Upper Rhine Valley: A migration crossroads of middle European oaks

TL;DR

This study investigates the post-glacial migration, hybridization, and genetic diversity of indigenous oak species in the Upper Rhine Valley using chloroplast DNA markers, revealing multiple refugia, hybridization, and recent increases in genetic diversity due to human activities.

Contribution

It provides detailed phylogeographic insights into oak migration patterns and hybridization in the region, highlighting the impact of human activities on genetic diversity.

Findings

Three maternal lineages corresponding to glacial refugia were identified.

Hybridization among oak species was widespread and reflected in chloroplast DNA patterns.

Recent human activities have increased genetic diversity in Q. robur stands.

Abstract

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