Hyrcanian forests—Stable rear-edge populations harbouring high genetic diversity of Fraxinus excelsior, a common European tree species

Aim The Hyrcanian forests, in a region of lowland and montane temperate pure and mixed broadleaf forests located in Iran, near the southern shores of the Caspian Sea, form part of the Caucasus biodiversity hotspot. In this region, species experienced suitable and stable environmental conditions over historic periods and even some Arcto‐Tertiary relict species could survive. Although the ranges of several European tree species expand to the Hyrcanian forests, its role has mostly been overlooked in phylogeographic studies so far. Here, we used common ash (Fraxinus excelsior) to study the genetic diversity, population genetic structure and time of divergence between European and Hyrcanian populations. Location Six populations from the Hyrcanian forests at the southern shore of the Caspian Sea (Iran) and three selected populations in Europe (Norway, Denmark and Italy). Method We amplified four genomic, seven genic and four plastid SSR (simple sequence repeat) markers in 268 Fraxinus excelsior samples. Results In particular, EST‐SSRs (expressed sequence tag–SSRs), that is genic markers, showed significantly higher genetic diversity in the Hyrcanian forests than in European populations. Population divergence between European and Hyrcanian populations dated back to the end of the middle to upper Pleistocene. A recent reduction in population size was detected in all study populations. Within the Hyrcanian region, a substructure was detected at nuclear and plastid SSRs, with a western to central and a smaller eastern subcluster. Two new plastid haplotypes were described in the easternmost Hyrcanian populations. Main conclusions Our results confirm that the Hyrcanian forests harbour high genetic diversity that might be of great value for the evolutionary potential of Fraxinus excelsior currently facing global climate change and ash dieback. We argue that the Hyrcanian forests could be an important genetic reservoir also for other European tree species.