A subgeneric classification of Fagus (Fagaceae) and revised taxonomy of western Eurasian beeches

We propose a long-overdue subgeneric classification of Fagus and revision of its western Eurasian taxa based on population- level sampling of morphological and molecular data. The molecular sequence data bolstering this classification derive from nuclear-encoded genetic markers. Fagus subg. Fagus comprises twelve species, of which two occur in North America, four in western Eurasia, and six in East Asia. Fagus subg. Englerianae Denk & G. W. Grimm comprises three East Asian species. Application of a new species concept using fine-scale data justifies the recognition of previously overlooked cryptic taxa. Beech trees from western Eurasia are currently treated as one species, F. sylvatica L., with two subspecies. Despite this, several previous studies pointed out distinct differences between beech populations from Europe, Turkey, the Caucasus region, and the Hyrcanian forest region including the Talysh Mountains of Azerbaijan and the forests south of the Caspian Sea in Iran. All newly generated data indicate that the various Oriental beeches reflect divergences and speciation processes predating the split between the European beech and its eastern siblings. Therefore, based on morphological and molecular differences, we recognize four species in western Eurasia, the European F. sylvatica L., the chiefly northwestern Turkish F. orientalis Lipsky, the Caucasian F. hohenackeriana Palib., and the Hyrcanian F. caspica Denk & G. W. Grimm. Diagnostic genotypic characteristics are included in the descriptions