The extent to which co-occuring or neighboring species exchange genes has important implications for the distribution of genetic and phenotypic variation affecting species’ responses to their environment, as well as for species identity and conservation.  Oaks (Quercus spp.) have a relatively high capacity to hybridize, with viable offspring being produced from many inter-specific crosses, but the frequency of oak hybridization in the wild has long been a contentious issue, as most early studies were based solely on morphological features, which can be highly variable within species.  Genetic data are beginning to shed new light on this question, showing in some cases high levels of cryptic gene flow, and in others evidence of strong barriers to hybridization.  In this study we couple genetic structure data with the results of parentage analyses for two stands in North Carolina: one in the Piedmont containing Q. rubra, Q. velutina, and Q. falcata, the other in the Southern Appalachians containing Q. rubra, Q. velutina, and Q. coccinea. 

Results/Conclusions

Fst and STRUCTURE analyses reveal extremely weak genetic differentiation between co-occuring species at both sites, suggesting that, despite differentiation in physical characteristics between morpho-species, gene flow has prevented differentiation in neutral allele frequencies.  When all individuals are included in parentage analyses, ~20% of seedlings are identified as having a parent categorized as belonging to a different species.  While some of these heterospecific parent-offspring pairs may be due to misidentification of young seedlings, true hybridization rates are likely >7% at these sites.  We discuss the potential significance of inter-specific gene flow in oaks, especially regarding their responses to global change, and suggest directions for future research.