Genetic diversity and differentiation of introduced red oak (Quercus rubra) in Germany in comparison with reference native North American populations

Northern red oak (Quercus rubra) was introduced to Europe in the late seventeenth century and has since become the most important deciduous non-native tree species in Germany. Despite its importance, little is known about the origin and patterns of genetic variation in German red oak stands. To be able to make recommendations regarding the adaptive potential of red oak stands, which might be related to their origin and the selection of provenances, with respect to climate change, a better understanding of the genetic diversity and structure of German red oak stands is needed. Individuals from 62 populations in Germany and North America were genotyped at five chloroplast microsatellite loci to characterize chloroplast haplotype diversity and geographic structure. Compared to reference populations from the natural distribution range, German red oak stands demonstrated a relatively low genetic differentiation among populations and represented only a fraction of the haplotype diversity found in North America. For several stands located in southern Germany, considerably higher haplotype diversity than in other German stands was found. While most German stands showed signatures of founder effects, the diversity of stands in southern Germany might have been increased due to admixture and multiple introductions of different North American provenances. Overall, we conclude that German stands originated from a limited geographic area, possibly located in the northern part of the native distribution range.