fulltext - DiVA

140 BENGT PETTERSSONcalcareous lakes have been filled in by sedimentsand overgrown with calcicolous fen vegetation. Therate of development has been different, and severalbasins are still occupied by Chara lakes, especiallyin the districts with outcropping strata of limestone-poor in other respects than calcium. Raised bogshave had a very restricted occurrence.Both islands have suffered greatly from theintense artificial drainage which has markedlylowered the subsoil water surface. Only little of theoriginal fen vegetation remains on the islands butenough to show that much of the fen vegetationhas been rather similar; for instance Cladiummariscus is a typical element of the fen vegetation.For various reasons Cladium is at present increasingin frequency and a bun dance in many districts onGotland (BENGT PETTERSSON 1946 a, 1958),.aphenomenon unknown from Oland.The Cladium swamps are very poor in speciesnumber but are nevertheless interesting because ofthe succession that may be observed in their vegetation.Only where the succession has reached itslater stages the vegetation may sometimes be richer.Slightly elevated areas and sections of richer vegetation,dominated by moss carpets (especiallyCampylium helodes, C. stellatum, and Drepanccladusrevolvens sens. lat.), alternate with lower 2-nd wetterportions where dense stands of Cladium crowd outother species. Moss carpets of Scorpidium scorpioidesoften cover the bottom of the wet sections.On Gotland there are still numerous placeswhere percolating calcareous water comes to thesurface and spring mires have developed. Schoenusferrugineus often dominates here, accompanied bya lot of other species. On Oland there remains butlittle of such vegetation which has always been ofminor importance there.Below Gotland's coastal cliffs of argillaceouslimestone thick layers of calcareous tufa have beenformed in some places. Beside some common speciesof the spring mires Cratoneurum comrnutatum, C.falcatum and Barbula tophacea are the most conspicuousspecies, growing in thick tufts. On nakedwet limestone dried up in summer the small semiaquaticbryophytes Seligeria oelandica and Trochobryumcarniolicum (BENGT PETTERSSON 1950) arefound in many places, the latter only on Gotland.Shore vegetationThe coastal vegetation is on the whole similar onthe two islands, especially the salt marsh vegetation.ENGLUND (1942) has made a thorough study of theshore vegetation and flora on Gotland with elaboratedistribution maps of most species. For Olandwe have no comparable investigation but STERNER(1938) has mapped several shore plants.Some southern halophytic species are restrictedon Gotland to the southern part, Plantago coronopusand Obione pedunculata reaching here their ·northernlimit. On Oland they are more evenly distributedwhich partly may be due to the slightly highersalinity in the sea water, partly to the more southernsituation.On both islands the salt marshes and the shorevegetation as a whole have been greatly influencedby grazing sheep and cattle. Some areas have alsobeen used for hay making. Where this influence hasceased changes in the vegetation have occurred.A special effect of the calciferous soil is generallyapparent only in the upper belts of the shore zonation,where for instance Sesleria coerulea plays animportant role. If there were no human activitiesaffecting the vegetation, brushwood or forest wouldoccupy most of the upper zone, even areas whichat present are devoid of trees and bushes.Though situated in close proximity to each otherand having in many respects similar physiographicconditions, Gotland and Oland have a rather differentvegetation. The similarities between the islandsare nevertheless striking. This is due not only toprimary conditions but to the fact that the historyof man on the two islands has many features incommon, for instance a rather coincident developmentduring the Iron Age. The ancient ruraleconomy and organization have also been similaron both islands, as well as the more recent courseof development which is characterized by greatchanges in the vegetation. In spite of the profoundhuman transformations of the botanical landscapefrom pre-agricultural time onwards, essential elementsof an original flora and vegetation havesurvived.Acta Phytogeogr. Suec. 50