Lichen communities in the British Isles: A preliminary conspectus

326 P. W. James, D. L. Hawksworth and F. Rose
away from the more humid western and southern counties, the Lobarion
tends to be relatively species poor. Often only Lobaria pulmonaria (and
sometimes L. scrobiculafa) represent that genus, Sticta species are usually
absent, and the cyanophilous species may be represented only by Parmeliella
plumbea (or Pannaria rubiginosa in north-east Scotland, e.g. Darnarvay
Forest). The same pattern of impoverishment appears to occur in this
alliance in the forests of north, central and eastern Europe (e.g. Denmark,
Poland).
Conversely, in south-west England, North Wales and, more particularly,
western Scotland, Brittany and the French Pyrenees, additional species
absent further east become locally common (e.g. Lopadium pezizoides,
Parmeliella atlantica, Pseudocyphellaria crocata, P. intricata, Sticta
canar;ensis (5. dufourii morphotype\, S. fuliginosa; in a few areas of western
and southern England and Wales, also Rinodina isidioides). An especially
species-rich facies of this alliance occurs in moist valley-bottom woods in
sheltered lowland situations in western Scotland, particularly on inclined
boughs of old Corylus or Salix atrocinerea in wet carr woodland; this
includes fewer crustose lichens but all four British Lobaria species (all
fertile in western Scotland; all but L. laeteoirens rarely so elsewhere),
Pannaria, Parmeliella and Pseudocyphellaria species and, occasionally, also
Heterodermia obscurata. In such constantly wet conditions Cetreliq
oliztetorum s.1., Menegazzia terebrata, Parmelia endochlora, P. laez,tigata
and P. taylorensis (all species usually found only in the Parmelion laeztigatae,
see p. 327) may also occur, together u,ith Leptogium species (particularly
L. burgessii), Parmelia sinuosa (on twigs) and Ulota species. This nodum,
which is very distinctive when well developed, shows affinities with both
the Parmelion laevigatae (although on far less acid bark) and, through
Lecanora jamesii communities, to the Graphidion scriptae (generally of
smooth-barked twigs and small branches). In such sites in western
Scotland and south-west Ireland, small twigs and branches often support
Graphidion with Lecanactis homalotropum and Thelotrema subtile (see p.
31s).
A euoceanic moss-dominated facies of the Lobareoz in south-west Ireland
characteristically includes Leptogium brebissonii, L. burgessii, Lobaria
pulmonaria, Porina hibernica, Pseudocyphellaria lacerata and Sticta
canariensis (the S. dufourii morphotype occurring in deep shade).
In western Ireland (particularly in the Burren, Co. Clare; e.g. Poulavallan,
Den of Clab) a facies of the Lobarion occurs on Corylus in relict
Corylus woodlands of the limestone pavements. In this community
Lobaria species are now lacking (currently restricted to mixed Quercus
forest areas in Ireland) but Pannaria, Parmeliella and Sticta species are
abundant and Leptogium burgessii, Normandina pulchella. and (Jsnea
10. Lichen Communitt)es in the Ilritish Isles 327
specics occur on larger stems. Thelotrema subtile is present in these communities
in Connemara and south-west Ireland.
Fraxinus (Jlmus forest, especially when on basic soils over limestone,
basic volcanic rocks, or on rich alluvial soils, supports a further distinctive
community of the Lobarion in which, in addition to the taxa marked "
in Table II, there is a particularly rich development of species of Pannaria
(including P. sampaiana), Parmeliella, Collema (C. fasciculare, C. furfuraceum,
C. nigricans and C. subflaccidum) and Leptogium (L. azureum, L.
burgessii, L. cyanescens, L. hibernicum and L. saturninum); Lithographa
dendrographa may also belong here. This commr,rnity is now well developed
only in a few areas of western Scotland where it occurs both in open forest
and on isolated well lit old trees (e.g. Loch Melfort; Ellary Woods, Loch
Caolisport; Rassal Wood, Kishorn; Mull; Sunart; Loch Arkaig); fragments
do, however, persist in southern (e.g. New Forest, Cranborne
Chase) and r,r,estern England, Wales, the Lake District (e.g. Gowbarrorv)
and parts of western Ireland. Also, as members of this community thcre
may be the very rare British species Arctomia delicatula (euoceanic),
Collemtt occultatum and Pannaria ignohilis (a valley species of the central
Flighlands, often found on trees which have Bryoria capil.laris on their
twigs).
All. 7. Parrnelion laeoigatae
Parmelian laedgatae all, nov.; type: Parmelietum laeoigatae P, James et al.
(monotype).
This new alliance contains a single association:
Parmehletum laeztigatae ass. nov. (Table III, Fig. 4)
This is a well marked community characteristic and general on the westcrn
side of the British Isles, including parts of south-west Ireland, and is also
present in western Brittany. It is largely restricted to Betula ard Quercus
(although it can be encountered on Alnus and mossy rocks) ancl confinecl to
exposed, often upland, woodland sites q'ith high rainfall (127-229 cm
year 1) and with at least 180 wet days year-l. The only sites for this
community outside the 180 wet-day isoline (see Ratcliffe, 1968; Coppins,
7976) are in north Pembrokeshire but, were more meteorological clzrta
available, we suspect they also would be within this isoline. 'l'he Parmelietum
laeoigatae is esscntially an association of well lit hardwoocl forests, and
appears to be correlated with exposure to heavy rain which leaches thc
upper horizons of the bark. The pH of bark supporting this community is