Responses of flower phenology and seed production under cattle grazing impact in sandy grasslands

The impact of cattle grazing on selected characteristic and dominant plant species of three sandy grassland communities in northwestern Germany (Spergulo-Cory-nephoretum typicum, S.-C. cladonietosum and Diantho-Armerietum) is investigated with regard to the loss of above-ground diaspores in the course of a vegetation period. Special attention is given to the importance of the seed bank in the soil as compensation potential. The flower and fruit phenology of the plant species was analyzed by counting. A fence was erected so that data samples outside and within an exclosure could be compared. Extracted soil samples and a germination test give information about the diaspore reservoir in the soil at the beginning of the investigation. The comparison of grazed and ungrazed stands yielded the following results. The Spergulo-Corynephoretum typicum is poor, the S.-C. cladonietosum richer in palatable inflorescences and infructescences (e.g. Carex arenaria). In the former only 12-24% of the inflorescences and infructescences are grazed (Carex arenaria, Corynephorus canescens), in the latter 45-51% (Carex arenaria). The Spergulo-Corynephoretum can regenerate itself from the diaspore potential to a slight extent if there are gaps, e.g. caused by cattle trampling. The Diantho-Armerietum is quite intensively grazed, entailing a major reduction of flowers and fruits of certain plant species (Agrostis capillaris: inflorescences by 71%, infructescences 72%, Diantbus deltoides: flowers by 61%, fruits 22%). In contrast, two species increase flower and fruit numbers (by 36-77%) in the grazed sites (Agrostis vinealis, Ranunculus bulbosus). Faeces microsites are important elements for patch dynamic systems in the Diantho-Armerietum. At faeces microsites in the Diantho-Armerietum, which constitute about one-third of the plot areas, many flowers and fruits develop. Flower and fruit development at the faeces microsites and the seed bank in the soil ensure a generative regeneration of the Diantho-Armerietum. Gap dynamics, patch dynamics of faeces microsites and seed bank processes are driving forces for the generative regeneration of the investigated plant communities.