Upper Rhine Valley: A migration crossroads of middle European oaks

1 The indigenous oak species (Quercus spp.) of the Upper Rhine Valley have migrated to their 2 current distribution range in the area after the transition to the Holocene interglacial. Since 3 post-glacial recolonization, they have been subjected to ecological changes and human 4 impact. By using chloroplast microsatellite markers (cpSSRs), we provide detailed 5 phylogeographic information and we address the contribution of natural and human-related 6 factors to the current pattern of chloroplast DNA (cpDNA) variation. 626 individual trees 7 from 86 oak stands including all three indigenous oak species of the region were sampled. In 8 order to verify the refugial origin, reference samples from refugial areas and DNA samples 9 from previous studies with known cpDNA haplotypes (chlorotypes) were used. Chlorotypes 10 belonging to three different maternal lineages, corresponding to the three main glacial 11 refugia, were found in the area. These were spatially structured and highly introgressed 12 among species, reflecting past hybridization which involved all three indigenous oak species. 13 Site condition heterogeneity was found among groups of populations which differed in 14 terms of cpDNA variation. This suggests that different biogeographic subregions within the 15 Upper Rhine Valley were colonized during separate post-glacial migration waves. Genetic 16 variation was higher in Quercus robur than in Quercus petraea, which is probably due to 17 more efficient seed dispersal and the more pronounced pioneer character of the former 18 species. Finally, stands of Q. robur established in the last 70 years were significantly more 19 diverse, which can be explained by the improved transportation ability of seeds and 20 seedlings for artificial regeneration of stands during this period.