Abstract
The grasslands on slopes in Southern Limburg belong to the most species-rich grasslands in the Netherlands and are included in EU Habitat Directive as protected habitat types. They are located on slopes with a clear gradient in soil material with gravel deposits from the river Maas on top of the
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calcareous bedrock (especially in the western part of Limburg). In the 20th century, the number and area of these grasslands decreased due to changing land use and the small, remaining reserves became fragmented, isolated, and lost characteristic species. Around 1980 restoration management started. In the first years, restoration seemed successful, but on the long-term however, the floristical quality and species-richness have declined even further. Further insight into the causes of this decline and opportunities for restoration were the main aims of a O+BN research project and the current thesis is derived from this research. Analyses of the long-term vegetation development in calcareous grassland (Chapter 2) showed that distinct calcareous grassland species declined and more common grassland species increased. Generally, the vegetation has developed towards more nutrient-rich circumstances. Eutrophication is one of the main factors hampering restoration and nitrogen is one of the most important limiting nutrients for terrestrial plant growth, especially under nutrient-poor circumstances. Experimental studies confirmed repressed potential nitrification activity in the matgrass sward vegetation, compared to the two adjacent vegetation zones (Chapter 3) and a four-month growth experiment revealed a negative effect of characteristic matgrass sward species on potential nitrification activity (Chapter 4). High ammonium concentrations and high ammonium:nitrate ratio are known to have toxic effects on the characteristic plant species, and therefore this can have contributed to the decline of this species-rich grassland vegetation. Restoration management for the accelerated re-creation of species-rich grasslands was studied on agricultural grasslands with a history of intensive use in an experimental design (Chapter 6) and on grasslands that have been abandoned for a long time (Chapter 7). The combination of sod cutting with hay transfer (the transfer of freshly mown material from a nearby well-developed grassland) was the most successful measure. In calcareous grasslands that had been fertilized in the past, the development on the longer term (25 years after the fertilisation has stopped) showed that the effects of nitrogen application have disappeared after 10 years, whereas the effects of phosphate application are still visible (Chapter 5). The results of this thesis and additional results of the O+BN research show that two approaches are needed to ensure optimal conservation and restoration. Firstly, the management in the reserves should be optimalized to ensure more nutrient removal. Secondly, the dispersal opportunities for species should be increased by enlarging and connecting the remaining grasslands, therefore reducing fragmentation and isolation. On the short term, sod-cutting in combination with the transfer of mown material seems to be a useful first step towards restoration on former agriculturally used grasslands. Additional research on the found repressed nitrification should reveal if this effect has a negative effect on the biodiversity in these grasslands and how this can be counterbalanced
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