong>gton and nor<strong>the</strong>rn Idaho. Technical<br />
Bullet<strong>in</strong> <strong>of</strong> <strong>the</strong> Agricultural Experiment Station Wash<strong>in</strong>gton State (60), 1-104.<br />
DAUBENMIRE, R. (1970). Steppe vegetation <strong>of</strong> Wash<strong>in</strong>gton. Technical Bullet<strong>in</strong> <strong>of</strong> <strong>the</strong> Agricultural Experiment<br />
Station Wash<strong>in</strong>gton State (62), 1-131.<br />
DAVIES, S. J. J. F. (1976). Studies on <strong>the</strong> flower<strong>in</strong>g season and fruit production <strong>of</strong> some arid zone shrubs<br />
and trees <strong>in</strong> western Australia. Journal <strong>of</strong> Ecology 64, 665-87.<br />
DELEUIL, G. (1951). Orig<strong>in</strong>e des substances toxiques du sol des associations sans thkrophytes du<br />
Romar<strong>in</strong>o-Ericion. Compte rendu hebdomadaire des shnces de Z’Acadlmie des sciences, Paris 232,2038-9.<br />
DIELS, L. (1937). Beitrage zur Kenntnis der Vegetation und Flora von Ecuador. Biblio<strong>the</strong>ca botanica<br />
116, 1-190.<br />
DUCHAUFOUR, P. H. (1960). Stations, types d’humus et groupements bcologiques. Rewue forestihe<br />
frangaise, Nancy 12, 484-94.<br />
EHRENDORFER, F. (1973). Adaptive significance <strong>of</strong> major taxonomic characters and morphological<br />
trends <strong>in</strong> angiosperms. In Taxonomy and Ecology (ed. V. H. Heywood), pp. 317-27. Academic Press,<br />
London.<br />
ELLENBERG, H. (1954). Landwirtschaftliche Pflanzensoziolcp.e, vol. I. Ulmer, Stuttgart.<br />
ELLENFIERG, H. (1963). Vegetation Mitteleuropas mit den Alpen. Ulmer, Stuttgart.<br />
FENNEF~, M. (1975). Factors affect<strong>in</strong>g <strong>the</strong> distribution <strong>of</strong> strict calcicoles. Ph.D. dissertation, University <strong>of</strong><br />
Cambridge.<br />
FLOWER-ELLIS, J. G. K. (1971). Age structure and dynamics <strong>in</strong> stands <strong>of</strong> bilberry (Vacc<strong>in</strong>ium myrtillus<br />
L.). Research Notes, Department <strong>of</strong> Forest Ecology and Forest Soils, Royal College <strong>of</strong> Forestry,<br />
Stockholm (9), i-x, 1-108.<br />
FOGGIE, A. (1960). Natural regeneration <strong>in</strong> <strong>the</strong> humid tropical forest. Caribbean Forester 27, 73-81.<br />
FORCIER, L. K. (1975). Reproductive strategies and <strong>the</strong> co-occurrence <strong>of</strong> climax tree <strong>species</strong>. Science<br />
189, 808-10.<br />
FOWELLS, H. A. & SCHUBERT, G. H. (1956). Seed crops <strong>of</strong> forest trees <strong>in</strong> <strong>the</strong> P<strong>in</strong>e region <strong>of</strong> California.<br />
United States Department <strong>of</strong> Agriculture Technical Bullet<strong>in</strong> ( I I~o), 1-48.<br />
FRANKIE, G. W., BAKER, H. G. & OPLER, P. A. (1974). Comparative phenological studies <strong>of</strong> trees <strong>in</strong><br />
Tropical Wet and Dry forests <strong>in</strong> <strong>the</strong> lowlands <strong>of</strong> Costa Rica. Journal <strong>of</strong> Ecology 62, 881-913.<br />
I37
P. J. GRUBB<br />
FRANKLYN, J. F. & DYRNESS, C. T. (1969). Vegetation <strong>of</strong> Oregon and Wash<strong>in</strong>gton. United States<br />
Department <strong>of</strong> Agriculture Forest Service Research Paper (PWN-So), i-vi, 1-216.<br />
FREE, J. B. (1968). Dandelion as a competitor to fruit trees for bee visits. Journal <strong>of</strong> Applied Ecology<br />
5, 16978.<br />
FRIEDMAN, J. & ORSHAN, G. (1975). The distribution, emergence and survival <strong>of</strong> seedl<strong>in</strong>gs <strong>of</strong> Artemisia<br />
herba-alba Asso <strong>in</strong> <strong>the</strong> Negev - Desert <strong>of</strong> Israel <strong>in</strong> relation to distance from <strong>the</strong> adult <strong>plant</strong>s. Yournal <strong>of</strong><br />
Ecology 63, 627-32.<br />
GENTRY, A. H. (1974). Flower<strong>in</strong>g phenology and diversity <strong>in</strong> tropical Bignoniaceae. Biotropica 6,<br />
64-8.<br />
GENTRY, A. H. (1976). Bignoniaceae <strong>of</strong> Sou<strong>the</strong>rn Central America: distribution and ecological specificity.<br />
Biotropica 8, I 17-3 I.<br />
GILBERT, 1. M. (1959). Forest succession <strong>in</strong> <strong>the</strong> Florent<strong>in</strong>e Valley. Proceed<strong>in</strong>gs <strong>of</strong> <strong>the</strong> Royal Society <strong>of</strong><br />
Tasmania 93, I 29-5 I.<br />
GILLETT, J. B. (1962). Pest pressure, an underestimated factor <strong>in</strong> evolution. In Taxonomy and Geography<br />
(ed. D. Nichols). Publications <strong>of</strong> <strong>the</strong> Systematics Assciciation 4, 37-46.<br />
GODWIN, H. (1941). Studies <strong>in</strong> <strong>the</strong> ecology <strong>of</strong> Wicken Fen. IV. Crop-tak<strong>in</strong>g experiments. Journal <strong>of</strong><br />
Ecology 29, 83-106.<br />
Pi. B. rony6esa (GOLUBEVA, I. V.) (1968). Amamma ceMemozl ~ ~O~~KTEBHOCTH IIOII~JIXUE~ Knesepa<br />
ropHoro (Trifolimmontanum L.) H KOB~IJIS~ nepnmoro (Stipapennata L.)B paawrx t$moqeHoTHsecIcHx<br />
YCJIOBH~X ~ ~ O B O memi & (The dynamics <strong>of</strong> <strong>the</strong> seed productivity <strong>of</strong> Trzfalium montanum L. and Stipa<br />
pennata L. populations under different phytocoenotic conditions <strong>of</strong> a meadow steppe). Bornanuvecicic<br />
wcypHan (Botanicheskii zhurnal SSSR) 53, 1604-1 I.<br />
G~MEZ-POMPA, A. (1971). Posible papel de la vegetaci6n secundaria en la evoluci6n de la flora<br />
tropical. Biotropica 3, 125-35.<br />
GOODALL, D. W. (1970). Statistical <strong>plant</strong> ecology. Annual Review <strong>of</strong> Ecology and Systematics I, 99-124.<br />
GRAHAM, S. A. (1941). Climax forests <strong>of</strong> <strong>the</strong> Upper Pen<strong>in</strong>sula <strong>of</strong> Michigan. Ecology 22, 355-62.<br />
GRANT, U. (1949). Poll<strong>in</strong>ation systems as isolat<strong>in</strong>g mechanisms <strong>in</strong> angiosperms. Evolution, Lancaster,<br />
Pennsylvania 3, 82-97.<br />
GRATKOWSKI, H. (1961). Brush seedl<strong>in</strong>gs after controlled burn<strong>in</strong>g <strong>of</strong> brushlands <strong>in</strong> southwestern<br />
Oregon. Journal <strong>of</strong> Forestry 59, 885-8.<br />
