Restorative grazing as a tool for directed succession with diaspore inoculation: the model of sand ecosystems
Stroh, Michael; Storm, Christian; Zehm, Andreas; Schwabe, Angelika
In this study we examine the restoration of sand grassland target communities (belonging to Koelerio-Corynephoretea, Festuco-Brometea) on bare ground. A field experiment was established in 1998. Soil seed bank and seed rain were assessed. Directed and spontaneous succession was monitored for 3 years. For diaspore transfer (inoculation) 1. mown material, 2. raked material or 3. sods from intact sand habitats were used. The experimental plots were grazed by sheep from 1999 to 2001; in 2001 also by donkeys. Comparisons with grazed-only plots and controls were made. Additionally, spontaneous succession was determined in two non-managed sand grassland plots. Spontaneous succession leads to ruderal communities (first Stellarietea phase, followed by increasing dominance of Artemisietea and Agropyretea species, e.g. Poa angustifolia, Calamagrostis epigejos). A few target species were able to establish themselves, but they play a minor role, because they are only present in small numbers in both soil seed bank and seed rain. Grazing-only (without inoculation) is not sufficient to stop the ruderalisation trend. However, after each of the three diaspore inoculation treatments nearly all target species established themselves. Ruderalisation was suppressed by inoculation combined with grazing. Nevertheless, many ruderal species occur and represent a latent ruderalisation potential. Ruderal species are grazed preferentially by sheep. Several ruderal species that are rejected by sheep are grazed by donkeys (e.g. Cirsium arvense, Calamagrostis epigejos). A compensation for biomass loss after grazing (as supposed by the grazing optimization hypothesis) was not observed (except for some perennial Fabaceae species). Thus, grazing appears to be a suitable measure to inhibit ruderalisation. Most sand-specific, endangered species are not grazed and thus are able to spread. The analysis of plant tissue has shown that sheep prefer nitrogen-rich plant species (Fabaceae) and plant parts > 2 % N. Plant parts containing < 1% N are avoided. This may enhance oligotrophication of ruderalised habitats and alter ecosystem functions.