Abstract
Oak species are generally interfertile within taxonomic sections, even if species’ distribution ranges are not overlapping. Despite interspecific gene flow in natural populations, oak species form genetically distinct clusters and maintain ecological adaptations. However, especially in contact zones between species and in sympatric stands, hybrids and introgressive forms are comparatively frequent. Here, we describe for the first time hybrid forms between the related red oaks Quercus imbricaria Michx. and Quercus coccinea Münchh. based on leaf morphological assessment and genetic assignment analyses corroborating bidirectional gene flow between both species. Additionally, we observed introgression for 20 % of the individuals identified by morphologic traits as ‘pure taxa’ Q. imbricaria, Q. coccinea, Quercus rubra L. and Quercus velutina Lam. One individual, morphologically identified as intermediate between Q. imbricaria and Q. rubra, was genetically assigned to Q. imbricaria possibly as result of trait introgression by repeated backcrossing of a Q. imbricaria x Q. rubra hybrid with Q. imbricaria. Our results confirm frequent introgression between red oak species in sympatric stands with the potential to increase the genetic variation and adaptive potential within species.