Vortrag, DGL-Jahrestagung 2014, Madgeburg: 29.09.2014 - 03.10.2014
Crenal ecosystems like springs feature an island position in the terrestrial and aquatic environment according to their local abiotic constancy under pristine conditions. This includes constancy in water supply, temperature and nutrient availability. By providing rather unique habitats, springs are inhabitied by highly specialized communities. In comparison with streams and forest floors in their vicinity, such communities are also very diverse. Due to their narrow ecological requirements, species assemblages are expected to be vulnerable to antropogenic changes of the environment. Severe environmental impacts have stressed Central European ecosystems during the last decades. First of all, the emission of acidifying pollutants, which culminated in the 1970s and 1980s, resulted in a strong acidification of soils and in consequence also of springs and headwaters with violent repercussions in terrestrial and aquatic systems. The legal framework in the 1980s in combination with the German reunification and the subsequent substitution of outdated technology strongly reduced acid depositions and, thus, acidification stress of Central European ecosystems. Nevertheless, the still detectible effects of acidification in combination with current climate warming are likely to be still effective on crenal communites. In recent years and in the near future climatic changes are developing additional ecological pressures. However, little is known about the combined effects of acidification and climate change on the community structure of crenal habitats. In this study, we investigated large-scale and long-term effects of modified environmental conditions on crenal plant communities in the lower mountain ranges of Central Europe. The study is affected at the landscape scale and covers a period of 24 years. We analysed spatial and temporal patterns of dominance relations and compositional community turnover using realized niche space and generalized dissimilarity modelling based on spring hydrochemistry. Spatial patterns were analysed for 235 springs, investigated at the start of the monitoring in 1989. Temporal patterns were analysed for a subset of 102 springs, monitored from 1989 to 2013. Water temperature and acidity regime act as the major drivers of helocrenic spring plant community composition in Central European mountain ranges. This is found to be true for spatial patterns but also for temporal trends. Spatial turn-over in community composition was explained best by water temperature followed by paramaters related to the acidity regime (Al, Cd, Ca and Mg). Also long-term, compositional turnover was explained best by the variation of pH, parameters related to acidification (Al, Ca, Mg and Fe) and water temperature. In consequence, the combined anthropogenic impact of both, acidification and climate warming can alter species occurrence patterns and community assemblages of crenal habitats. However, uncertainty exists about the speed of adaptation due to spatial isolation and knowledge is missing on species dispersal and persistance under suboptimal conditions. Our approach is aiming to disentangle the interactions of abiotic and biotic changes by using Central European helocrenic springs as a model ecosystem.