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Maritime sands 107Fig. l. Recently formed foredunecaused by the invasion of Ammophilaarenaria along the shoreafter the grazing animals wereremoved. The low dune is partiallydemolished by wave abrasionduring autumnal gales. In thebackground pinewood and grovesof alder which also immigratedafter the grazing pressure hadceased and deflation had abated.North Gotland, Faro. Jan. 21,1962. Photo Bengt Pettersson.foreune are . Festuca arenaria, Agropyron junceiforme,Hieracium umbellatum and Galium verum.0. ANDERSSON (1950b) has found several southernspecies of large fungi which are ecologically specializedto grow on the unstabilized soil of the foreduneor in its 'close vicinity, e.g. Phallus hadrianus.The external morphology of the foredune displaysa great variation. Sometimes it is very low, andsand from the se'a can be transported onto it by theaction of waves during heavy storms. The aeolianeffect is of greater importance, in particular wherethe sand contains fine-grained fractions. Themobility of the soil is an obstacle to the colonizationof lichens and bryophytes.MoBILE DUNES.-In a few areas the sandy soilinside the foredune has been affected by winderosion, and mobile dunes have been formed.Wherever this has occurred the process had itsorigin in human influence, above all timber-cutting,grazing, and cultivation, often as co-operativefactors (cf. HESSELMAN 1908 a).It may be mentioned here that a layer of dunesand on a site on North Gotland has been datedby the radiocarbon method to 2940±90 B.P.(U-429) (OLSSON and KILICCI 1964, p. 301). This8-652151 APhS 50sand layer built up during the Bronze Age wasabout 2 metres thick and rested on a dark bed ofvarying thickness containing charcoal and potsherds.Closed coniferous wood covered the sandysoil.The mobile dunes have often once been devoidof vegetation, but are colonized especially byAmmophila in later stages when the factors thatwere responsible for the mobility of the sand havebeen eliminated. The originally smooth surface ofthe dunes is changed by the vegetation which isaccumulating drifting sand. In this manner astrongly undulating landscape is developed, wherethe highest mounds are grown over with Ammophilaand between them more or less naked blowoutsform valleys and hollows (cf. Fig. 2 and 3).All the time colonization is going on by lowgrowingspecies, especially Corynephorus canescens,Cq,rex arenaria, Festuca arenaria, F. polesica, Violacanina, and Thymus serpyllum. These species donot resist deflation as much as Ammophila and areoften covered with sand or detached by erosion.In areas where the erosion .or accumulation areless intense bryophytes and lichens are able tocolonize. The earliest are for instance Ceratodonpurpureus, Bryum spp., Tortula ruralis var.ruraliformis, Stereocaulon tomentosum, CorniculariaActa Phytogeog.r. Suec. 5()