Differences in growth, survival and phenology in Quercus rubra and Q-ellipsoidalis seedlings

Related oak species with different ecological requirements often hybridize where they co-occur. Even though interspecific gene flow is considered to be common in closely related oaks, species identity in sympatric oak species with different local adaptations is generally maintained with a low number of hybrids and introgressive forms in the adult tree generation. Quercus rubra and Q. ellipsoidalis offer a good model to study characters that are related to different local adaptations and reproductive isolation of the species. Both species are interfertile, but grow in different micro-environments with Q. ellipsoidalis as the most drought tolerant red oak species occurring often on very dry sites. In an earlier study, genetic assignment analysis at 15 highly variable microsatellite markers revealed a low number of hybrids (0-2%) and introgressive forms (0-4%) in neighboring Q. ellipsoidalis and Q. rubra populations in both the adult tree and seedling generation. In the present study, pronounced differences in growth and survival, the timing of bud burst and leaf senescence between seedlings of both species in a common garden experiment suggested reproductive isolation between species and genetic differences in fitness-related traits. Future studies should focus on the analysis of fitness traits in parental environments using reciprocal transplant studies.