GREIG-SMITH, P., AUSTIN, M. P. & WHITMORE, T. C. (1967). The application <strong>of</strong> quantitative methods<br />
to vegetation survey. I. Association-analysis and pr<strong>in</strong>cipal component ord<strong>in</strong>ation <strong>of</strong> ra<strong>in</strong> forest.<br />
Journal <strong>of</strong> Ecology 55, 483-503.<br />
GRIME, J. P. (1966). Shade avoidance and shade tolerance <strong>in</strong> flower<strong>in</strong>g <strong>plant</strong>s. In Light as an Ecological<br />
Factor (ed. R. Ba<strong>in</strong>bridge, G. C. Evans and 0. Rackham). Symposia <strong>of</strong> <strong>the</strong> British Ecological Society<br />
6, 187-207.<br />
GRIME, J. P. (1973 a). Competitive exclusion <strong>in</strong> herbaceous vegetation. Nature 242, 344-7.<br />
GRIME, J. P. (1973b). Competition and diversity <strong>in</strong> herbaceous vegetation. Nature 244, 3 I I.<br />
GRIME, J. P., MACPHERSON-STEWART, S. F. & DEARMAN, R. S. (1968). An <strong>in</strong>vestigation <strong>of</strong> leaf palatability<br />
us<strong>in</strong>g <strong>the</strong> snail Cepaea nemoralis L. Journal <strong>of</strong> Ecology 56, 405-20.<br />
GRUBB, P. J. (1976). A <strong>the</strong>oretical background to <strong>the</strong> conservation <strong>of</strong> ecologically dist<strong>in</strong>ct groups <strong>of</strong><br />
annuals and biennials <strong>in</strong> <strong>the</strong> chalk grassland ecosystem. Biological Conservation 10, 5376.<br />
GRUBB, P. J. & STEVENS, P. F. (1976). The forests <strong>of</strong> <strong>the</strong> Fatima bas<strong>in</strong> and Mt Kerigomna and a Review <strong>of</strong><br />
Montane and Subalp<strong>in</strong>e Forests elsewhere <strong>in</strong> Papua New Gu<strong>in</strong>ea. Department <strong>of</strong> Biogeography and<br />
Geomorphology Publication (BG/5). Australian National University, Canberra. (In <strong>the</strong> press.)<br />
GRUBB, P. J. & TANNER, E. V. J. (1976). The montane forests and soils <strong>of</strong> Jamaica: a re-assessment.<br />
Journal <strong>of</strong> <strong>the</strong> Arnold Arboretum 57, 313-68.<br />
GUEVARA, S. Sz G~MEZ-POMPA, A. (1972). Seeds from surface soils <strong>in</strong> a tropical region <strong>of</strong> Veracruz,<br />
Mexico. Journal <strong>of</strong> <strong>the</strong> Arnold Arboretum 53, 312-35.<br />
GUYOT, L. (1960). Sur la presence dans les terres cultivkes et <strong>in</strong>cultes des semences dormantes des<br />
espbces adventices. Bullet<strong>in</strong> du Service de la carte phytogkographique B 5, 197-254.<br />
HAIZEL, K. A. & HARPER, J. L. (1973). The effects <strong>of</strong> density and <strong>the</strong> tim<strong>in</strong>g <strong>of</strong> removal on <strong>in</strong>terference<br />
between barley, white mustard and wild oats. Journal <strong>of</strong> Applied Ecology 10, 23-31.<br />
HARBERD, D. J. (1961). Observations on population structure and longevity <strong>of</strong> Festuca rubra L. New<br />
Phytologist 60, 184-206.<br />
HARLAN, H. V. & MARTINI, M. L. (1938). The effect <strong>of</strong> natural selection <strong>in</strong> a mixture <strong>of</strong> barley varieties.<br />
Journal <strong>of</strong> Agricultural Research 57, I 89-99.<br />
HARPER, J. L. (1961). Approaches to <strong>the</strong> study <strong>of</strong> <strong>plant</strong> competition. In Mechanisms <strong>in</strong> Biological<br />
Competition (ed. F. L. Milthorpe). Symposia <strong>of</strong> <strong>the</strong> Society for Experimental Biology 15, 1-39.<br />
HARPER, J. L. (1965). Establishment, aggression and cohabitation <strong>in</strong> weedy <strong>species</strong>. In The Genetics <strong>of</strong><br />
Colonis<strong>in</strong>g Species (ed. H. G. Baker and G. L. Stebb<strong>in</strong>s), pp. 243-68. Academic Press, New York.<br />
HARPER, J. L. (1967). A Darw<strong>in</strong>ian approach to <strong>plant</strong> ecology. Journal <strong>of</strong> Ecology 55, 247-70.
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> ‘39<br />
HARPER, J. L. (1969). The role <strong>of</strong> predation <strong>in</strong> vegetational diversity. Diversity and Stability <strong>in</strong> Ecological<br />
Systems. Brookhaven Symposia <strong>in</strong> Biology 22, 48-61.<br />
HARPER, J. L., CLATWORTHY, J. N., MCNAUGHTON & SAGAR, G. R. (1961). The evolution and ecology <strong>of</strong><br />
closely related <strong>species</strong> liv<strong>in</strong>g <strong>in</strong> <strong>the</strong> same area. Evolution, Lancaster, Pennsylvania 15, 209-27.<br />
WER, J. L., LOVELL, P. H. & MOORE, K. G. (1970). The shapes and sizes <strong>of</strong> seeds. Annual Review <strong>of</strong><br />
Ecology and Systematics I, 327-56.<br />
~ E R J. , L.,WILLIAMS, J. T. & SAGAR, G. R. (1965). The behaviour <strong>of</strong> seeds <strong>in</strong> soil. I. The heterogeneity<br />
<strong>of</strong> soil surfaces and its role <strong>in</strong> determ<strong>in</strong><strong>in</strong>g <strong>the</strong> establishment <strong>of</strong> <strong>plant</strong>s from seed. Journal <strong>of</strong><br />
Ecology 53, 273-86.<br />
HEINRICH, B. & RAVEN, P. H. (1972). Energetics and poll<strong>in</strong>ation ecology. Science 176, 597-602.<br />
HEWETSON, C. E. (1956). A discussion on <strong>the</strong> ‘climax’ concept <strong>in</strong> relation to <strong>the</strong> tropical ra<strong>in</strong> forest.<br />
Empire Forestry Rm’ew 35, 274-1.<br />
&m, G. (1852). Verhalten der Waldbaumegegen Licht und Schatten. Erlangen.<br />
HOCKING, B. (1968). Insect-flower associations <strong>in</strong> <strong>the</strong> high Arctic with special reference to nectar.<br />
Oikos 19, 359-88.<br />
HORN, H. S. (1975). Forest succession. Scientific American 232 (s), 90-8.<br />
HUFFAKER, C. B. & KWNET, C. E. (1959). A ten-year study <strong>of</strong> vegetational changes associated with<br />
biological control <strong>of</strong> Klamath weed. Journal <strong>of</strong> Range Management 12,69-82.<br />
HUTCHINSON, G. E. (1957). Conclud<strong>in</strong>g remarks. Population Studies: Animal Ecology and Demography.<br />
Cold Spr<strong>in</strong>g Harbour Symposia on Quantitative Biology w, 415-7.<br />
