Enhancing plant biodiversity in species-poor grassland through plant material transfer - the impact of sward disturbance

Questions: Is the transfer of plant material a feasible measure for the enrichment of species-poor alluvial grassland? Which is the proper degree of grass sward disturbance to enhance germination and establishment of plant material species? Does the establishment success depend on soil nutrient status of the restoration sites? Location: Northern Upper Rhine valley, southwest Germany. Methods: Seed-containing plant material was transferred to species-poor meadows. We tested two methods of sward disturbance: rotovation or ploughing before the transfer of plant material vs. a control, i.e. no disturbance but transfer of plant material. We studied vegetation development on nine sites for 3 yr. Results: Three years after the transfer of plant material, 101 species were successfully re-established on the restoration sites, including 28 Red List species. In contrast to our expectations, establishment was similar on rotovated and ploughed plots, both harbouring higher numbers and abundances of plant material species than the control plots. In contrast, the cover of resident grassland species differed significantly between ploughed plots (lowest cover), rotovated plots (intermediate cover) and control plots (highest cover) in the last 2 yr of vegetation recording. While elevated phosphorus was in line with reduced establishment of plant material species, potassium and total nitrogen had no negative effect. Conclusions: The transfer of plant material is an appropriate method to enhance plant biodiversity in species-poor grasslands. We show that a sward disturbance by rotovation or ploughing is crucial for species to re-establish. Our results suggest that ploughing is the more promising disturbance measure because it more persistently suppresses grassland vegetation.