Establishing a semi-natural grassland: Effects of harvesting time and sowing density on species composition and structure of a restored Arrhenatherum elatius meadow

Species-rich semi-natural grasslands play an essential role in the conservation of European biodiversity. To restore them, existing grasslands may be used as a seed source. A key factor for successful restoration is the transfer of all species of the plant community to the site being restored. This approach, however, often poses practical problems due to variations in species phenology, so that only time-staggered harvesting allows the seeds of many species to be collected. Poor harvesting methods may reduce the number of species transferred from the donor to recipient site. The effects of harvesting at various time points by various methods were evaluated here in a restoration experiment on an ex-arable field in Northern Italy. Propagation materials from the first or second regrowth (or both) of a meadow dominated by Arrhenatherum elatius were collected by four harvesting methods. The materials were spread at the sowing density between 830 and 14360 seeds m2. Species composition and structure of the vegetation were examined during 6 years. Untransferred species were almost exclusively those not present at harvesting as mature seed. Compared with materials from one regrowth period, materials from both regrowth periods significantly increased the number of transferred species per plot (26.5 vs. 28.5, respectively) and the absolute transfer rate (64% vs. 75%). Higher sowing density yielded a greater number of positive than negative effects. It favored stable establishment of donor site species and significantly reduced the presence of weeds. Nonetheless, due to the initial dominance of species with high sowing density, evenness of the restored plots was lower than that at the donor site but increased with time, at least under low- and medium-sowing-density conditions. Multiple harvesting at time points when several species with mature seed are present increases the species transfer rate. Medium sowing density positively affects restoration development because it promotes the establishment of donor site species with lower sowing density, prevents species with the highest density from dominating the vegetation during the first few years, and reduces weed cover.