Filling up the gaps-Passive restoration does work on linear landscape elements

Open landscapes in many parts of Europe have been negatively affected by large-scale drainage and amelioration to support agricultural production. In continental alkali grasslands, amelioration and establishment of drainage ditch systems were typical in the 1950s and 60s. Drainage ditches caused a considerable fragmentation and degradation of natural grasslands; thus several projects aimed at eliminating these linear landscape elements. In a multi-site study, we explored the drivers of grassland recovery after soil-filling of drainage ditches in landscape-scale restoration projects in Hortobágy National Park, East-Hungary. Ditch embankments, formerly built from the excavated soil, were used to fill the 8-m wide ditches and grazing was applied to facilitate the recovery of grasslands similar to the surrounding matrix. Three age classes were selected for the study: 1-, 6- and 8-year-old filled ditches; with nine sites per age group, surrounded by three grassland types (27 ditches in total). We recorded the percentage cover of vascular plant species in 18 plots per ditch, 486 plots in total. We found that the species pool of the filled ditches became more similar to the reference grasslands with increasing successional age and increasing distance to the central zone of the ditches regardless of grassland type. Species richness of the filled ditches became more similar to that of the reference grasslands with increasing successional age. However, we found that several target species, especially salt-tolerant pioneers, could establish even in the first year. Grassland recovery was most successful in sites adjacent to dry grasslands characterised by soils with high salt content, which favoured specialist species and suppressed non-target species. Cover of non-target species was higher in wet meadows with moist, nutrient-rich soils which favoured generalists and non-target species. Our study revealed that passive restoration after soil filling of disused drainage ditches can effectively support grassland recovery even within less than ten years, when restoration sites are surrounded by natural grasslands.