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low-alpine belt most of the other species of thealliance show a rather weak power of competitionin relation to the larger dwarf shrubs and are mainlyconfined to the areas of bare soil, e.g. Lycopodiumselago, Festuca ovina, Carex Bigelowii, Juncus trifidus,Luzula arcuata (incl. L. confusa) and L. spicata.The Empetrion alliance thus consists of a rathersmall number of vascular plants, all of which arecharacterized by hardiness against unfavourablewinter conditions and tolerance towards an acidsoil reaction. They are therefore widely spread inthe mountain range, and the field layer of thealliance probably shows rather small variation.s indifferent parts of the Swedish Scandes. In ' the_Chionophobous plant communities 259The bottom layer in the Diapensia-Loiseleuriacommunities of the most wind-exposed hilltops isoften composed of the small hepatics Gymnomitriumcorallioides and Prasanthus suecicus. Wind-hardylichens as Alectoria ochroleuca, A. nigricans, A.divergens, Cetraria nivalis, C. cucullata, Corniculariaaculeata and Thamnolia vernicularis are of greatmportance. At less wind exposure and better snowprotection, near the transition between Empetrionand Myrtillion, the Alectoria and Cetraria speciesare reduced in quantity and Cladonia species suchas C. silvatica (agg.), C. rangiferina, C. alpestrist and C. uncialis are increasing. Alpine heaths trulyrich in lichens, with Cetraria nivalis and Cladoniaalpestris as most important dominants, are in Swe­den best devloped in the northernmost and amountains of north-western Dalarna Diapensia_ ·lapponica is lacking but Arctostaphylos uva-ursi isthere a rather important component of the alpine' ·· part of the southernmost sectors of the mountainheaths. In parts of Lappland Calamagrostis lappo- area; where a comparatively continental climatenica and Hierochloe alpina play an important role. favours the lichens. The intensity of the reindeerWhile most of the alpine plants with a restricted grazing is also of a certain importance (cf. STEEN'sarea in the Scandes are more or less calcicolous paper). Among the bryophytes Rhacomitriumand favoured by a circumneutral soil reaction, lanuginosum and Polytrichum spp. are often foundH ierochloe alpina is a good example of an acidophilousin exposed situations. With increasing humiditynorthern unicentric species.and snow protection Dicranum spp., HylocomiumIn Empetrion differential species absent in Myrtillionsplendens, Pleurozium Schreberi, Ptilidium ciliare,are few as most of the Empetrion species etc., often form a closed moss carpet. Among thedo not avoid localities with a moderate snow cover bryophytes big foliaceous lichens such a-s Nephromain winter. However H ierochloe alpina, Luzula arcuataarcticum and Peltigera spp. are intermingled withand Diapensia lapponica seem to be differential. Cladonia bellidiflora and C. elongata.On the other hand several Myrtillion species likeVaccinium myrtillus itself need winterly protection The Dryadion allianceby snow and therefore can be used as differential The rich heaths of Dryadion contain a largerspecies against Empetrion. Among them Lycopodiumnumber of species and show more regional variation,alpinum, Deschampsia flexuosa, A nthoxan­as compared to Empetrion and Myrtillion. As mostthum odoratum, Phleum commutatum, Alchemilla of the rare alpine plants of the Scandes are neutrophilousalpina, Sibbaldia procumbens, Trientalis europaea,or even basiphilous, the Dryadion commu­Solidago virgaurea, Gnaphalium norvegicum and G. nities have been interesting objects of investigationsupinum may be mentioned. An important eodominantfor floristic phytogeographers. On circum-neutralto V accinium myrtillus is Phyllodoce soil Dryadion communities usually grow on thecoerulea. In the southern part of the mountain range types of localities that on an acid substratumCalluna vulgaris is a common Myrtillion species. would be expected to be occupied by mpetrionCassiope tetragona, which in northern Lappland is and Myrtillion vegetation. As to snow protectionan important dominant in certain heaths of Dryadion,Dryadion has probably about the same amplitudesometimes grows in stands of typical poor as the two other alliances takem together, and theheath vegetation completely without calcicolous lower border of Myrtillion is often equivalent toplants (see TH. C. E. FRIES 1913 and NoRDHAGEN the lower border of Dryadion (GJJEREVOLL 1956).1955).Many field layer species of the poor heaths haveActa Phytogeogr. Suec. 50