Enkele bijzondere mosvondsten in de Knopbies-vegetaties van de Muy op Texel

Two “rich-fen” bryophytes, Ctenidium molluscum and Rhizomnium pseudopunctatum, were discovered in Schoenus nigricans-fens in dune-slacks on the Isle of Texel. Relevés of the vegetation with the two species are presented in the text. It is presumed that the rare occurrence of both mosses in dune-slacks is related to the rarity of the hydrology-type of the slacks in question. Both slacks are influenced by drainage measurements in nearby cultivated parts of the same area. As a consequence large-scale and prolonged inundation in winter, which is the normal situation in wet dune-slacks, has become impossible. On the other hand the drainage is not so strong that it would cause fatal drought in the slacks; ground water is available to the plants all year round. The drainage merely ensures a constant flow of baserich ground water from neighbouring dunes to these slacks. The combination of these two factors results in the development of a “mixed” vegetation with calcifuge elements on tops of old Schoenus-tussocks and more basiphilous plants in lower parts of the vegetation. Rhizomnium pseudopuncatum grows on the sides of the tussocks, as well as on low hummocks, but avoids the higher tussock-tops. Ctenidium molluscum grows in a vegetation that is grazed by cattle in summer and autumn. Therefore the pattern of tussocks and interspaces in this Schoenusvegetation is less pronounced than in the ungrazed vegetation with Rhizomnium pseudopunctatum and the moss-layer mainly contains calcicoles. Nevertheless Ctenidium too showed a preference for an intermediate position between tops and depressions.