Das Projekt "Effect of experimentally applied drought and warming stress on three oak species and provenances using C and O stable isotopes in leaves, shoots, stem and roots" wird vom Umweltbundesamt gefördert und von Eidgenössische Forschungsanstalt für Wald, Schnee und Landschaft, Eidgenössisches Institut für Schnee- und Lawinenforschung durchgeführt. The predicted climate change will have a profound effect on ecosystems in Switzerland, including forests. Tree species, for example oaks, which are more adapted to warmer and dryer conditions are expected to profit from the changing climate. Models predict a large increase in the distribution of oaks that are currently covering only 2Prozent of Swiss forests. Unfortunately, no long-term trials exist in Switzerland that could be used to test for climate sensitivity of the various oak species. At the Swiss Federal Institute WSL, therefore, an experiment in 16 open-top chambers was carried out to test how the three native oak species react to drought and warming. Regarding drought tolerance and temperature preference, the natural distribution suggests that Q. pubescens is the most drought-tolerant, followed by Q. petraea and Q. robur. The warming treatment is achieved by selecting two opening angles of the chamber window. Under less opened windows air and soil surface temperatures increases by 1 to 3 C. Automatically closing roofs exclude rain. An artificial drought treatment is achieved by reducing the mean local precipitation from April to October (600 mm) to less than half. While the control and warming treatment, receive water in regular intervals, the drought treatment and the warming and drought combination are only periodically watered. To prevent irreversible damage under drought stress trees usually close the stomata to minimize water loss via transpiration. This, however, reduces the net photosynthesis and thus the carbon fixation. The potential to optimize between water loss and carbon fixation is a key in understanding tree responses to climatic change. Besides stomata closure, photosynthesis is determined by other factors, including temperature. Both, stomata closure and photosynthetic capacity affect the fractionation of the stable carbon and oxygen isotopes. Therefore, the determination of the isotope ratios ?13C and ?18O in plant tissue allows to draw conclusion on how drought and warming affect the trees. The following samples are collected and are intended to be analyzed: a) fresh leaves, b) wood section from shoots, c) stemwood and d) root samples. The proposed study with the stable C and O isotopes will be an ideal complement to the other measurements, such as shoot elongation, biomass change, gas exchange, root growth and leaf size and nutrient content. Based on these results, potentially useful oak species and provenances for future seeding or planting can be identified and recommendations for the forest practice can be made. In addition, the derived isotope ratios would allow the identification of drought stressed trees.