Climate change may endanger not only yield and fulfilling the social functions of European forests, but even the survival of several tree species. The study emphasises the complexity of climatic factors and physiological mechanisms, which may potentially endanger the persistence of tree populations and which cannot be reduced to problems of drought and temperature increase. A substantial inter-population variation in traits associated with the response to climatic stress, observed in provenance experiments, is a prerequisite for the choice of proper forest reproductive material (FRM) in reforestation as a strategy of climate-change mitigation. Assisted migration, i.e., transfer of FRM from source regions, currently characterised by such climate characteristics, which are expected in the target regions in the future, requires knowledge of key stress factors (depending on the climate scenario), physiological processes associated with the adaptation to this stress, identification of genes and eventually epigenetic mechanisms, controlling adaptation processes, and finally mapping of genetic and/or epigenetic variation in key genes. For most tree species, such information is not yet available. Therefore, assisted migration under such information uncertainty needs to be complemented by in situ gene conservation measures to preserve the possibility of reversing the effects of eventual erroneous decisions on FRM transfer.