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260 KARL-GORAN BRINGERFig. 1. Dryadion: richly floweringDryas octopetala on Mt. Daunatjakkonear Fattjaur, Vilhelmina,Asele Lappmark. 1947. Photo 0.Rune.a wide ecological amplitude and are also of greatimportance in Dryadion, e.g. Empetrum hermaphroditum,V accinium uliginosum, Arctostaphylosalpina and Betula nana while others normally areabsent, as Liycopodium alpinum, Deschampsiaflexuosa, Oarex brunnescens, Vaccinium myrtillus,Oalluna vulgaris and the Hieracium alpinum group.Other species, e.g. Lycopodium selago, Juncus trifidus,Agrostis borealis, Loiseleuria procumbens andPhyllodoce coerulea, appear in transitional vegetationtypes but do not occur in the best developedDryas heaths on limestone and dolomite, wherethe soil has a pH value at or above 7.Dryadion has a great number of good differentialspecies, absent in the poor heath alliances. Someof them such as Oar ex atrata, 0. rupestris, K obresiamyosuroides, Salix reticulata, Silene acaulis, Dryasoctopetala and Saxifraga oppositifolia are regularlyfound in Dryas stands all through the SwedishScandes. Other species have a more restricteddistribution, as Potentilla nivea, Draba nivalis andOxytropis lapponica which have one northern andone southern Swedish area, or Oarex nardina,Rhododendron lapponicum, Euphrasia lapponica,Campanula uniflora and Arnica alpina, which inSweden are restricted to the northern part of themountain range.On wind-swept hills of limestone or calcareousschists with no or very scanty snow cover in winteroccurs a vegetation of Dryas tufts heavily erodedand affected by deflation and nearly always intermingledwith Oarex rupestris. On the bare soilbetween the Dryas stands a few plants resistantto severe cold and drought but with a weak powerof competition may be found, such as Oarex glacialis,Draba spp. and Euphrasia lapponica. Theseemingly stronger Carex nardina and Arnica alpinaalso grow in such places. Among the cryptogamsa number of crustaceous lichens of the generaCaloplaca, Rinodina, Ochrolechia, Lecidea andPertusaria grow on earth or litter and are verycharacteristic of this type of Dryas stands. Manylichens and bryophytes of Dryadion, particularlyspecies from the wind-exposed, poorly snow-protectedexamples of the alliance, are also found inlimestone areas in southern Sweden, especially inthe alvar vegetation of the Baltic islands blandand Gotland. Examples are Ditrichum flexicaule,Rhytidium rugosum, Tortella tortuosa, Cetraria juniperinav. terrestris, Thamnolia vermicularis, Oladoniasymphycarpia and Toninia coeruleonigricans.With decreasing exposure the areas of bare soildisappear and the vegetation becomes better closed.Together with the earlier mentioned dwarf shrubs,Rhododendron lapponicum and Oassiope tetragonanow are important dominants, but occurring inSweden only in northern Lappland. With a bettersnow protection and greater humidity of the soil.Acta Phytogeog1·. Suec. 50