Triple isotope analyses of carbon, oxygen, and hydrogen in plant material provide retroperspectively information on plant functional responses to changing climatic conditions. Applied on tree-ring archives, this allows reconstruction of climatic conditions and functional responses of the past and helps to predict trajectories of tree functioning in future. During this project, I will enhance the potential of triple isotope analysis by (1) developing new methods for hydrogen isotope analysis, (2) determining leaf-level hydrogen isotope fractionation processes in assimilates (i.e. sugars and starch), (3) refining source-to-sink isotope fractionations, and (4) applying the new methods and knowledge on large tree-ring data sets from Europe and Siberia. Ultimately, the project aims to provide novel mechanistic ways to reconstruct functional responses of trees in high temporal and spatial resolution by improving and combining isotope fractionation models of different elements.