Langfristige Veränderungen von Flora und Vegetation des Grünlandes in der Dümmerniederung (Niedersachsen) unter dem Einfluss von Naturschutzmaßnahmen

Abstract

Wet grasslands represent endangered ecosystems which since the 1950ies have severely decreased in area and habitat quality, especially in the fen areas of northern Germany. Since the 1980ies, large efforts have been undertaken to restore these grasslands. These measures are considered as very difficult because of the irreversible degradation of the peaty soils by melioration and intensive agricultural usage. The effects of re-wetting including temporal inundation and of the impoverishment in nutrients on wet grasslands have been extensively studied. There is also vast experience about the effects of experimental management techniques. However, there is a lack of long-term studies spanning 15 years or more. The surroundings of lake Dümmer (western Lower Saxony) have been rich in wet grasslands. Here, large vegetation surveys were made around 1950 just before the start of melioration, as well as during the period of intensive agricultural use and during the still ongoing process of restoration since the early 1990ies. These vegetation data are accompanied by land use data, mappings of target species and permanent plot surveys conducted since the mid 1990ies. On the whole, these data are unique for Germany and allow an examination of the restoration-driven successional dynamics under changing environmental conditions as well as of the development of species richness. These results also allow us to evaluate the possibilities and limits of large-scale restoration of species-rich wet grasslands. The data analysis included spatial analysis with a Geographic Information System, phytosociologial table work, ordination of vegetation plots with Detrended Correspondence Analysis and an ecological interpretation with the help of Ellenbergs indicator values. Calthion meadows and Agrostis canina grasslands were the most common vegetation types around 1950, but severely decreased after melioration. Molinion meadows and Nardetalia communities completely disappeared. Restoration management caused a pronounced change in vegetation: arable land, new-sawn grasslands and Elymus repens grasslands disappeared, while flood grasslands and Carex acuta sedge fens increased in abundance. Reeds with Phalaris arundinacea and Glyceria maxima almost developed periodically. The ongoing nutrient impoverishment allowed Agrostis canina grasslands to re-settle, resulting in an increase of species typical for small-sedge fens. Differences in site factors, especially in nutrient contents, between the present and historical conditions appear to have considerable effects on the distribution of many species, but are less obvious in the actual distribution of certain vegetation types. Nearly all plant species that are listed in the Red data book of Lower Saxony and that were noticed in the study area in recent times as well as the target species of conservation increased in abundance and spread out over large parts of the study area following restoration. Some highly rare species which were extinct in the study area and its surroundings for many years re-established most likely via long-distance dispersal, namely Bromus racemosus, Dactylorhiza incarnata and D. majalis. The occurrence of target species was positively influenced by a high level of re-wetting and nutrient impoverishment, whereas land use proved to be less important. Most species appear to be able to deal with either mowing or grazing, or a combination of both. Only few target species like Oenanthe fistulosa and Senecio aquaticus that need a comparatively high level of nutrients showed a slight decrease in abundance during recent years. The populations of Caltha palustris stabilized in size and were not affected negatively by long-time inundation. Finally, some species which are problematic for land use increased in abundance in response to restoration, especially Deschampsia cespitosa and the toxic plant Equisetum palustre. The species richness of the wet grasslands has shown a steady linear increase, and there are no indications for a weakening of this trend. Heavily degraded peaty soils showed significantly lower species numbers than less degraded peat. In conclusion, extensive restoration efforts cannot guarantee success in re-establishing species-rich wet grasslands with endangered target species, but in this study they had strongly positive effects on total species richness and on the abundances of most target species. Main factors of success are large-scale, consequent impoverishment, ongoing usage of the grasslands and an adjustable rewetting. The level of peat degradation was determined as a major limit of restoration success. A low level can enable the restoration of more or less species-rich grassland and a recolonisation of a couple of target species, a lot of them endagered.