HUTCHINSON, G. E. (1959). Homage to Santa Rosalia or Why are <strong>the</strong>re so many k<strong>in</strong>ds <strong>of</strong> animals?<br />
American Naturalist 93, 145-59.<br />
HUTCHINSON, T. C. (1967). Comparative studies <strong>of</strong> <strong>the</strong> ability <strong>of</strong> <strong>species</strong> to withstand prolonged periods<br />
<strong>of</strong> darkness. Journal <strong>of</strong> Ecology 55, 291-9.<br />
Iwm, H. & TOTSIJKA, T. (1959). Ecological and physiological studies on <strong>the</strong> vegetation <strong>of</strong> Mt Shimagare.<br />
11. On <strong>the</strong> crescent-shaped “Dead Trees Strips” <strong>in</strong> <strong>the</strong> Yatsugatake and <strong>the</strong> Chichibu Mounta<strong>in</strong>s.<br />
Botanical Magaz<strong>in</strong>e, Tokyo 72, 255-60.<br />
JACKSON, W. D. (1968). Fire, air, water and earth - an elemental ecology <strong>of</strong> Tasmania. Proceed<strong>in</strong>gs <strong>of</strong> <strong>the</strong><br />
Ecological Society <strong>of</strong> Australia 3, 9-16.<br />
JACQUARD, P. (1968). Manifestation et nature des relations sociales chez les vggktaux superieurs.<br />
Oecologia Plantarum 3, 137-68.<br />
JANZEN, D. H. (1969). Seed-eaters versus seed size, number, toxicity and dispersal. Evolution, Lancaster,<br />
Pennsylvania 23, 1-27.<br />
JANZEN, D. H. (1970). Herbivores and <strong>the</strong> number <strong>of</strong> tree <strong>species</strong> <strong>in</strong> tropical forests. Am*can<br />
Naturalist zoq, 501-28.<br />
JANZEN, D.H. (1971). Seed predation by animals. Annual Review <strong>of</strong> Ecology and Systematics 2,<br />
465-92.<br />
JARVIS, M. S. (1963). A comparison between <strong>the</strong> water relations <strong>of</strong> <strong>species</strong> with contrast<strong>in</strong>g types <strong>of</strong><br />
geographical distribution <strong>in</strong> <strong>the</strong> British Isles. The Water Relations <strong>of</strong> Plants (ed. A. J. Rutter and<br />
F. H. Whitehead). Symposia <strong>of</strong> <strong>the</strong> British Ecological Society 3, 289312.<br />
JEFFREY, D. W. & PIGOTT, C. D. (1973). The response <strong>of</strong> grasslands on sugar-limestone <strong>in</strong> Teesdale to<br />
application <strong>of</strong> phosphorus and nitrogen. Journal <strong>of</strong> Ecology 61, 85-92.<br />
JONES, E. W. (1945). The structure and reproduction <strong>of</strong> <strong>the</strong> virg<strong>in</strong> forest <strong>of</strong> <strong>the</strong> North Temperate<br />
Zone. New Phytologist 4, 130-48.<br />
JONES, E. W. (19554). Ecdogical studies on <strong>the</strong> Ra<strong>in</strong> Forest <strong>of</strong> sou<strong>the</strong>rn Nigeria. IV. The plateau<br />
forest <strong>of</strong> <strong>the</strong> Okomu Forest Reserve. Journal <strong>of</strong> Ecology 43, 564-94, 4.83-1 17.<br />
B. r.KapnOB(I(ARPOV,V. G.) (r96r).OB~K0a~ypeE~~KopEelt<br />
aerrrenbmxn TaembIx TP~ B enoBm necax (On <strong>the</strong> <strong>in</strong>fluence <strong>of</strong> tree root competition on <strong>the</strong><br />
photosyn<strong>the</strong>tic activity <strong>of</strong> <strong>the</strong> herb layer <strong>in</strong> spruce forest). AOKM~M AKadeMUU Hap CCCP (Dokladp<br />
Akudemii nauk SSSR) 140, 1205-8.<br />
KARR, E. J., LINCK, A. J. & SWANSON, C. A. (1959). The effect <strong>of</strong> short periods <strong>of</strong> high temperature<br />
dur<strong>in</strong>g day and night periods on pea yields. American Journal <strong>of</strong> Botany 46, 91-3.<br />
JSEATINGE, T. H. (1975). Plant community dynamics <strong>in</strong> wet heathland. Journal <strong>of</strong> Ecology 63, 16372.<br />
KBRNER VON ~ILAUN, A. (1902). TheNaturalHistory <strong>of</strong> Plants (trans. F. W. Oliver). Blackie, London.<br />
KBVAN, P. G. (1972). Insect poll<strong>in</strong>ation <strong>of</strong> high arctic flowers. Journal <strong>of</strong> Ecology 60, 831-47.<br />
KING, T. J. (1975). Inhibition <strong>of</strong> seed germ<strong>in</strong>ation under leaf canopies <strong>in</strong> Arenaria serpyllifolia, Veronica<br />
arvm‘s and Cerastium holosteoides. New Phytologist 75, 87-90.<br />
KNAPP, R. (1974). Mutual <strong>in</strong>fluences between <strong>plant</strong>s, allelopathy, competition and vegetation changes.<br />
In Vegetation Dynamics (ed. R. Knapp), Part 8 <strong>of</strong> Handbook Vegetation Science, pp. 111-22. Junk,<br />
The Hague.<br />
KNIGHT, G. H. (1964). Some factors affect<strong>in</strong>g <strong>the</strong> distribution <strong>of</strong> Endymion nonscriptlrs (L.) Garcke <strong>in</strong><br />
Warwickshire woods. J o m l <strong>of</strong> Ecology 42,405-zr.<br />
JTepeSbeBHa ~~EMEJuIWOEFIyIo
P. J. GRUBB<br />
KOLLER, D, (1969). The physiology <strong>of</strong> dormancy and survival <strong>of</strong> <strong>plant</strong>s <strong>in</strong> desert environments. In<br />
Dormancy and Surptival (ed. H. W. Woolhouse). Symposia <strong>of</strong> <strong>the</strong> Society for Experimental Biology<br />
23,449-69.<br />
KOLLER, D. (1972). Environmental control <strong>of</strong> seed germ<strong>in</strong>ation. In Seed Biology, vol. 2 (ed. T. T.<br />
Kozlowski), pp. 1-101. Academic Press, New York.<br />
KORCHAGIN, A. A. & KARPOV, V. G. (1974). Fluctuations <strong>in</strong> coniferous taiga <strong>communities</strong>. In Vegetation<br />
Dynamics (ed. R. Knapp), Part 8 <strong>of</strong> Handbook <strong>of</strong> Vegetation Science, pp. 225-31. Junk, The<br />
Hague.<br />
KREBS, C. J. (1972). Ecology: an Experimental Analysis <strong>of</strong> Distribution and Abundance. Harper and<br />
Row, New York.<br />
KUJALA, V. (1926). Untersuchungen uber die Waldvegetation <strong>in</strong> Sudund Mittlelf<strong>in</strong>nland. I. Zur<br />
Kenntnis des okologisch-biologischen Charakters der Pflanzenarten unter spezieller Beriicksichtigung<br />
der Bildung von Pflanzenvere<strong>in</strong>. A. Gefasspflanzen. Metsatienteellisen Koelaitoksen Julkaisuja 10<br />
(I), 1-140.<br />
KYDD, D. D. (1964). The effect <strong>of</strong> different systems <strong>of</strong> cattle graz<strong>in</strong>g on <strong>the</strong> botanical composition <strong>of</strong><br />
permanent downland pasture. Journal <strong>of</strong> Ecology 52, 139-49.<br />
