Final Activity and Management Report Summary - BIWACLIM (Evaluating the effects of species composition and biodiversity on ecosystem water fluxes in a changing climate) Oxygen isotope composition (?18O) of plant materials responds to environmental and physiological change. When analysed in biological archives such as tree ring cellulose, ?18O signals in plant materials have therefore been suggested to be a potentially powerful tool that can be used to assess plant physiological or environmental change over time. However, several key uncertainties remain in the interpretation of ?18O signals in plant materials, which has prevented the general application of ?18O in ecological studies. The main goal of this Marie Curie Activity was to address these uncertainties using a combination of controlled experiments and model-based simulations. With this work, I was able to show that ?18O values in plant materials cannot be used for cross-species assessments of plant water relations e.g. transpiration as had been previously often hypothesized. Instead, the research that I was able to perform within the framework of the Marie Curie Fellowship showed that ?18O values of plant cellulose originating from a single species - such as cellulose in tree-ring time-series - can be integrated into a single environmental index of atmospheric drought - the leaf-to-air vapour pressure difference (VPD). The implications of this finding for both paleoclimatic and ecosystem research are substantial because VPD exerts strong controls on a plant's physiological performance it will also be a pivotal determinant of an ecosystem's carbon and water balance. Using ?18O values in plant material such as tree-ring time-series as a proxy for VPD has therefore an enormous potential for understanding temporal and spatial variation of one of the key environmental drivers that influences the biogeochemical dynamics of ecosystems be they past or present.
