Veranderingen in de mosvegetatie van de Nieuwkoopse Plassen (incl. De Haak) tussen 1967/1972 en 2009/2013: 2. Veranderingen in veenmosrietland en moerasheide

Changes in the moss vegetation of the Nieuwkoopse Plassen (De Haak included) between 1967/1972 and 2009/2013. II. Changes in Sphagnum reedlands and fen-bogs. A sample of 50 historic relevés in Sphagnum reedlands and fen-bogs assessed in the Nieuwkoopse Plassen in 1967 was repeated in 2013. On average, the ground water level dropped with 0,5 dm in this period, and there was a obvious diminution of macro-ions in the younger Sphagnum reedlands. Also, between 1960-1980, there was a peak in the acid deposition. Statements on the relation of changes in moss vegetation and chemical characteristics of the substrate are hardly possible because of the too incidental measurements. In general, the moss layer reflects the drying up of the substrate. Further, a group of species more or less constant in the 1967 relevés, particularly Pallavicinia lyellii, S. fimbriatum, Pohlia nutans, Calypogeia fissa, Dicranum bonjeanii, and Cephalozia connivens has strongly decreased in 2013. A conspicuous phenomenon is the decrease of Polytrichum commune var. commune, possibly linked to the reduced acid deposition. Also, Sphagnum fallax has decreased considerably, probably by its spidery shape which is less resistant as to dessication; this matches with the increase of massive species like Sphagnum palustre and S. magellanicum. Within 46 years, many relevés with a management of summer or winter mowing without burning (of the remaining litter) show a change into an advanced fen-bog vegetation, with moss species like Cephalozia macrostachya, Kurzia pauciflora, Sphagnum magellanicum, S. papillosum, Polytrichum juniperinum var. strictum and Leucobryum glaucum, The relevés with a winter mowing and burning management in 2013 and the years before haver turned into reedland with Calamagrostis canescens and usually very much Molinia caerulea; the moss layer is often reduced, with Sphagnum (mainly palustris), and species such as Brachythecium rutabulum and Mnium hornum.