Long-term effects of large-scale grazing on the vegetation of a rewetted river valley

•Large-scale grazing is a relevant tool for improving plant diversity in rewetted river valleys.•Land-use history impacts vegetation development.•Establishment of thorny shrubs was most pronounced on former arable fields.•Fine-tuning of management is necessary to preserve species-rich fen communities.

Large-scale, low-density grazing is a standard management concept for conserving or enhancing biodiversity in cultural landscapes of central Europe. Documentation of results concerning effects of this type of grazing on biodiversity and functioning of degraded river valleys, however, is rare. For a period of ten years, we investigated vegetation development of three pastures in a river valley of northern Germany, where rewetting was combined with large-scale cattle grazing for restoration of the valley’s biodiversity as well as its water and nutrient regulation. The study yielded ambiguous results concerning achievement of restoration goals. Changes in plant diversity were dependent on site conditions and on previous land-use of the associated pastures. On mineral soils, species richness, total species number and β-diversity increased. On fen soils that formerly were species-poor wet grasslands, an increase in species richness was observed, whereas at sites formerly of species-rich vegetation, species richness decreased. Development of β-diversity showed no clear trend on fen sites. Establishment of woody species as indicator for enhanced structural heterogeneity was successful on mineral soils of abandoned arable fields with an open sward, and also on fen soils close to mature nursery trees. We conclude that a fine-tuning of factors influencing grazing behaviour of cattle is necessary for achieving simultaneous targets of nature conservation and resource protection. Restoration of river valleys in cultural landscapes is however a lengthy process, thus, long-term monitoring is indispensable for avoiding mismanagement.