Lichen communities in the British Isles: A preliminary conspectus

388 P. W. James, D. L. Hazlhszlorth and F. Rose
Species
Arthonia lohata aggr.
Buellia leptoclinoides
Chiodecton petraeum
Lecanactis monslrosa.
Lecanora atra
L. gansaleoides
L. praeposlera
L. tenera
Lecidea orosthea
Pertusaria fla.oicans
Ramalina siliquosa
Roccella fuciformis
I?. phycopsis
S c lerophy tor t cir cuntscriptunt
Tanrr XV.
Sclerophytetum circumscriplae ass. nov.
1.-
2.2
+.1
1.-
2.1
+
1.7
J.J
Stands
+ ))
+ J.J
1.2
f,1
1.+
2.7
2.2
1' Isles of scilly, st Mary: sheltered dry acid rock. incl. 90o, aspect 315., 0.5 x 0.5
m, 1968, P.W.J.
2. Channel Islands, Al9":l:I, La Roche peninsula: acid rock, aspect 45., 1 X 2
m, cover 600/6, April, 1975, p.W.J. ; type yecord.
Roccella fuc{ormis and, R. phycopsis are present as far north as the islancl of
skomer, Pembrokeshire, whereas Buehia reptocrinoides, chiodecton myrticola
and c. pett'aeum are restricted to south-west England and the channel
Islands. Arthonia atlantica is confined to western Ireland. Further notes
on the ranges of many of the species in this association within the British
Isles are included in James (1970).
It is of interest to note that in south-west Brittany, northern spain and
Portugal, this association is not confined to crevicls but is also able to
extend on to exposed rock faces.
V err uc arietum maur ae DF..
soensh. z.tcixtsociol. stiltsh. Haydl.. 2, 51 (rgzs) las ,,verrucaria maura-Ass.,,).verrucaria
maura-I'ichina confinis-Associatio''DR., Beih. bot. cbl. 49, gl (1932)
Thc verrucarietum maurae is the association of the littoral (brack) zone on
all siliceous rocky coasts of Britain. The 15 species which .o-pri." it ur"
more or less regularly inundated by sea-water on shertered shtres or lie
10. Lichen Communities in the British Isles 389
within the splash zone on more exposed coasts; only I'ichina pygmaea is
regularly submergcd by thc tide. Althoush considerable patterns of
zonatiorr may occur within the Verrucarietum maurae thcse are often
obscurcd due to variations in shoreline topographv and the density of
algal cover. This results in the upper and lower elements of this association
becoming spatially close as on respectively drier exposcd and rvctter, seaweed
sheltered surfaces in juxtaposition. The Verrucarietu.m maurae is
dominated by species of Verrucaria thc identification of which still presents
considcrable problems (Fletcher, 1975a). Verrucaria maura generally
delimits the upper part of the zone and extends into thc Caloplacetum
marinae; on some shores this species is associated with V. ampltibia.'fhe
maximum immersion time for any species in the association is that of
52o/o of one year recorded for V. striatula andV. mucosa (Fletcher, 1973a);
these two species constitute the lowest ertension of the zone. Intermediate
spccies are Arthopyrenia halodTttes, Arthopyrenia sp., Verrucart:.a degelii
(northern Scotland only), V. ditmatsica, V. erichsenii, V. microspora atrct.
and V. sandstedei. Arthopyrenia halodytes exhibits a very wide range of
substrate tolerance, occurring on acid or limestone rocks, on a r,vide range of
shells of molluscs and on soft chalk (as in south-east England).
F. Aquatic Cornrnunities
The lichen flora of siliceous rocks in, or at the margins of, lakes and strearns
includes a small but distinctive assemblage of species, many of w-hich are
almost exclusive to the habitat. The horizontal zonation patterns formed
by particular lichen associations lvithin or below the splash zone are
comparable with those occurring on coastal rocks (see Santesson, 1939;
Wirth, 1972). Recently the zonation formcd has proved of value even in
the determination of river channel capacity (Gregory, 1976). l'he ccology
and phytosociology of the freshwater communities in the British Isles have
received little study although the taxonomy of the aquatic species of
Verrucaria and Polyblastia, two of the most difficult genera represcnted,
has been critically reviewed by Swinscow (1968, 1971). According to the
information available there appear to be at lcast lour associations which
merit consideration. As with the case of the marine and maritime lichen
communities, no attempt has been made herc' to assign the associations of
aquatic habitats to alliances.
Some of the ecological factors which account for the delimitecl distribution
of aquatic lichens have been discussed by Ried (1960a,b). Probably
the frequency and sum extent of submersion are t\4-o important controlling
factors for thc survival of most species; only Verrucat'ia aquatilis, V.