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110 BENGT PETTERSSONHEIM 1946, p. 113). Phleum arenarium represents asuboceanic type among the therophytes (HoRN AFRANTZIEN 1946).The sand is mostly covered by moss carpets inwhich Tortula ruralis var. ruraliformis is often_dominant, accompanied by several lichens, especiallyCladonia spp. and Cornicularia aculeata. Somesouthern gasteromycetes are also present, e.g.Tulostoma brumale, Geastrum nanum, G. minimum,and Discis-eda bovista.An interesting fact is that there are two differenttypes of habitats in which Tulostoma brumale isfound on Gotland. It generally grows on sand inclose proximity to the coast but is also found in afew localities in the interior, here only in limestonepavement areas and accompanied by a lot ofother calcicolous species. The dual types of substrateof this gasteromycete may be compared withthe two different habitats of the submediterraneandwarf shrub Fumana procumbens on Gotland. Thisspecies grows typically as a chasmophyte on thelimestone pavements (BENGT PETTERSSON 1958, p.106 and Pl . X) but has a few localities on gravellysand on the coasts. This may be an example of interactionbetween pedological and climatic factors.Tulostoma brumale is also found in North Uppland,at its northernmost locality in the world(cf. 0. ANDERSSON 1950 b, Figs. 28 and 29). Thehabitat is a sandy patch next to the sea, thusillustrating the preference for coastal habitatswhich is characteristic of many southern speciesat their northern limit.There is a floristic connection between the Koeleriaglauca grass heath in Skane and scatteredsandy areas on the coast farther north. Oland andGotland have several southern psammophytes incommon with Skane. Farther to the north thenumber of true psammophytes decreases. They arereplaced by species that frequently occur inmeadowland and woods (cf. ENEQUIST 1944).VARIOUS HUMAN FACTORS.-The sand vegetationis easily disturbed by mechanical action becauseof the porous loose soil. This implies that manyspecies of the sandy shore vegetation are favouredby trampling people or cattle. Becaui:le of this theirexistence is threatened in case the shores are completelyprotected against human activities. Withinmilitary training fields the digging in the soil andthe driving of heavy carriages procure patches ofnaked sand where species weak in competition areable to surviye.Such species also often show a preference forground managed according to old-fashioned agriculturalpractice that includes the creation of£allows and disused arable land. Gnaphalium luteaalbumhas earlier had several localities but it hasprobably become extinct in Sweden, according toRICKMAN (1963). Helichrysum arenarium is oftengrowing abundantly in such areas, temporarilyused as grazing land. The planting of coniferousforest on the earlier open areas of sand occasionscomplete disappearance of these species. The surfacelayer of the sandy soil is rapidly leached outthrough the influence of the litter.At least since the beginning of the 18th century(cf. ALB . NILSSON 1905, p. 328) the coastal dunesin South Sweden have been planted with trees inmany places on account of the damage they did toboth woodland and cultivated fields. EspeciallyPin us mugo has been used to check the drifting sand.The disturbances of soil and vegetation on sandyshores caused by the more and more increasingseaside life may often be regarded as a positivefactor securing the survival of those species whichare weak in competition and dependent on freesand. The trampling of people is in many respectsvicarious for grazing cattle and sheep. Althoughthe effects are not identical in detail the immigrationof wood will be impeded and the mobility ofthe sand maintained. Plants with a pronouncedresting period in summer, mainly geophytes andtherophytes, will be highly favoued by the newfactors. In some places, for instance, the earlyfloweringOrchis sambucina has become abundantin sandy . areas heavily trampled by people insummer.Acta Phytogeog?'. Suec. 50