Welches sind die Langzeit-Effekte von Beweidung mit Großherbivoren in naturschutzfachlich wertvollen Steppenrasen auf sandigem Substrat? : Ergebnisse einer Studie über 12 Jahre
To examine the necessity of livestock grazing for managing threatened sand vegetation so as to en-sure and develop its nature-conservation value, we investigated a grazing system in a model nature reserve (German upper Rhine valley) characterized by threatened steppic sandy grassland (Allio-Stipetum vegetation complex: Habitats Directive Annex I, priority type 6240, additionally Koelerion glaucae: priority type 6120). The area has been grazed by sheep and additionally by donkeys since late summer of the year 2000. We established ungrazed (Au) and grazed plots (Ag) in the Allio-Stipetum complex in a systematic grid-plot design before grazing impact started. We sampled phytosociological relevés yearly in the Au/Ag plots for 12 vegetation periods. Addi-tionally, we sampled relevés on former farmland adjacent to the Allio-Stipetum complex (systematic grid-plot design) over ten vegetation periods. These former fields (Fg) were integrated in the grazing system since the beginning of the study. A constancy table was produced for Au, Ag, Fg. To determine the portions of target species (Koelerio-Corynephoretea, Festuco-Brometea species) we calculated target-species ratios (proportion of target species in comparison with the total species number; qualitative or quantitative approach: TSRqual/quant). We tested the effects of grazing, year and interactions on structural and phytodiversity characteristics of the relevés by mixed linear models. The results of the long-term experiment confirmed significant beneficial effects of grazing on habi-tat-typical structure and phytodiversity: e. g. reduction of litter, support of species which are short in height ( 50 cm), support of the diversity of all plant species, of non-graminoid herbs and of target plant species, reduction of Calamagrostis epigejos, positive effects on TSRqual and TSRquant values. Similar developments, e.g. for phytodiversity and Calamagrostis epigejos cover, were observed on the former fields, but there are still seed limitations and high ruderal-plant dominances. In general TSRqual/quant show remarkable increases, but do not reach the values of the Ag plots. Concerning aims of nature conservation, the habitat-typical vegetation structure and phytodiversity of the Allio-Stipetum shows an excellent development as a consequence of the used grazing manage-ment. However, occasionally small populations of target species did not increase (e.g. Koeleria glauca). The former fields show a development towards valuable sandy grassland.Allgemein gilt Beweidung als ein in der Regel wirkungsvolles Instrument für die Entwicklung von verschiedenen Vegetationstypen mit hoher Phytodiversität (Rosenthal et al. 2012). Um die Effekte der Beweidung für das Management von gefährdeten Sandrasen-Komplexen zu prüfen, untersuchten wir ein Beweidungssystem mit Schafen und zusätzlich mit Eseln im Naturschutzgebiet "Griesheimer Düne und Eichwäldchen"(nördliche Oberrheinebene, Hessen, 45 ha). Besonders bezeichnend sind hier großflächige Vorkommen des Allio-Stipetum-Vegetationskomplexes (Abb. 1, 2, 3). Das Gebiet wurde seit dem Jahre 2000 beweidet
