Differentiation and hybridization of Quercus frainetto, Q. petraea, and Q. pubescens (Fagaceae): insights from macro-morphological leaf traits and molecular data

Macro-morphological leaf traits and genetic assignments were combined to study the differentiation and hybridization of three sympatric and inter-fertile white oak species (Quercus frainetto, Q. petraea, and Q. pubescens). The sampling was performed in a single forest stand of central Italy (Mount Vairano) in which the cover percentages of each of these three oak species were almost equal. The individuals classified as pure species and the hybrid individuals were divided into two subsets (A and B) which were subsequently statistically analysed. The results regarding the subset of pure individuals showed a clear separation between the three species on the basis of differences observed in the following leaf traits: basal leaf shape, petiole ratio, petiole length, number of intercalary veins, pubescence of the petiole, leaf area, number of lobes, lamina length, and percentage of venation. Regarding the subset of hybrid individuals, as expected, a wider pattern of leaf traits compared to that exhibited by the pure individuals was observed. The leaf traits of the pure species that had provided the greater genetic contribution in the hybridization process were easily identifiable. Quercus pubescens and its hybrids exhibited a higher degree of leaf traits variability when compared with those observed for Q. petraea and Q. frainetto.