Relationship analyses of the flora of the Dutch, German and Danish Wadden Islands

Flora and vegetation of the Dutch, German and Danish Wadden Islands were compared with the aid of similarity computer programs. The Wadden District, often described as an exclusively Dutch phytogeographical entity, was found to cover the whole area. From a comparison with the floras of the neighbouring Dune and Half District and the more distant Kempen District, it was concluded that the Wadden District forms a distinct unity by its relatively large internal cohesion. Within the Wadden District three groups of islands could be discerned: a western group including the Dutch islands and Borkum, a central group of German islands and a northern group of partly German, partly Danish islands. Nordstrand, Pellworm and Skallingen usually showed only slight similarities with the last group. Texel was clustered sometimes with the northern group. Ecological differentiation chiefly determines the presence or absence of species in the islands, and thus the degree of (dis)similarity of their floras and vegetation types. Surface, position, soil structure and man mainly contribute to this ecological differentiation. Local climate plays a minor role.