Landscape change and impoverishment in North German grasslands since the 1950s

Landscape and agricultural management has changed in Central Europe dramatically since the large-scale intensification processes (e.g. meliorations, synthetic fertilizer and herbicide use) started in the 1950/60s. Today, most croplands and remaining grasslands are used very intensively and many former grassland areas were converted to cropland resulting in an overall decline of grassland area in Germany. The reverse case of converting cropland to grassland is a regional phenomen on, e.g. in the uplands or initiated by compensation measures, but the species composition of the recent grasslands differs from continuously managed grasslands. To detect melioration effects on phytodiversity, landscape structure and the fragmentation of the remaining grasslands and species composition it is necessary to analyse long-term data dating back to the onset of major agricultural industrialisation in the middle of the 20th century. Floodplain grasslands (wet and mesic species-rich grassland) were common and formerly widespread in the 1950/60s, but nowadays are threatened in Central Europe. There are no replicated studies at multiple locations that refer to the time before agricultural intensification started, and historical and spatially explicit vegetation data is rare. Although the extent of grasslands has increased in some upland regions, the ecological quality of these grasslands is arguable. Additionally, species-rich calcareous grasslands on steep slopes are threatened by abandonment (succession) and increasing nutrient supply. The aim of this thesis was to detect landscape changes in six (+ one nature protected) floodplain study sites and one upland study site in Northern Germany by quantifying changes in grassland area, grassland replacement by other habitat types, habitat continuity, fragmentation and their effects on plant species and functional trait diversity as well as species composition. The studies on floodplain sites were based on fine-scale vegetation maps and corresponding vegetation releves from the 1950/60s. In 2008, vegetation mapping of the almost 2.500 ha (+ 293 ha) historical area was repeated and vegetation releves were sampled in the former grassland at stratified randomly selected sampling points. The case study in the uplands of Lower Saxony is based on recent vegetation releves randomly distributed among grasslands covering 394 ha within the 2.500 ha of the study area. Additional environmental data for each releve were surveyed in the field or were extracted from historical, geological and soil maps. The continuity of each grassland habitat was determined using historical and recent topographical maps and aerial photographs. The first study in the floodplain grasslands pointed out that the wet and species-rich mesic meadows in the unprotected grassland sites decreased dramatically by over 80%. They were mainly replaced by species-poor and intensively managed grasslands, arable land or set-asides. Caused by this strong area decline, the fragmentation of remaining wet meadows as well as of species-rich mesic meadows also increased dramatically. The nature protected study site at the Havel River showed relatively small changes in the extent of wet and species-rich mesic grasslands indicating local effects such as fertilisation and drainage, but no overregional factors such as climate warming or atmospheric N deposition being responsible for the detected changes. Our second study in the floodplain grasslands showed that the species richness at the plot-level declined by 30-50%, similar to losses in functional diversity. The species composition of floodplain grasslands changed tremendously in all study regions in the last five to six decades. The Ellenberg indicator values (EIV) for nutrient availability increased over time and the observed differences between the unprotected and the one protected study site support the assumption that the changes were mainly caused by local factors and not by overregional influences. The trend is on-going to species-poor communities which are dominated by mowing-tolerant and competitive grasses in all unprotected study sites. Early flowering and insect-pollinated plants decreased strongly with related effects on the fauna. The third study in the floodplain grasslands connects the dramatic loss of floodplain grassland area and increasing fragmentation with the impoverishment in plant species richness since the 1950/60s. Therefore, a list of 78 characteristic floodplain grassland species was defined from 482 vascular plant species recorded in total and the analysis focussed on these specialist plants. We found a 30-66% diversity loss at the plot-level over time. The tested landscape metrics (total meadow area, mean patch size and landscape proximity index) had no significant influence on the richness of characteristic species at the landscape level. The distance to the nearest suitable habitat of characteristic meadow species had a slightly negative effect on the richness at the plot-level. Ellenberg indicator values for nutrient and moisture showed even for the characteristic meadow species that local management intensity and corresponding habitat deterioration, but not fragmentation, were the driving forces for species shift and species decline. In contrast to the floodplain study site, the grassland area of our representative upland case study doubled since the 1950/60s. Species diversity and species composition differed strongly between six detected grassland types ( 27 spp. per releve), while species-rich grasslands are rare (6% of the grassland area). The number of insect-pollinated plants as well as species richness declined with increasing land use intensity (as indicated by the EIVs for nutrient and mowing tolerance) in accordance with the results of the floodplain grassland studies. Species-rich grasslands only occured on areas grazed traditionally by sheep with a habitat continuity of more than 100 years, whereas medium-rich grassland occured mainly on former arable fields with a habitat continuity of less than 30 years. The presented study showed dramatic losses in the extent of floodplain grasslands. Remaining areas were fragmented and suffered major losses in species and functional trait diversity. The case study in the upland meadows of lower Saxony shows, that the ecological quality of most grassland is low even if the percentage of grassland area is relatively high.