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The Growth on RockBy EDVARD voN KRUSENS TJERNAThe vegetation growing on rock is of great sociologicalinterest, especially in glaciated countries,where such vegetation is often prominent. Due tothe crustal uplift in Fennoscandia, bare rock appearsfrequently, not only along the coasts, but inseveral inland places as well, owing to the fact thatoverlying deposits have been totally washed awayin Post-glacial times.The vegetation growing on a naked rock surfaceis of. course primarily related to the kind of rock,its chemical composition, state of weathering andsurface micro-structure (SJ6GREN 1964). It is alsoinfluenced by macroclimate (cf., e.g., DEGELIUS1935, AHLNER 1948) and by _microclimatic conditionsdepending largely on relief and exposure. Finally,it is dependent on snow cover, access ofwater of different chemical quality, additions oflitter, dust, fertilizers and bird droppings, etc. Theage and developmental stage of the vegetation isalso important (SJOGREN 1964). Due to the completeor almost complete absence of soil in the stageof primary colonization, specialized rock mossesand lichens often play an important role, but in thelater stages of succession, the moss or lichen cushionscontain humus and mineral particles to avarying degree.Bedrock is exposed either as more or less steep,rugged cliffs or with flat, horizontal or shelvingsurfaces. Both kinds of surfaces occur on largeboulders, too. Even on an identical substrate steepsurfaces have quite different species in comparisonwith flat ones, owing to differences above all inlight, but also in humidity, etc. Between the acidophilousspecies and communities on non-calcareousrock (granites, gneisses, leptites, etc.) and the basiphilousones on calcareous rock there are severalcommunities showing intermediate types in compositionby species.In southern Sweden the moss vegetation on thehabitats mentioned can be grouped according toTable I (cf. v. KRUSENSTJERNA 1945, p. 87).This system was first worked out for centralUppland but is on the whole applicable for southernSweden, save the Cambro-Silurian districts. G.TABLE I. Moss (lichen) communities on bare rock.Non-calcareous rockLightsupplyAcidLess acid, influenced bydust, bird droppings, etc.Calcareous rockBasicGoodRhacomitrionRhacomitrium lan'uginosumGladonia spp.Grimmion commutataeGrimmia commutataH edwigia ciliataTortellionTortella spp.Ditrichum . flexicauleLecanora cinereaLecanora muralisLecanora calcareaPoorPlagiothecionAnti trichionEncalyptionPlagiothecium denticulatumAntitrichia curtipendulaEncalypta streptocarpaI sopterygium elegansM nium cuspidatumI sopterygium ( Taxiphyllum)Isopterygium pulchellumdepressumActa Phytogeog1'. Suec. 50