Bidirectional gene flow between Fagus sylvatica L. and F. orientalis Lipsky despite strong genetic divergence

Fagus sylvatica L., one of the most common Central European deciduous tree species, is one of the species globally suffering from changing and stressful environmental conditions. It has shown symptoms of crown damage, a reduction in growth and increased mortality following recent severe droughts. Fagus orientalis Lipsky, a closely related species with higher drought tolerance, originates from southeastern Europe, Turkey, the Greater Cau-casus region, and the Hyrcanian forest. For Germany, F. orientalis has been proposed as an alternative to F. sylvatica by translocation of pre-adapted planting material as a tool to mitigate negative effects of climate change in certain locations. This approach can be beneficial but might also harbor risks. We studied admixtures between the two beech species and the direction of gene flow, taking advantage of F. orientalis trees planted over 100 years ago in the forest district of Memsen, Germany. Additionally, we used a range-wide dataset of F. sylvatica and F. orientalis provenances to determine the origin of the introduced trees. Using a combination of nuclear microsatellites and one chloroplast marker with species-specific variants, we showed that interspecific gene flow was going in both directions. In most cases, F. sylvatica was the pollen donor, likely due to its greater abundance and production of vast amounts of pollen. The planted trees from Memsen originated from the Greater Caucasus region and showed strong genetic divergence from German F. sylvatica populations, clearly differing in leaf morphological traits. In the future, the suitability of different provenances for the mitigation of climate change impacts should be carefully assessed by testing gene flow patterns as well as hybrid performance in additional stands and in comparison to F. sylvatica provenances from southern Europe.