LAMBERT, R. G. & LINCK, A. J. (1958). Effects <strong>of</strong> high temperature on <strong>the</strong> yield <strong>of</strong> peas. Plant Physiclogy,<br />
Lancaster, Pennsylvania 33, 347-50.<br />
LAUER, E. (1953). uber die Keimtemperatur von Ackerunkrautern und deren E<strong>in</strong>fluss auf die Zusammensetzung<br />
von Unkrautgesellschaften. Flora, Jena N.F. 140, 551-95.<br />
LOUCKS, 0. L. (1970). Evolution <strong>of</strong> diversity, efficiency and community stability. American Zoologist<br />
10, 17-25.<br />
LOWRY, W. P. (1966). Apparent meteorological requirements for abundant cone crop <strong>in</strong> Douglas-Fir.<br />
Forest Science 12, 185-92.<br />
MAAREL, E. VAN DER (1971). Plant <strong>species</strong> diversity <strong>in</strong> relation to management. In The Scientific<br />
Management <strong>of</strong> Animal and Plant Communities for Consematwn (ed. E. DufFey and A. S. Watt).<br />
Symposia <strong>of</strong> <strong>the</strong> British Ecological Society 11, 45-63.<br />
MACARTHUR, R. H. (1955). Fluctuations <strong>of</strong> animal populations, and a measure <strong>of</strong> community stability.<br />
ECOk?Y 36, 533-6.<br />
MCNAUGHTON, S. J. (1967). Relationships amongst functional properties <strong>of</strong> Californian grassland.<br />
Nature 216, 168-9.<br />
MCNAUGHTON, S. J. (1968~). Autotoxic feedback <strong>in</strong> relation to germ<strong>in</strong>ation and seedl<strong>in</strong>g growth <strong>in</strong><br />
Typha latifolia. Ecology 49, 367-9.<br />
MCNAUGHTON, S. J. (1968b). Structure and function <strong>of</strong> Californian grassland. Ecology 49, 86272.<br />
MCPHERSON, J. K. & MULLER, C. H. (1969). Allelopathic effects <strong>of</strong> Adenostoma fasciculatum, Chamise’,<br />
<strong>in</strong> <strong>the</strong> Californian chaparral. Ecological Monographs 39, 177-98.<br />
MCWILLIAM, J. R., CLEMENTS, R. J. & DOWLING, P. M. (1970). Some factors <strong>in</strong>fluenc<strong>in</strong>g <strong>the</strong> germ<strong>in</strong>ation<br />
and early seedl<strong>in</strong>g development <strong>of</strong> pasture <strong>plant</strong>s. Australian Journal <strong>of</strong> Agricultural Research<br />
21, 19-32.<br />
MAJOR, J. & PYOTT, W. T. (1966). Buried, viable seeds <strong>in</strong> two Californian bunchgrass sites, and <strong>the</strong>ir<br />
bear<strong>in</strong>g on <strong>the</strong> def<strong>in</strong>ition <strong>of</strong> a flora. Vegetatio 13, 253-82.<br />
MARKS, P. L. (1974). The role <strong>of</strong> p<strong>in</strong> cherry (Prunuspensylvanica L.) <strong>in</strong> <strong>the</strong> <strong>ma<strong>in</strong>tenance</strong> <strong>of</strong> stability <strong>in</strong><br />
nor<strong>the</strong>rn hardwood ecosystems. Ecological Monographs 4, 73-88.<br />
MARSHALL, D. R. & JAIN, S. K. (1969). Interference <strong>in</strong> pure and mixed populations <strong>of</strong> Avenafatua and<br />
A. barbata. Journal <strong>of</strong> Ecology 57, 251-70.<br />
MARTIN, M. H. (1968). Conditions affect<strong>in</strong>g <strong>the</strong> distribution <strong>of</strong> Mercurialis perennis L. <strong>in</strong> certa<strong>in</strong><br />
Cambridgeshire woodlands. Journal <strong>of</strong> Ecology 56, 777-93.<br />
MAY, R. M. (1975). Stability and Complexity <strong>in</strong> Model Ecosystems. 2nd ed. University Press, Pr<strong>in</strong>ceton,<br />
New Jersey.<br />
MEDWAY, LORD (1972). Phenology <strong>of</strong> a tropical ra<strong>in</strong> forest <strong>in</strong> Malaya. BiologicalJournal <strong>of</strong> <strong>the</strong> L<strong>in</strong>nean<br />
Society 4, I 17-46.<br />
MILES, J. (1973). Early mortality and survival <strong>of</strong> self-sown seedl<strong>in</strong>gs <strong>in</strong> Glenfeshie, Inverness-shire.<br />
Journal <strong>of</strong> Ecology 61, 93-8.<br />
MILES, J. (1974). Effects <strong>of</strong> experimental <strong>in</strong>terference with stand structure on establishment <strong>of</strong> seedl<strong>in</strong>gs<br />
<strong>in</strong> Callunetum. Journal <strong>of</strong> Ecology 62, 675-87.<br />
MonwIoB A. A. (MOLEANOV, A. A.) (1967). reorpa@ari nnonoHomeHm rnameimx npesecHbrx<br />
nopon B CCCP (Geographical Distribution <strong>of</strong> <strong>the</strong> Fruit-bear<strong>in</strong>g <strong>of</strong> Ma<strong>in</strong> Woodland Trees <strong>in</strong> <strong>the</strong> USSR).<br />
&i3naTenbcTeo cmayKaB (Izdatel’stvo‘ Nauka’), Moscow.<br />
MOONEY, H. A. & DUNN, L. E. (1970). Convergent evolution <strong>of</strong> Mediterranean-climate evergreen<br />
scleropyll shrubs. Evolution, Lancaster, Pennsylvania ~ q, 292-303.<br />
MORGAN, G. H. (1965). A colony <strong>of</strong> Senecio <strong>in</strong>tegrifolius at Burham, Kent, with some notes on <strong>in</strong>vestigations<br />
<strong>of</strong> <strong>the</strong> growth <strong>of</strong> <strong>the</strong> <strong>plant</strong>s. Bullet<strong>in</strong> <strong>of</strong> <strong>the</strong> Kent Field Club 3, 26-49.
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> 141<br />
MORRIS, R. F. (1951). The effects <strong>of</strong> flower<strong>in</strong>g on <strong>the</strong> foliage production and growth <strong>of</strong> balsam fir.<br />
Forestry Chronicle 27, 40-57.<br />
MOSQUIN, T. (1971). Competition for poll<strong>in</strong>ators as a stimulus for <strong>the</strong> evolution <strong>of</strong> flower<strong>in</strong>g time.<br />
Oikos 22,398-402.<br />
MOTT, J. J. (1972). Germ<strong>in</strong>ation studies on some annual <strong>species</strong> from an arid region <strong>of</strong> Western<br />
Australia. Journal <strong>of</strong> Ecology 60, 293-304.<br />
MOTT, J. J. (1974). Factors affect<strong>in</strong>g seed germ<strong>in</strong>ation <strong>in</strong> three annual <strong>species</strong> from an arid region <strong>of</strong><br />
Western Australia. Journal <strong>of</strong> Ecology 62, 699709.<br />
NEWMAN, E. I. (1963). Factors controll<strong>in</strong>g <strong>the</strong> germ<strong>in</strong>ation date <strong>of</strong> w<strong>in</strong>ter annuals. Journal <strong>of</strong> Ecology<br />
51, 625-38.<br />
