Cliff Vegetation in the Eastern Mediterranean

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Abstract
A preliminary account is given of the major rock habitats in the Mediterranean, distinguishing pavement, sloping, vertical and overhanging rock, ledge and step-crevice. The plant communities that occupy these habitats in Palestine are outlined; each habitat is characterized by distinct and specialized communities containing many rare spp. of a relict nature. The chamaephyte (especially suffruticose) is the dominant life form, becoming increasingly abundant on vertical and overhanging rock; annuals are virtually absent. The prevalent method of seed dispersal is by wind, except on the step-crevice where bird-dispersed spp. predominate. Saxatilism and the woody habit, though they may sometimes date from Tertiary times, are considered advanced characters in chasmophyte communites. Certain gamopetalous spp. play the major role in Mediterranean crevice associations. Cliff communities are biologically closed associations and are edaphic and/or physiographic climaxes; they.suggest that the Mediterranean climate (whose Quaternary variations are discussed) is drier than in recent times. A cliff offers a refuge from favorable climatic change, competition with hillside communities, and grazing; the latter may increase the development of the chasmophytic habit by eliminating non-saxatile biotypes. The rarity of so many saxatile spp. is due to genetic homogeneity resulting from the climatic and topographic changes of the Pleistocene. Having considered the roles played by geographical and ecological isolation, and by natural selection and genetic drift, it is concluded that chasmophytes are in general a blind alley in the scheme of evolution.