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108 BENGT PETTERSSONFig. 2. Arnagrop, a part of thevast mobile dune area on GotskaSandon, N of Gotland. Grazingand logging once started the deflation.Since these factors wereeliminated about half a centuryago the development has beenquite natural. The dead trunks ofthe sand-buried pinewood appearagain after the dune has passed.Ammophila arenaria is the earliestcolonizer, collecting sand andbuilding up "organogenic" dunes.June 22, 1937. Photo BengtPettersson.aculeata, and Cladonia spp. To some extent thesespecies endure accumulation of a sandy cover.If no measures are taken to check the transportof sand by the wind the recolonization of suchdunes will proceed very slowly, but the final stageis a closed coniferous forest in most districts.Vast areas of mobile dunes are found on NorthGotland, on the islands of Faro ad Gotska Sandon,where the dunes still intrude upon pine forest.The dunes form parabolic chains which had theirorigin in some damage done to the soil within arestricted area. Later on the deflation widenedthe hollow, and the sand was moved in severalradiating directions landwards. Where the velocityof the wind was reduced by the forest, accumulationbegan, creating high dunes with a gently slopingwindward side and a steeper leeward slope. Gradu.ally the sheltering pines were suffocated, and thedune was able to move further on.Between the mobile dune systems and the fore.dune, on Gotska Sandon, there is an almost barrenzone of gravel and pebbles mixed with sand. Thismaterial is arranged in concentric raised beachridges. The effect of the wind is so vehement onthis deflation area that the epilithic vegetation isvery scanty, eliminated by the scouring sand drift.On the whole, the bryophyte and lichen vegetationis insignificant here, and Carex arenaria and Thymusserpyllum occur in thinly dispersed stands. Therichest epilithic vegetation is found in youngstands of pine which have immigrated after thegrazing sheep had been removed.INLAND DUNES.-The coastal dune systems onGotska Sandon have on the whole arisen during the19th century through over-exploitation of theground. In the interior of this island there areseveral dune ridges of uncertain age. They are coveredby old pine forest, but many open sandy areasstill exist, mostly facing south or south-west.Both deflation and rain water erosion take partin the disintegration of the dunes here. The earliestcolonizers of the naked sand are crustaceous lichens>especially Lecidea uliginosa, L. granulosa, and Stereo.caulon condensatum, accompanied by crusts ofbryophytes, especially Ceratodon purpureus andCephaloziella sp. The successional stages are manyfoldand intricate. The crustaceous lichens aresucceeded by numerous fruticose Cladonia spp ..Before the forest immigrates, the surface of thesand is covered by lichen cushions mainly ofCladonia silvatica (sens. lat.) and 0. rangiferina.DuNE Rows.-The arrangement of coastal dunes.in southernmost Skane described by WARMING(1909, p. 355-63) is found also in other districtsin South Sweden. Relatively low ridges of dune sand.parallel the coast, displaying different stages ofsuccession, owip.g to different age. A general explanationof this arrangement has not yet been found.Acta Phytogeogt·. Suec. 50