NEWMAN, E. I. (1964). Factors affect<strong>in</strong>g <strong>the</strong> seed production <strong>of</strong> Teesdalia nudicaulis. I. Germ<strong>in</strong>ation date.<br />
Journal <strong>of</strong> Ecology 52, 391-404.<br />
NEWMAN, E. I. (1965). Factors affect<strong>in</strong>g <strong>the</strong> seed production <strong>of</strong> Teesdalia nudicaulis. 11. Soil moisture<br />
<strong>in</strong> spr<strong>in</strong>g. Journal <strong>of</strong> Ecology 53, 211-32.<br />
NEWMAN, E. I. (1973). Competition and diversity <strong>in</strong> herbaceous vegetation. Nature 244, 310.<br />
NEWMAN, E. I. & ROVIRA, A. D. (1975). Allelopathy among some British grassland <strong>species</strong>. Journal <strong>of</strong><br />
EcologY 63, 727-37.<br />
NICHOLSON, D. I. (1960). Light requirements <strong>of</strong> seedl<strong>in</strong>gs <strong>of</strong> five speices <strong>of</strong> Dipterocarpaceae. Malayan<br />
Forester 23, 344-56.<br />
Om, H. (1965). Flora <strong>of</strong> Japan (English edition edited by F. G. Meyer and E. H. Walker). Smithsonian<br />
Institution, Wash<strong>in</strong>gton D.C.<br />
OINENEN, E. (1967~). Sporal regeneration <strong>of</strong> bracken (Pteridium aquil<strong>in</strong>um (L.) Kuhn.) <strong>in</strong> F<strong>in</strong>land <strong>in</strong> <strong>the</strong><br />
light <strong>of</strong> <strong>the</strong> dimensions and <strong>the</strong> age <strong>of</strong> its clones. Acta Forestalia Fennica 83 (I), 1-96.<br />
OINENEN, E. (19673). The correlation between <strong>the</strong> size <strong>of</strong> F<strong>in</strong>nish braken (Pteridium aquil<strong>in</strong>um (L.)<br />
Kuhn.) clones and certa<strong>in</strong> periods <strong>of</strong> site history. Acta Forestalia Fennica 83 (2), 1-51.<br />
OLATOYE, S. L. & HALL, M. A. (1973). Interaction <strong>of</strong> ethylene and light on dormant weed seeds. In<br />
Seed Ecology (ed. W. Heydecker), pp. 233-49. Butterworths, London.<br />
OSHIMA, Y., KIMURA, M., IWAKI, H. & KUROIWA, S. (1958). Ecological and physiological studies on <strong>the</strong><br />
vegetation <strong>of</strong> Mt. Shimagare. I. Prelim<strong>in</strong>ary survey <strong>of</strong> <strong>the</strong> vegetation <strong>of</strong> Mt. Shimagare. Botanical<br />
Magaz<strong>in</strong>e, Tokyo 71, 289-301.<br />
PARHAM, M. R. (1970). A comparative study <strong>of</strong> <strong>the</strong> m<strong>in</strong>eral nutrition <strong>of</strong> selected halophytes andglycophytes.<br />
Ph.D. <strong>the</strong>sis, University <strong>of</strong> Norwich.<br />
PEMADASA, M. A. & LOVELL, P. H. (1974). Factors controll<strong>in</strong>g <strong>the</strong> flower<strong>in</strong>g time <strong>of</strong> some dune annuals.<br />
Journal <strong>of</strong> Ecology 52, 869-80.<br />
PERTULLA, U. (1941). Untersuchungen iiber die generative und vegetative Vermehrung der Blutenpflanzen<br />
<strong>in</strong> der Wald Ha<strong>in</strong>wiesen und Ha<strong>in</strong>felsenvegetation. Annales Academiae scientiarum Fennicae<br />
58, (I), 1-388.<br />
PIGOTT, C. D. (1968). Biological Flora <strong>of</strong> <strong>the</strong> British Isles: Cirsium acaulon (L.) Scop. Journal <strong>of</strong><br />
Ecology 56, 597612.<br />
PIGOTT, C. D. (1975). Natural regeneration <strong>of</strong> Tilia cordata <strong>in</strong> relation to forest-structure <strong>in</strong> <strong>the</strong> forest <strong>of</strong><br />
Biafowieza, Poland. Philosophical Transactions <strong>of</strong> <strong>the</strong> Royal Society B 270, I 51-79.<br />
PIJL, L. VAN DER (1960). Ecological aspects <strong>of</strong> flower evolution. I. Phyletic evolution. Evolution,<br />
Lancaster, Pennsylvania 14, 403-16.<br />
PIJL, L. VAN DER (1961). Ecological aspects <strong>of</strong> flower evolution. 11. Zoophilous flower classes. Evolution,<br />
Lancaster, Pennsylvania 15, 44-59.<br />
PIJL, L. VAN DER (1971). Pr<strong>in</strong>ciples <strong>of</strong> Dispersal <strong>in</strong> Higher Plants. 2nd ed. Spr<strong>in</strong>ger, Berl<strong>in</strong>.<br />
PLATT, W. J. (1975). The colonization and formation <strong>of</strong> equilibrium <strong>plant</strong> <strong>species</strong> associations on badger<br />
disturbances <strong>in</strong> tall-grass prairie. Ecological Monographs 45,285-305.<br />
POORE, M. E. D. (1967). The concept <strong>of</strong> <strong>the</strong> association <strong>in</strong> tropical ra<strong>in</strong> forest. Journal <strong>of</strong> Ecology 55,<br />
46P-47P.<br />
POORE, M. E. D. (1968). Studies <strong>in</strong> Malaysian ra<strong>in</strong> forest. 1. The forest on triassic sediments <strong>in</strong> Jengka<br />
Forest Reserve. Journal <strong>of</strong> Ecology 56, 143-96.<br />
PUTWAIN, P. D. & HARPER, J. L. (1970). Studies <strong>in</strong> <strong>the</strong> dynamics <strong>of</strong> <strong>plant</strong> populations. 111. The <strong>in</strong>fluence<br />
<strong>of</strong> associated <strong>species</strong> on populations <strong>of</strong> Rumex acetosa L. and R. acetosella L. <strong>in</strong> grassland. Journal <strong>of</strong><br />
Ecology 58, 251-64.<br />
Pa60THOB T. A. (RABOTNOV, T. A.) (1950). XKH3HeHHbIfi IlHKn MHorOneTHHx TpaSHHECTbIX PaCTeHd<br />
B nyrobIx qe~osax (Life cycles <strong>of</strong> perennial herbage <strong>plant</strong>s <strong>in</strong> meadow coenoses). Tpyh<br />
EomanuqecKozo Nncmumyma AKadeMuu Hayrc CCCP (. TruQ . Botanicheskogo <strong>in</strong>stitutu Akademzy nauk<br />
SSSR, series 3) 6, 7-204.<br />
RABOTNOV, T. A. (1969~). Plant regeneration from seed <strong>in</strong> meadows <strong>of</strong> <strong>the</strong> USSR. Herbage Abstracts
P. J. GRUBB<br />
RABOTNOV, T. A. (1969b). On coenopopulations <strong>of</strong> perennial herbaceous <strong>plant</strong>s <strong>in</strong> natural coenoses.<br />
Vegetatio 19, 87-95.<br />
RABOTNOV, T. A. (1974). Differences between fluctuations and seasons. In Vegetation Dynamics (ed<br />
R. Knapp), Part 8 <strong>of</strong> Handbook <strong>of</strong> Vegetation Science, pp. 19-24. Junk, The Hague.<br />
RAMSBOTTOM, J. (1953). Mushrooms and Toadstools. Coll<strong>in</strong>s, London.<br />
