Distribution, hybridisation and morphological variation in Alnus rohlenae (Betulaceae) an endemic species of the Balkan Peninsula

The diploid A. glutinosa (L.) Gaertn . is widespread throughout the European continent, except in the Iberian and Balkan Peninsulas where tetraploid populations have been discovered. We focused on the tetraploid species described as A . rohlenae Vít, Douda and Mandák that occupies the western part of the Balkan Peninsula, where it has likely completely replaced the diploid species. While the distribution range of the diploid A . glutinosa s. str. is well known, the exact distribution range of the tetraploid A . rohlenae is unknown. Here, we report the first exact distribution of the tetraploid A. rohlenae and the anticipated hybrid zones in which it is in contact with diploid populations using flow cytometry and morphometrics. Tetraploids are located primarily in the mountainous parts of the study area and towards the lowlands are gradually being replaced by diploids, forming a contact zone. We compare the main morphological characteristics of both species. Due to the geographical proximity of the study species, the morphological differences between them are clear outside the contact zones. However, within the contact zones, we recorded hybridisations that obscure the morphological differences between species, probably due to the presence of triploid hybrids.