Genetic analysis of European common ash (Fraxinus excelsior L.) populations affected by ash dieback

European common ash is an important component of mixed forest ecosystems in Bavaria and is considered a valuable tree species under climate change. The first aim of the present study was to assess the genetic diversity within and between ash populations in Bavaria in comparison with neighboring regions. Because ash stands have been heavily attacked by ash shoot disease in the last few years, the second aim of the study was to detect genetic differences between susceptible and less susceptible trees (trees with defoliation up to 30%) within populations. Altogether 41 populations were investigated using nuclear and chloroplast microsatellites. The results showed high genetic variation within and high genetic differentiation between ash populations. Higher chloroplast microsatellite variation was detected instead populations from south-eastern Bavaria compared to other regions. The comparison of susceptible and less susceptible groups of individuals within each population revealed medium to high genetic differences in some cases. For the observed heterozygosity, higher values were found for the group of less susceptible trees compared to the group of all trees or to the group of susceptible trees within populations. This could be a first indication that individuals with a higher heterozygosity might be able to withstand ash dieback in a better way compared to homozygotic individuals. Within the group of less susceptible trees a relatively wide genetic base exists whereupon a future breeding programme can be built. Thus it is of utmost importance for the species to maintain less affected trees in order to keep the genetic potential for future adaptation processes within ash populations, seed production and regeneration.