PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest

vation that species such as Brewer spruc e
(Picea breweriana), Port-Orford-cedar
(Chamaecyparis lawsoniana), vine maple
(Ater circinatum), and rhododendron
(Rhododendron macrophyllum) are restricte d
to areas with low potential transpiration . Th e
most valuable index, however, was the rati o
of actual to potential transpiration (T/PT)
Which integrates for water, the demand, th e
supply, and the control b the plant . Where
sot moisture was never inadequate and th e
evaporative stress remained low, the ratio wa s
1 .0. Where water became limiting and hig h
evaporative stress was common, ratios of 0 . 3
were calculated . Under these conditions, both
forest composition and growth were dramatically
affected .
Measurement and Interpretation of Soi l
Fertility and Plant Nutritio n
Soil profiles from each stand were reconstructed
to a depth of 60 cm and taken into
controlled environment chambers where seedlings
of Douglas-fir and Shasta red fir were
grown for a period of 5 months (Waring an d
Youngberg 1972) . The dry weight yields at
the end of the experiment were used as a bio -
assay of soil fertility, representing the supply
of nutrients available to conifers . Foliar analyses
were made on reference trees, first durin g
the time of maximum demand when ne w
foliage was being produced and again after al l
shoot and diameter growth had ceased in th e
fall. In the first period, 1-year-old foliage wa s
assessed because it represents a major sourc e
of mobile nitrogen, phosphorus, and potassium.
Three categories of nutrient availability
can be identified : (1) where no nutritional
stress occurs during the year ; (2) where nutrition
is adequate only when shoot and
diameter growth has ceased ; and (3) where
nutrition is inadequate year around (Warin g
and Youngberg 1972) . These three categorie s
may be further refined and are of consider -
able value in reaching decisions concernin g
fertilization. The composition of vegetatio n
is, however, insensitive to this classification of
nutrient availability .
Only on soils where an imbalance of nutrients
or toxic amounts of certain heavy metals
were present did special plant communitie s
develop. In this paper, therefore, we hav e
assigned plants to one of three categories of
tolerance to infertile soils developed from
ultrabasic parent materials : In class 1 ar e
those which are tolerant ; class 2 is made up of
those species that are intolerant ; and in class 3
are those plants both tolerant and competitively
restricted to ultrabasic soils .
Vegetation as an Index to Environmen t
Can we use plants to define their environment,
or more precisely, the environment expressed
through reference plants? To test thi s
idea, we selected 47 species from more than
600 in the local flora and recorded thei r
distributional limits in relation to various
environmental plant indices (table 2) . For
some plants we knew only the approximat e
ranges and occasionally we had insufficien t
data to set particular index limits .
In forest ecosystems where plant composition
is known, the range of environmenta l
indices may be predicted from information i n
table 2 . In table 3, the plant response indice s
were thus predicted for the 25 stand s
described in table 1 . Therefore for the blac k
oak forest, stand 3, where Rhus diversiloba ,
Arbutus menziesii, Lonicera hispidula, and
Quercus chrysolepis grew, one can assess fro m
table 2 that the Temperature Growth Inde x
lies between 98 and 96, the Plant Moistur e
Stress Index at 25 .4, and the ratio of transpiration
to potential transpiration as 0 .29 .
Plants that were exclusively adapted t o
ultrabasic soils, such as Jeffrey pine, isolated
those ecosystems (stands 4, 5, and 25) . Those
forests with oak (stands 3, 8, and 21 )
exhibited the greatest water stress, th e
warmest environments, and the greatest control
of transpiration . At the other extreme ,
mountain hemlock forests (stands 6, 18, 15 ,
and 24) had the coolest environments and
lowest potential transpiration .
In figure 5, the midpoint of the predicted
Temperature Growth Index from table 3 is
plotted against the Temperature Growt h
Index calculated from temperature records .
The regression has an r2 of 0 .93, and is highl y
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