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Ohionophilous plant communities 265TABLE 1. Survey of the heath and meadow communities in the low-alpine and middle-alpine regions.Alliances poor in calciphilesAlliances rich in calci philesSnow conditionsHeath seriesMeadow seriesHeath seriesMeadow seriesWind -exposed,almostsnow-freeEmpe-trionJ unci ontrifidi(middle­Dryadion(e.g. withCarex nardina)alpine)Protectedbut earlysnow-free(Low scrub)Myrtillion(low-alpine only)Dryadion(e.g. withCassiope tetragona)Potentilleto­Polygonionvi vi pariLatesnow-freeRanun­culo­Cryptogrammo-Deschampsio­AnthoxanthionReticulato-Poion alpinaeRanunculo­Poion alpinaeAthyrion AnthoxalpestrisanthionVery latesnow-freeHerbaceonPolarionStellari­OxyrionOppositifolio­OxyrionExtremely latesnow.freePolytrichionnorvegiciDistichion capillacei(not every year)particularly in the middle-alpine region. It is builtup by numerous homogeneous sub-communities.Communities rich in calciphiles but poor n hygrophiles(heath series)The numerous communities belonging here arebeautifully developed in the mountains of NorthernScandinavia, but less frequent and less rich inspecies farther south in the Swedish parts of theScandes. In the northern Lappish mountains calcareousschists are widely distributed. On slopesand in depressions with a long-lasting snow coverheath species requiring a free exposure or at leasta fairly long growth period like, i.a. Dryas octopetala,Rhododendron lapponicum, Kobresia myosuroides,Oarex nardina, and Oassiope tetragona areunable to grow, and in the same way as on soilspoor in lime, chionophilous species take over.RETICULATO-POION ALPINAE.-On slopes dryingup quickly after exposure the Dryas octopetala­Oassiope tetragona heaths are succeeded downwardby a very distinct community dominated by Salixreticulata. This community is poor in grasses, theonly conspicuous one being Poa alpina (allianceReticulato-Poion alpinae). Salix reticulata is alsoan important constituent of the Dryas- Gassiopeheaths, but its amplitude with regard to time ofexposure is wide. The Salix reticulata zone of thesno-beds is easily visible from a distance due tothe silvery-greyish colour of the dominant speciesitself. In this community Pinguicula alpina mayoften play a conspicuous part.There is a striking difference in the bottom layerbetween the Dryas- Oassiope heaths and thisalliance. In the former Hylocomium splendens,Rhytidium rugosum, Rhytidiadelphus triquetrus, andTomentypnum nitens are the most important species,whereas in Reticulato-Poion alpinae they arereplaced in the first instance by Distichium capillaceum,furthermore by Webera spp. and Anthelia.PoLARION.-With very late exposure Salix reticulatais replaced by S. polaris which with regard torequirement of exposure is equivalent to S. herbacea,reaching its highest frequency in very late exposedsnow-beds which dry up quickly when the snow isgone.Like Salix herbacea, S. polaris may form denseActa Phytogeogr. Suec. 50