The number one question in ecology is why certain organisms occur where they do, and what the traits are which make them successful. This project aims at arriving at a functional explanation of the climatic limits of major European broad leaved tree taxa. It will focus on and explore their temperature-related limits and aims at reviving Europe's traditional strength in physiology based ecology by training a group of young scientists to answer such questions. The project builds upon many years of the PIs experience in mechanism oriented ecology (e.g. synthesis in Körner 2003) and should help trading those rapidly disappearing skills to a next generation of experimental ecologists. The project adopts a three-step approach: (1) Assess the current extreme postions of tree taxa along thermal gradients, using existing data bases and site visits (data mining, biogeography). (2) Associate those patterns with bioclimatic information, both available and newly acquired (climatology). (3) Empirically test hypotheses of causes of growth limitation and stress survival, both in the field and in the laboratory (eco-physiology). The project will account for ecotypic differentiation by using the uppermost (marginal) and central (optimal) positions of taxa and will explore plant establishment as well as adult plant performance. It will use in situ measurements, transplant and common gardens as well as phytotron testing. Genotypic control of phenology, frost hardiness, thermal constraints of growth and reproduction (fitness) will play a central role. The results will, for the first time, offer a mechanistic (rather then correlative) explanation for broad leaf tree species distribution in Europe and thus, will provide a basis for improved parameterization and evaluation of species distribution models in a climate change context.