Effects of water regime and agricultural land use on diversity and species composition of vascular plants inhabiting temporary ponds in northeastern Germany Auswirkungen von Wasserstand und landwirtschaftlicher Nutzung auf Diversität und Artenzusammensetzung höherer Pflanzen an temporären Gewässern in Nordostdeutschland

Fluctuations of the water level at the edges of temporary water bodies provide favourable living conditions for annual plant communities of the phytosociological class Isoeto-Nanojuncetea. Such communities of periodically flooded ponds within the agricultural landscape of NE Germany are particularly rich in rare plant species of that class. During the past decades drainage, fertilisation and herbicides in the surrounding arable fields have led to a severe decline in diversity of these species. To develop efficient conservation strategies it is essential to understand the factors driving the species composition. Therefore, we studied how varying water regimes, soil properties and agricultural practices affect the diversity and species composition of these temporary ponds. The study was carried out in seven ponds on a conventionally managed farm in NE Brandenburg. At each of these wetlands mixed soil samples were taken to determine the pH, total nitrogen and phosphorus concentration. The plant species were recorded in 177 plots, each covering 1 x 1 m². For each plot, the water level was recorded in April, July and August 2013, respectively, resulting in five ‘water level regimes’. Total species number and percentages of Isoeto-Nanojuncetea species were determined per plot, to evaluate water level effects on the vegetation. In addition, mean Ellenberg indicator values for light, moisture and nutrients were calculated to assess the environmental conditions. Kruskal-Wallis tests with subsequent multiple comparisons showed significant differences between water regimes in both total species richness and percentage of Isoeto-Nanojuncetea species. IsoetoNanojuncetea species established best at sites with fluctuating water levels. Fitting environmental variables to NMDS ordination axes indicated that the water regime was the most significant factor for floristic variation. In plots with a more uniform water regime the species composition was explained mainly by the soil fertility. Farming practices showed less pronounced effects on species composition. Our results suggest that maintaining water regimes with fluctuating water levels and keeping nutrient content low are crucial for the successful conservation of Isoeto-Nanojuncetea communities in arable landscapes.