Carpinus austrobalcanica – a new highly polyploid species from the Balkan Peninsula closely related to European hornbeam

The Balkan Peninsula was the primary glacial refugium for many temperate tree species and contributed overproportionally to the postglacial recolonisation of central and eastern Europe. This is also the case for Carpinus betulus, the European hornbeam, whose main glacial refugium was in the Balkans. During our fieldwork in the southern Balkans, we discovered trees with a morphology similar to that of C. betulus, but differing in their rough, highly rigged bark and the shape of their fruit involucre. The aim of this study was to investigate the evolutionary origin and differentiation of these morphologically distinct populations of C. betulus from the North Pindus Mountain range in northern Greece and southern Albania using an integrative approach. Our study combined phylogenetic analyses of plastid and nuclear internal transcribed spacer sequences, amplified fragment length polymorphism fingerprinting, relative genome size estimation, and multivariate morphometric analyses. After establishing the genetic and morphological divergence of the aforementioned populations, we described them as a new species, Carpinus austrobalcanica D.Lakušić, Kuzmanović, Stevanoski, Schönsw. & Frajman, sp. nov. We provide diagnosis, description, geographical distribution, and conservation status of this enigmatic newly described tree species locally endemic to the southern Balkans.