RATCLIFFE, D. (1961). Adaptation to habitat <strong>in</strong> a group <strong>of</strong> annual <strong>plant</strong>s.Journa1 <strong>of</strong> Ecology 49, 187-203.<br />
RICHARDS, P. W. (1969). Speciation <strong>in</strong> <strong>the</strong> tropical ra<strong>in</strong> forest and <strong>the</strong> concept <strong>of</strong> <strong>the</strong> niche. Biological<br />
Journal <strong>of</strong> <strong>the</strong> L<strong>in</strong>nean Society I, 149-53.<br />
RICHARDSON, A. E. U., TRUMBLE, H. C. & SHAPTER, R. E. (1932). The <strong>in</strong>fluence <strong>of</strong> growth stage and<br />
frequency <strong>of</strong> cutt<strong>in</strong>g on <strong>the</strong> yield and composition <strong>of</strong> a perennial grass - Phalaris tuberosa. Bullet<strong>in</strong><br />
<strong>of</strong> <strong>the</strong> Council for Scientific and Industrial Research (Australia)(66), 1-33.<br />
ROBBINS, J. S. & DOMINGO, C. E. (1953). Some effects <strong>of</strong> severe soil moisture deficits at specific growth<br />
stages <strong>in</strong> corn. Agronomy Journal 45, 618-21.<br />
ROLLET, B. (1969). La rtgtntration naturelle en for& dense humide sempervirente de pla<strong>in</strong>e de la<br />
Guyane Vtnkzutlienne. Bois et for& des tropiques I-, 19-38.<br />
RUTTER, A. J. (1955). The composition <strong>of</strong> wet heath vegetation <strong>in</strong> relation to <strong>the</strong> water-table. Journal <strong>of</strong><br />
Ecology. 43, 507-43.<br />
SAGAR, G. R. & HARPER, J. L. (1964). Biological Flora <strong>of</strong> <strong>the</strong> British Isles: Phntago mjor L., P. media L.<br />
and P. lanceolata L. Journal <strong>of</strong> Ecology 52, 189-221.<br />
SALISBURY, E. J. (1929). The biological equipment <strong>of</strong> <strong>species</strong> <strong>in</strong> relation to competition. Journal <strong>of</strong><br />
Ecology 17, 197-222.<br />
SALISBURY, E. J. (1941). The Reproductive Capacity <strong>of</strong> Plants. Bell, London.<br />
SALISBURY, SIR EDWARD (1974). Seed size and mass <strong>in</strong> relation to environment. Proceed<strong>in</strong>gs <strong>of</strong> <strong>the</strong> Royal<br />
Society B 186, 83-8.<br />
SARUKHAN, J. (1974). Studies on <strong>plant</strong> demography: Ranunculus repens L., R. bulbosus L. and R. acris L.<br />
11. Reproductive strategies and seed population dynamics. Journal <strong>of</strong> Ecology 62, 151-77-<br />
SARUKHAN, J. & HARPER, J. L. (1973). Studies on <strong>plant</strong> demography: Rammculus repens L., R. bulbosus<br />
L. and R. acris L. I. Population flux and survivorship. Journal <strong>of</strong> Ecology 61, 675-716.<br />
SCHULZ, J. P. (1960). Ecological studies on ra<strong>in</strong> forest <strong>in</strong> nor<strong>the</strong>rn Sur<strong>in</strong>ame. Verhandel<strong>in</strong>gen der<br />
Kon<strong>in</strong>klijke nederlandse akademie van wetenschappen. Afdeel<strong>in</strong>g natuurkunde (sectie 2) (53), 1-67.<br />
SCHWAPPACH, A. (1895). Die Samenproduktion der Wichtigsten Waldholzarten <strong>in</strong> Preussen. Zeitschift<br />
fur Forst- undJagdwesen 27, 14714.<br />
SEEGER, M. (I 913). Die Samenproduktion der Waldbaume <strong>in</strong> Baden. Naturwissenschaftliche Zeitschrift<br />
fur Forst- und Landeuirtschaft 12, 529-54.<br />
SHAMSI, S. R. A. &WHITEHEAD, F. H. (1974). Comparative eco-physiology <strong>of</strong> Epilobium hirsutum L. and<br />
Lythrum salicaria L. 11. Growth and development <strong>in</strong> relation to light. Journal <strong>of</strong> Ecdogy 62, 631-45.<br />
SHANTZ, H. L. & PIEMEISEL, R. L. (1917). Fungus fairy r<strong>in</strong>gs <strong>in</strong> eastern Colorado and <strong>the</strong>ir effect on<br />
vegetation. Journal <strong>of</strong> Agricultural Research IT, 191-245.<br />
SHEIKH, K. H. & RUTTER, A. J. (1969). The responses <strong>of</strong> Mol<strong>in</strong>ia caerulea and Erica tetralix to soil<br />
aeration and related factors. I. Root distribution <strong>in</strong> relation to soil porosity. Journal <strong>of</strong> Ecology 57,<br />
7 I 3-26.<br />
SHELDON, J. C. (1974). The behaviour <strong>of</strong> seeds <strong>in</strong> soil. 111. The <strong>in</strong>fluence <strong>of</strong> seed morphology and <strong>the</strong><br />
behaviour <strong>of</strong> seedl<strong>in</strong>gs on <strong>the</strong> establishment <strong>of</strong> <strong>plant</strong>s from surface-ly<strong>in</strong>g seeds. Journal <strong>of</strong> Ecology<br />
62,47-66.<br />
SKELLAM, J. G. (1951). Random dispersal <strong>in</strong> <strong>the</strong>oretical populations. Biometrika 38, 196-218.<br />
SMITH, F. L. & PRYOR, R. H. (1962). Effect <strong>of</strong> maximum temperature and age on flower<strong>in</strong>g and seed<br />
production <strong>in</strong> three bean varieties. Hilgardia 33, 669-88.<br />
SNOW, D. W. (1965). A possible selective factor <strong>in</strong> <strong>the</strong> evolution <strong>of</strong> fruit<strong>in</strong>g seasons <strong>in</strong> tropical forest.<br />
Oikos 15, 274-81.<br />
SNOW, D. W. (1968). Fruit<strong>in</strong>g seasons and bird breed<strong>in</strong>g seasons <strong>in</strong> <strong>the</strong> New World tropics. Journal <strong>of</strong><br />
Ecolcgy 56, 5P-6P.<br />
SOUGNEZ, N. (1966). Rtactions floristiques d'une lande humide aux fumures m<strong>in</strong>trales. Oecologia<br />
Plantarum I, 219-34.<br />
SPRUGEL, D. G. (1976). Dynamic structure <strong>of</strong> wave-regenerated Abies balsamea forests <strong>in</strong> <strong>the</strong> nor<strong>the</strong>astern<br />
United States. Journal <strong>of</strong> Ecology 64, 889-911.<br />
STEBBINS, G. L. (1951). Natural selection and <strong>the</strong> differentiation <strong>of</strong> angiosperm families. Evolution,<br />
Lancaster, Pennsylvania 5, 299-323.<br />
STEBBINS, G. L. (1970). Adaptive radiation <strong>of</strong> reproductive characteristics <strong>in</strong> angiosperms. I. Poll<strong>in</strong>ation<br />
mechanisms. Annual Review <strong>of</strong> Ecology and Systematics I, 307-26.<br />
STEBBINS, G. L. (1971). Adaptive radiation <strong>of</strong> reproductive characteristics <strong>of</strong> angiosperms. 11. Seeds and<br />
seedl<strong>in</strong>gs. Amml Rm'ew <strong>of</strong> Ecology and Systematics 2, 237-60.
