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262 OLAV GJ lEREVOLLridges in the low-alpine and middle-alpine belts.Sometimes Carex glacialis and Festuca ovina maydominate over small areas, the latter more oftenin poor heath vegetation. As the calcicolous lowalpinegrass arid dwarf shrub heaths show noremarkable qualitative differences in the field andbottom layers they are usually placed in the samealliance by Scandina vian phytosociologists.Middle-alpine chionophobous heathsIn the middle-alpine belt the areas with chionophobousvegetation are rather small compared tothe chionophilous communities, depending on theshort vegetation period and the large extensionof the long-lasting or even permanent snow-fields.Many of the dominant species of Empetrion andMyrtillion disappear or show a reduced vitality ator just above the upper limit of the low-alpine belt.On the tops of hills and crests of ridges scatteredstands of Empetrum hermaphroditum, Phyllodocecoerulea and Dryas octopetala still grow.· In thenorthern mountains Cassiope tetragona is the mostimportant dwarf shrub of the middle-alpine belt.Cassiope tetragona and also Dryas octopetala areable to grow on a far more acid substratum inthe middle-alpine belt than is usual in the lowalpinebelt. This possibly depends on the weakcompetition at higher altitudes, where the vitalityof other dwarf shrubs is more or less reduced(Du RIETZ 1942a).Instead of the dwarf shrub communities extensivegrass heaths are often found in the early snow-freeareas. They are composed of species which alsoare found in low alpine poor heath communitiesbut there seldom as dominants because of the competitionfrom the dwarf shrubs. These acidophilous,xerophilous and chionophobous grass heaths havebeen united in an alliance, Juncion trifidi, by NoRD­HAGEN (1936a). Juncus trifidus is the most importantdominant and grows together with Festuca ovina,F. vivipara, Luzula spicata, L. arcuata, Carex Bigelowiiand in northern Lappland Calamagrostislapponica and Hierochloe alpina. The grass heathsof J uncion trifidi are well separated from chionophilousgrass communities, e.g. through the occurrenceof some of the wind-hardy fruticulose lichensof Empetrion and Myrtillion.CniONOPHILo us PLANT CoMMUNITIEsBy Olav GjrerevollThe plant communities subjected to a more or lesssolid snow cover during the greater part of theyear, display a considerable variation. This variationis due partly to the time of exposure, partlyto the quality of the soil (calcareous or acid soils),and partly to the access of moisture. Consequentlyparallel series of mesophilous plant communitiesare found on calcareous and acid soils, respectively,varying from slightly chionophilous to extremelychionophilous, and in the same way two parallelseries of more hygrophilous communities with thesame amplitude as to exposure. This was clearlyemphasized by Du RIETZ ( 1942 a and b), GJ JERE­VOLL (1949, 1950, 1956), and others.This is the ecological background for the quadrupledivision of the vegetation dependent on latesnow into one heath series poor in calciphiles,another one rich in calciphiles, and two correspondingmeadow series. This division applies to thelow -alpine and middle-alpine regions.Communities poor in calciphiles and hygrophiles(heath series)DESCHAMPSio-ANTHOXANTHION .-The lower borderfor V accinium myrtillus is one of the mostsharp-cut boundaries found between plant communitiesin the mountains. The dense growth ofVaccinium myrtillus ceases abruptly. It is replacedby a distinct community predominated by grassesand sedges (alliance Deschampsio-Anthoxanthion).In the middle-alpine region, where V. myrtillus isabsent, the same alliance is found beneath theActa Phytogeogr. Suec. 50