Ambiguous leaf morphology of Central European white oaks and their hybrids in an atypical mixed stand.

Quercus pubescens on its only stand in Poland, in the Bielinek Reserve, was for years treated as atypical, being an effect of hybridizing with co-occurring Q. petraea and Q. robur. In the study, the leaves of these three species and their hybrids in Bielinek were characterized with 20 macro-morphological and eight micro-morphological traits, and statistically compared with Q. pubescens from Italy. Genetic assignments revealed a low number of hybrids. Quercus pubescens leaves from Bielinek and Italy were visually similar, the significant differences between them concerned mainly the size of the leaves. It was accepted that the characteristics of Q. pubescens from Bielinek were within the range of species variability and were similar to the doubtful species Q. virgiliana (Q. pubescens s.l.). Differences between Q. pubescens from Bielinek and Italy could have resulted from the acclimatization to the local conditions, or origination form a different refugium, and long-term isolation. The group of Q. petraea leaves joining morphological features of Q. petraea and Q. pubescens was detected, blurring the morphological differences between species and impeding the recognition of hybrids. This phenomenon is explained by the acclimatization of part of the population to the local micro-climate or by the ancient hybridization. [ABSTRACT FROM AUTHOR]