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong> '43<br />
STEENIS, C. G. G. J. VAN (1956). Basic pr<strong>in</strong>ciples <strong>of</strong> ra<strong>in</strong>-forest sociology. Proceed<strong>in</strong>gs <strong>of</strong> <strong>the</strong> Kandy<br />
Symposium on Study <strong>of</strong> Tropical Vegetation, pp. 159-63. UNESCO, Paris.<br />
STEWART, D. R. M. & STEWART, J. (1971). Comparative food preferences <strong>of</strong> five East African ungulates<br />
at different seasons. In The Scientific Management <strong>of</strong> Animal and Plant Communities for Consemation<br />
(ed. E. DutTey and A. S. Watt). Symposia <strong>of</strong> <strong>the</strong> British Ecologicat Society XI, 351-66.<br />
STILES, F. G. (1975). Ecology, flower<strong>in</strong>g phenology, and humm<strong>in</strong>gbird poll<strong>in</strong>ation <strong>of</strong> some Costa =can<br />
Heliconha <strong>species</strong>. Ecology 56, 285-301.<br />
SUMMERHAYES, V. S. (1941). The effect <strong>of</strong> voles (Microtus agrestis) on vegetation. Journal <strong>of</strong> Ecology 29,<br />
14-48.<br />
TAGAWA, H. (1969). A prelim<strong>in</strong>ary <strong>in</strong>vestigation <strong>of</strong> seed fall <strong>in</strong> an evergreen broadleaf forest <strong>of</strong> M<strong>in</strong>amata<br />
Special Research Area <strong>of</strong> IBP. Scientijk Reports, Kagoshima University 18, 27-41.<br />
TAGAWA, H. (1971). Studies on <strong>the</strong> biological production <strong>of</strong> laurel-leaved forest. Japanese IBP-PT<br />
Annunl Report pp. 39-60.<br />
TAMM, C. 0. (1956). Fur<strong>the</strong>r observations on <strong>the</strong> survival and flower<strong>in</strong>g <strong>of</strong> some perennial herbs: I.<br />
Oikos 7,273-92.<br />
TAMM, C. 0. (1972~). Survival and flower<strong>in</strong>g <strong>of</strong> some perennial herbs. 11. The behaviour <strong>of</strong> some<br />
orchids on permanent plots. Oikos 23, 23-8.<br />
TAMM, C. 0. (1g72b). Survival and flower<strong>in</strong>g <strong>of</strong> perennial herbs. 111. The behaviour <strong>of</strong> Prim& om's<br />
on permanent plots. Oikos 23, 159-66.<br />
TANNER, E. V. J. (1977). Four montane forests <strong>of</strong> Jamaica: a quantitative characterization <strong>of</strong> <strong>the</strong><br />
floristics and <strong>the</strong> soils and a discussion <strong>of</strong> <strong>the</strong> <strong>in</strong>terrelations. Journal <strong>of</strong> Ecology 65, (<strong>in</strong> <strong>the</strong> press).<br />
TEVIS, L. (1958). Interrelations between <strong>the</strong> harvester ant Veromessorpergandei (Mayr) and some desert<br />
ephemerals. Ecology 39, 695704.<br />
THURSTON, J. M. (1951). Some experiments and field observations on <strong>the</strong> germ<strong>in</strong>ation <strong>of</strong> wild oat<br />
(Aoena fatua and A. luakiciana) seeds <strong>in</strong> soil and <strong>the</strong> emergence <strong>of</strong> seedl<strong>in</strong>gs. Annals <strong>of</strong> Applied<br />
Biology 38, 812-32.<br />
THURSTON, J. M. (1969). The effect <strong>of</strong> lim<strong>in</strong>g and fertilizers on <strong>the</strong> botanical composition <strong>of</strong> permanent<br />
grassland, and on <strong>the</strong> yield <strong>of</strong> hay. In Ecological Aspects <strong>of</strong> <strong>the</strong> M<strong>in</strong>eral Nutrition <strong>of</strong> Plants (ed. I. H.<br />
Rorison). Symposia <strong>of</strong> <strong>the</strong> British Ecological Society 9, 3-10.<br />
TOUMEY, J. W. & KORSTIAN, C. F. (1937). Foundations <strong>of</strong> Silviculture upon an Ecological Basis, 2nd ed.<br />
Wiley, New York.<br />
TRABAUD, L. (1970). Quelques valeurs et observations sur la phyto-dynamique des surfaces <strong>in</strong>cendibes<br />
dans le Bas-Languedoc. Naturalia monrpeliensia, skie Botanique 21, 231-42.<br />
TUKEY, L. D. (1952). Effect <strong>of</strong> night temperature on <strong>the</strong> growth <strong>of</strong> <strong>the</strong> fruit <strong>of</strong> <strong>the</strong> sour cherry. Botanical<br />
Gazette 114, 155-65.<br />
TUKEY, L. D. (1955). Some effects <strong>of</strong> night temperatures on <strong>the</strong> growth <strong>of</strong>McIntosh apples. I. Proceed<strong>in</strong>gs<br />
<strong>of</strong> <strong>the</strong> American Societv - <strong>of</strong> - Horticultural Science 68, 32-43.<br />
Tub~, L. D. (1958). Effects <strong>of</strong> controlled tempera&& foilow<strong>in</strong>g bloom on berry development <strong>of</strong> <strong>the</strong><br />
Concord Grape (Vitis labrusca). Proceed<strong>in</strong>gs <strong>of</strong> <strong>the</strong> American Society <strong>of</strong> Horticultural Science 71,157-66.<br />
VILLIERS, T. A. (1972). Environmental control <strong>of</strong> seed germ<strong>in</strong>ation. In Seed Biology, vol. 2 (ed. T. T.<br />
Kozlowski), pp. 220-81. Academic Press, New York.<br />
WADE, L. K. & MCVEAN, D. N. (1969). Mt Wilhelm Studies. I. Department <strong>of</strong> Biogeography and<br />
Geomorphology Publication (BG/I), i-wi, 1-225. Australian National University, Canberra.<br />
WALTER, H. (1968). Die Vegetation der Erde <strong>in</strong> Gkophysiologkcher Betrachtung, vol. 2. Fischer, Jena.<br />
WALTER, H. (1971). Ecology <strong>of</strong> Tropical and Subtropical Vegetation. Oliver & Boyd, Ed<strong>in</strong>burgh.<br />
WANG. C.-W. (1961). The Forests <strong>of</strong> Ch<strong>in</strong>a. Maria Moors Cabot Foundation (Cambridge, Massachusetts)<br />
Publication (i5j, 1-313.<br />
WARDLE, J. (1970). The ecology <strong>of</strong> Noth<strong>of</strong>agus solandri. 3. Regeneration. New ZealandJoumal <strong>of</strong> Botany<br />
8, 571-608.<br />
WARDLE, P. (1959). The regeneration <strong>of</strong> Frax<strong>in</strong>us excelswr <strong>in</strong> woods with a field layer <strong>of</strong> Mercurialis<br />
perennis. Journal <strong>of</strong> Ecology 47.483-97.<br />
WATT, A. S. (1919). On <strong>the</strong> causes <strong>of</strong> failure <strong>of</strong> natural regeneration <strong>in</strong> British oakwoods. Journal <strong>of</strong><br />
Ecology 7, 173-203.<br />
WATT. A. S. (1923). On <strong>the</strong> ecology <strong>of</strong> British beechwoods with special reference to <strong>the</strong>ir regeneration.<br />
Journal <strong>of</strong> Ecology XI, 1-48.<br />
WATT, A. S. (1926). Yew communitites <strong>of</strong> <strong>the</strong> South Downs. Jountal <strong>of</strong> Ecology 14, 282316.<br />
WATT, A. S. (1934). The vegetation <strong>of</strong> <strong>the</strong> Chiltern Hills, with special reference to <strong>the</strong> beechwoods and<br />
<strong>the</strong>ir sera1 relationships. Journal <strong>of</strong> Ecology 22, 23~70,<br />
45-507.<br />
WATT, A. S. (1947). Pattern and process <strong>in</strong> <strong>the</strong> <strong>plant</strong> community. Journal <strong>of</strong> Ecology 35, 1-22.<br />
WATT, A. S. (1955). Bracken versus hea<strong>the</strong>r: a study <strong>in</strong> <strong>plant</strong> sociology. Journal <strong>of</strong> Ecology 43,490-506.<br />
WATT, A. S. (1957). The effect <strong>of</strong> exclud<strong>in</strong>g rabbits from Grassland B (Mesobrometum) <strong>in</strong> Breckland.<br />
Journal <strong>of</strong> Ecology 45, 86178.
