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258 KARL-GORAN BRINGERThe chionophobous dwarf shrub communities (see,e.g., NORDHAGEN 1936 a, 1943, KALLIOLA 1939 andDAHL 1956, in addition to works quoted below)have often by Scandina vian phyto-sociologists beendivided into three different vegetation units havingthe rank of alliances. Different authors have proposedvarious names and a variable delimitationof the units, as well as different ways of subdividingthem into associations. In conformity to Du RIETZ( 1942 a and b) the three alliances are in the followingcalled Empetrion, Myrtillion and Dryadion. Thenames allude to three of the most important dominantsamong the dwarf shrubs in the low-alpinebelt, Empetrum hermaphroditum, V accinium myrtillusand Dryas octopetala.The first two alliances, Empetrion and Myrtillion,are found in localities where the bedrock and soilmaterial consist of granites, quartzites, sandstonesand other rocks which through weathering aredeveloping a distinctly acid soil reaction. TheDryadion communities are met with in areas oflimestones, dolomites or calcareous schists, wherethe soil reaction is circum-neutral or slightly basic.On account of the geological conditions of theSwedish Scandes Empetrion and Myrtillion communitiesoccupy much larger areas than those ofDryadion. The latter alliance has its largest extensionin the west, near the border between theprovinces of Harjedalen and Jamtland and in someparts of central and northern Lappland, but iscompletely lacking in Dalarna and in most of theeastern part of the mountain range. The speciesof Empetrion and Myrtillion are rather few andusually widely distributed while Dryadion has acomparatively rich flora containing several of therare Scandinavian alpine plants. It is thus possibleto make a differentition into poor heath (Empetrionand Myrtillion) and rich heath (Dryadion)vegetation.Vegetation belonging to the alliances mentionedhas been described from different parts of theSwedish mountains by several authors, e.g. G. SA­MUELSSON (1917) from Dalarna, SMITH (1920) fromJamtland and Harjedalen (compare also NoRD­HAGEN's sizable monograph of 1928 on the vegetationof the Norwegian side of the Sylarna Mts., onthe border between Norwegian Sortrondelag andSwedish Jamtland), and from northern LapplandTH. c. E. FRIES (1913), TENGWALL (1920, 1925),Du RIETZ (1925c, 1942a and 1950e), G. BJORKMAN(1937), HEDBERG (HEDBERG et al. 1952), NoRD­HAGEN (1955), SELANDER (1955) and BRINGER(1961). Notes about alpine vegetation are also foundin floristically phytogeographical works, e.g., G.BJORKMAN (1939), ARWIDSSON (1943), SELANDER(1950a and b) and WISTRAND (1962).The Empetrion and M yrtillion alliancesIn the low-alpine poor heath vegetation thecommunities of Empetrion need little or no snowprotection in winter while Myrtillion does notdevelop unless a good winterly snow cover is provided.On small hills,, ridges and crests poor in limea characteristic zonation is usually found withEmpetrion communities on the top, and Myrtillionheaths on the better protected slopes. With increasingthickness and later melting of the snowcoverthe Myrtillion dwarf shrub heaths are passingover into chionophilous grass communities. As thedistribution of the snow-cover shows a great conformityfrom year to year the frontiers betweenthe different zones seem to be rather stable.In a transitional zone between Empetrion andMyrtillion, the combination of not too poor protectionby snow and early melting in the springcauses favourable conditions and a rather longvegetation period. Here a zone of low shrubs isoften developed with Juniperus communis, Betulanana, Salix glauca and S. lapponum. The branchesof the bushes do not reach above the normal levelof the winter snow blanket, for when exposed tothe drought and the abrasion of the wind and thedrifting snow, they are regularly killed off.On surfaces particularly exposed to wind andfrost heaving, only small stands of Diapensialapponica or, more frequently, Loiseleuria procumbensand a few other species occur, often alternatingwith patches of bare soil. Other Empetrion dwarfshrubs, with a greater power of forming closedstands, are Empetrum hermaphroditum (a northernspecies which in southern Sweden is replaced byits close relative E. nigrum), Arctostaphylos alpina,A. uva-ursi (in some districts only), Vacciniumuliginosurn, V. vitis-idaea and Betula nana. In theActa Phytogeogr. Suec. 50