Human-aided translocation of individuals within the species' range, assisted gene flow (AGF), has been suggested as a climate change mitigation strategy, especially for foundational species, such as forest trees. The benefits and risks of AGF largely depend on the genetic divergence between host and donor populations, their rate and direction of hybridization, and the climate distance that the transfer involves. In this study, we explored the use of Oriental beech (Fagus sylvatica subsp. orientalis), growing from Iran to the Balkans, for AGF in populations of European beech (F. sylvatica subsp. sylvatica), which grow throughout Europe and are increasingly affected by climate warming. Using 16 microsatellite loci and samples from 13 and 6 natural populations of Oriental and European beech, respectively, we identified 5 distinct genetic clusters in Oriental beech with a divergence (FST) of 0.15 to 0.25 from European beech. Using this knowledge, we tracked the origin of 11 Oriental beech stands in Western Europe, some established in the early 1900s. In two stands of Greater Caucasus origin, we additionally genotyped offspring and found evidence for extensive hybridization, with 41.3% and 17.8% of the offspring having a hybrid status. Further, climate data revealed a higher degree of seasonality across the Oriental beech growing sites than across the planting sites in Western Europe, with some sites additionally having a warmer and drier climate. Accordingly, in one of these stands, we found evidence that bud burst of Oriental beech occurs four days earlier than in European beech. These results suggest that AGF of Oriental beech could increase the genetic diversity of European beech stands and may even help the introgression of variants that are more adapted to future climatic conditions. Our study showcases an evaluation of the benefits and risks of AGF and calls for similar studies on other native tree species.

The publication of Liang et al. (2016, Science) seems to demonstrate very clearly that increasing tree species richness substantially increases forest productivity. To combine data from very di erent ecoregions, the authors constructed a relative measure of tree species richness. This relative richness however confounds plot-level tree species richness and the polar-tropical gradient of tree species richness. We re-analysed their orginal data, computing a regional measure of tree species richness and addressing several other issues in their analysis. We nd that there is virtually no effect of relative tree species richness on productivity when computing species richness at the local scale. Also, di erent ecoregions have very di erent relationships between tree species richness and productivity. Thus, neither the "global" consistency nor the actual e ect can be con rmed. F Conclusion 33Bibliography 33

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