I44<br />
P. J. GRUBB<br />
WATT,A. S. (1971). Factors controll<strong>in</strong>g <strong>the</strong> floristic composition <strong>of</strong> some <strong>plant</strong> <strong>communities</strong> <strong>in</strong> Breckland.<br />
In The Scientific Management <strong>of</strong> Animal and Plant Communities for Conservation (ed. E. Duffey and<br />
A. S. Watt). Symposia <strong>of</strong> <strong>the</strong> British Ecological Society XI, 137-52.<br />
WATT, A. S. (1974). Senescence and rejuvenation <strong>in</strong> ungrazed chalk grassland (Grassland B) <strong>in</strong><br />
Breckland: <strong>the</strong> significance <strong>of</strong> litter and <strong>of</strong> moles. Journal <strong>of</strong> Applied Ecology 11, I 157-71.<br />
WATT, A. S. & FRASER, G. K. (1933). Tree roots and <strong>the</strong> field layer. Journal <strong>of</strong> Ecology 21, 404-14.<br />
WEATHERELL, J. (1957). The use <strong>of</strong> nurse <strong>species</strong> <strong>in</strong> <strong>the</strong> afforestation <strong>of</strong> upland heaths. QwrterZy Journal<br />
<strong>of</strong> Forestry 51, 298-304.<br />
WEBB, L. J. (1958). Cyclones as an ecological factor <strong>in</strong> tropical lowland ra<strong>in</strong>-forest, North Queensland.<br />
Australian Journal <strong>of</strong> Botany 6, 220-8.<br />
WEBB, L. J., TRACEY, J. G. & HAYDOCK, K. P. (1967). A factor toxic to seedl<strong>in</strong>gs <strong>of</strong> <strong>the</strong> same <strong>species</strong><br />
associated with liv<strong>in</strong>g roots <strong>of</strong> <strong>the</strong> non-gregarious subtropical ra<strong>in</strong> forest tree Grmillea robusta. Journal<br />
<strong>of</strong> Applied Ecology 4, 13-25.<br />
WEBB, L. J., TRACEY, J. G. & WILLIAMS, W. T. (1972). Regeneration and pattern <strong>in</strong> <strong>the</strong> subtropical ra<strong>in</strong><br />
forest. Journal <strong>of</strong> Ecology 60, 675-95.<br />
WEBB, L. J., TRACEY, J. G., WILLIAMS, W. T. & LANCE, G. N. (1970). Studies <strong>in</strong> <strong>the</strong> numerical analysis<br />
<strong>of</strong> complex ra<strong>in</strong>-forest <strong>communities</strong>. V. A comparison <strong>of</strong> <strong>the</strong> properties <strong>of</strong> floristic and physiognomicstructural<br />
data. Journal <strong>of</strong> Ecology 58, 203-32.<br />
WELBANK, P. J. (1963). A comparison <strong>of</strong> competitive effects <strong>of</strong> some common weed <strong>species</strong>. Annals <strong>of</strong><br />
Applied Biology 51, 107-25.<br />
WENT, F. W. (1949). Ecology <strong>of</strong> desert <strong>plant</strong>s. 11. The effect <strong>of</strong> ra<strong>in</strong> and temperature on germ<strong>in</strong>ation and<br />
growth. Ecology 30, 1-13.<br />
WENT, F. W. (1957). Environmental control <strong>of</strong> <strong>plant</strong> growth. Chronica Botanica (17), i-xvii, 1-343.<br />
WESSON, G. & WAREING, P. F. (1969). The <strong>in</strong>duction <strong>of</strong> light sensitivity <strong>in</strong> weed seeds by burial. Journal<br />
<strong>of</strong> Exberimental Botany 20, 414-25.<br />
WESTHOFF, V. (1971). The dynamic structure <strong>of</strong> <strong>plant</strong> <strong>communities</strong> <strong>in</strong> relation to <strong>the</strong> objectives <strong>of</strong><br />
conservation. In The ScientiJic Management <strong>of</strong> Animal and Plant Communities for Conservation (ed.<br />
E. Duffey and A. S. Watt). Symposia <strong>of</strong> <strong>the</strong> British Ecological Society XI, 3-14.<br />
WHITE, J. H. & FRENCH, N. (1968). Lea<strong>the</strong>rjacket damage to grassland. Journal <strong>of</strong> <strong>the</strong> British Grassland<br />
Society 23, 326-9.<br />
WHITEHEAD, F. H. (1971). Comparative autecology as a guide to <strong>plant</strong> distribution. In T/ze Scientific<br />
Management <strong>of</strong> Animal and Plant Communities for Conservation (ed. E. Duffey and A. S. Watt).<br />
Symposia <strong>of</strong> <strong>the</strong> British Ecological Society XI, 16776.<br />
WHITFORD, H. N. (1949). Distribution <strong>of</strong> woodland <strong>plant</strong>s <strong>in</strong> relation to succession and clonal growth.<br />
Ecology 30, 199-208.<br />
WHITMORE, T. C. (1974). Change with time and <strong>the</strong> role <strong>of</strong> cyclones <strong>in</strong> tropical ra<strong>in</strong> forest on Kolombangara,<br />
Solomon Islands. Institute Paper, Commonwealth Forestry Institute, Oxford (46), 1-78.<br />
WHITMORE, T. C. (1975). Tropical Ra<strong>in</strong> Forests <strong>of</strong> <strong>the</strong> Far East. Clarendon Press, Oxford.<br />
WHITTAKER, R. H. (1965). Dom<strong>in</strong>ance and diversity <strong>in</strong> land <strong>plant</strong> <strong>communities</strong>. Science 147, 25-60.<br />
WHITTAKER, R. H. (1967). Gradient analysis <strong>of</strong> vegetation. Biological Rezkus 49, 207-64.<br />
WHITTAKER, R. H. (1969). Evolution <strong>of</strong> diversity <strong>in</strong> <strong>plant</strong> <strong>communities</strong>. Diversity and Stability <strong>in</strong><br />
Ecological Systems. Brookhaven Symposia <strong>in</strong> Biology 22, 178-95.<br />
WHITTAKER, R. H. (1972). Evolution and measurement <strong>of</strong> <strong>species</strong> diversity. Taxon 21, 213-51.<br />
WHITTAKER, R. H. (1975). Communities and Ecosystems, 2nd. ed. Macmillan, New York.<br />
WHITTAKER, R. H., LEVIN, S. A. &ROOT, R. B. (1973). Niche, habitat and ecotope. American Naturalist<br />
107, 321-38.<br />
WILLIAMS, J. T. (1969). Biological Flora <strong>of</strong> <strong>the</strong> British Isles: Chenopodium rubrum L. Journal <strong>of</strong> Ecology<br />
57,831-41.<br />
WILLIAMS, J. T. & HARPER, J. L. (1965). Seed polymorphism and germ<strong>in</strong>ation. I. The <strong>in</strong>fluence <strong>of</strong><br />
nitrates and low temperatures on <strong>the</strong> germ<strong>in</strong>ation <strong>of</strong> Chenopodium album. Weed Research 5,<br />
141-50.<br />
WILLIAMSON, G. B. (1975). Pattern and sera1 composition <strong>in</strong> an old-growth beech-maple forest.<br />
Ecology 56, 727-3 I.<br />
WILLIS, A. J. (1963). Braunton Burrows: <strong>the</strong> effects on <strong>the</strong> vegetation <strong>of</strong> <strong>the</strong> addition <strong>of</strong> m<strong>in</strong>eral<br />
nutrients to <strong>the</strong> dune soils. Journal <strong>of</strong> Ecology 51, 353-74.<br />
WILSON, J. Y. (1959). Vegetative reproduction <strong>in</strong> <strong>the</strong> bluebell, Endymion nonscriptus (L.) Garcke.<br />
Garcke. New Phytologist 58, 155-63.<br />
WINKWORTH, R. E. (1967). The composition <strong>of</strong> several arid sp<strong>in</strong>ifex grasslands <strong>of</strong> central Australia <strong>in</strong><br />
relation to ra<strong>in</strong>fall, soil water relations and nutrients. Australian Journal <strong>of</strong> Botany 15, 107-30.<br />
WIT, C. T. DE (1960). On competition. Verslagen van landbouwkundige onderzoek<strong>in</strong>gen, The Hague<br />
(66.8), i-iv, 1-82.
Ma<strong>in</strong>tenance <strong>of</strong> <strong>species</strong>-<strong>richness</strong> <strong>in</strong> <strong>plant</strong> <strong>communities</strong><br />
WIT, C. T. DE (1961). Space relationships with<strong>in</strong> populations <strong>of</strong> one or more <strong>species</strong>. In Mechanisms <strong>in</strong><br />
Biological Competith (ed. F. L. Milthorpe). Symposia <strong>of</strong> <strong>the</strong> Society <strong>of</strong> Experimental Biology 15,<br />
314-29.<br />
WUENSCHER, J. E. (1969). Niche specification and competition model<strong>in</strong>g. Journal <strong>of</strong> Theoretical Biology<br />
25,436-43.<br />
WUENSCHER, J .E. (1974). The ecological niche and vegetation dynamics. In Vegetation and Environment<br />
(ed. B. R. Stra<strong>in</strong> and W. D. Bill<strong>in</strong>gs), Part 6 <strong>of</strong> Handbook <strong>of</strong> Vegetation Science, pp. 39-45. Junk,<br />
The Hague.<br />
YARRANTON, G. A. & MORRISON, R. G. (1974). Spatial dynamics <strong>of</strong> a primary succession: nucleation.<br />
Journal <strong>of</strong> Ecology 62, 417-28.<br />
I45