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266 OLAV. GJJEREVOLLcarpets, but the carpets do not look so barren asthose of S. herbacea because S. polaris is usuallyaccompanied by numerous scattered vascularplants. The most important companions of Salixpolaris are Silene acaulis, Saxifraga oppositifolia,Thalictrum alpinum, and Viola biflora. The lattermay sometimes grow abundantly. The alliance ispoor in grasses and sedges, Carex Lachenalii andPoa alpina being the most important ones.In the bottom layer mosses and hepatics areconspicuous, whereas the lichens are of minorimportance. The Salix polaris communities (alliancePolarion) may cover quite extensive areas particularlyin Northern Scandinavia.Communities rich n calciphiles and hygrophiles(meadow series)RANUNCULO-POION ALPINAE.-On slopes withlong-lasting snow fields it is very usual to findcommunities of Salix 1·eticulata and S. polaris abovethe snow, i.e. in places that dry up rapidly. Belowthe snow fields, in places subject to irrigation, thesituation is considerably different. Hygrophilousspecies become predominant, i.a. Trollius europaeus,Ranunculus acris, R. nivalis, Oxyria digyna, Petasitesfrigidus and Phippsia algida, in addition toSaxifraga oppositifolia which has an extremelywide amplitude regarding moisture.The late snow-free areas are first and foremostcharacterized by Trollius europaeus and Ranunculusacris (alliance Ranunculo-Poion alpinae). Theluxuriant Trollius europaeus meadows are veryspectacular in many parts of the low-alpine region.This is a community developed most significantlyin Northern Scandinavia where it covers considerableareas. In the subalpine birch woods Trolliusis one of the most characteristic species of theLactuca alpina - Aconitum septentrionale communities,but unlike the other tall-growing herbs itascends in great numbers quite high above the ,birch line.In the middle-alpine region Trollius is of slightor no importance, and the well-watered late snowfreeareas are dominated by Ranunculus acris,Viola biflora, and Poa alpina. From a physiognomicalpoint of view this community is well defined,but sociologically it is very heterogeneous. Nume- -rous hygrophilous mosses and hepatics are present,but do not play any conspicuous part under theusually well-developed field layer. In a similarlyheterogeneous community Petasites frigidus mayoccur as a predominant species.0PPOSITIFOLI0-0XYRION.-With very late exposuremany of the most important species ofRanunculo-Poion alpinae will disappear, e.g.Trollius europaeus, Saussurea alpina, Petasitesfrigidus, Angelica archangelica, and the vitality ofmany other species, i.a. Ranunculus acris, is heavilyreduced. Several other species attain their highestfrequency within this alliance, e.g. Saxifraga oppositi}olia(alliance Oppositifolio- Oxyrion).Important constituents are a number of hygrophilousspecies such as Cerastium arcticum, C. cerastioides,Arabis alpina, Oxyria digyna, Ranunculusnivalis, R. sulphureus, Saxifraga rivularis, andPhippsia algida.The alliance displays a number of different communitiesof lower rank. Saxifraga oppositifolia itselfmay occur as the predominant species withinseveral sub-communities with different companions.Whereas in Ranunculo-Poion alpinae the cryptogamsare of less importance, they play a significantpart in Oppositifolio-Oxyrion, esp. Distichiumcapillaceum.In Northern Sweden an interesting sub-communityis met with showing abundant occurrenceof different Marchantiaceae, such as Preissia quadrata,Peltolepis grandis, and Sauteria alpina. Whenthese species are in the fructiferous state, thecommunities give a peculiarly fascinating impression.They seem to be among the most lime-requiringsnow-bed communities that exist, the pH valuesalways being very high (6.5-7.8).The influence of solifluction and water erosionis often strong within this alliance. Extensive areasare covered with a less coherent vegetation due tothese ecological factors. Some species reach theirhighest vitality under these circumstances, i.a.the bicentric Cerastium arcticum and the northernunicentric Draba crassifolia.In fairly flat areas drenched with water frommelting snow fields during the whole season ormost of it, Ranunculus nivalis is frequently foundActa Phytogeogr. Suec. 50