Research addressing the effects of global change on ecosystem ecology has mainly focused on carbon cycling, while the consequences for ecosystem water pools and fluxes have received little attention. Given the important regulating role of the vegetation for ecosystem hydrology it is hypothesized that global change will not only affect ecosystem water relations directly (e.g. by altered precipitation patterns) but also indirectly through changes in plant water demand or altered plant species composition.The main goal of the proposed work will be to act here and to address the effects of species composition or biodiversity for ecosystem water pools and fluxes in a changing climate. In addition to established ecophysiological tools, a particular emphasis will be on the use of 18O isotopes in ecosystem water pools and fluxes to address the main goal of the study. 18O isotopes have been suggested as an indispensable tool for the understanding of ecosystem hydrology that allow a much deeper insight into patters and processes than conventional methods. Despite the high potential, however, uncertainties remain in the interpretation of d18O signals that have prevented the general application of d18O in ecological studies.In the outgoing phase of the proposed activity, these uncertainties will be addressed in experiments and observational studies. In particular, the potential of d18O in leaf water of different plants as an integrated measure of the plant's transpiration as well as the use of d18O in ecosystem water vapour to partition ecosystem evapotranspiration into the component fluxes for the validation of SVAT models will be evaluated. The improved understanding of d18O signals will finally be applied during the return phase of the activity to an ongoing climate change experiment in European grasslands to address the main scientific goal of the proposal, the effects of species composition and diversity on ecosystem water relations in a changing climate.