Pollen-mediated gene flow in isolated and continuous stands of bur oak, Quercus macrocarpa (Fagaceae)

* Premise of the study: Pollination patterns determine the reproductive neighborhood size of plants, the connectivity of populations, and the impacts of habitat fragmentation. We characterized pollination in three populations of Quercus macrocarpa occurring in a highly altered landscape in northeastern Illinois to determine whether isolated remnant stands were reproductively isolated. * Methods: We used microsatellites to genotype all adults and 787 acorns from two isolated savanna remnants and a stand in an old-growth forest. One isolated remnant occurred in a highly urbanized/industrialized landscape, and one occurred in an agricultural landscape. Parentage assignment was used to assess pollen-mediated gene flow. * Key results: Pollen donors from outside the study sites accounted for between 46% and 53% of paternities and did not differ significantly among sites, indicating that similar high levels of gene flow occurred at all three sites. Within stands, the mean pollination distance ranged from 42 to 70 meters, and when accounting for outside pollinations, mean pollination distances were well over 100 meters. Genetic diversity of incoming pollen was extremely high in all three stands. The number of effective pollen donors, Nep, calculated from paternity assignment was higher than that estimated by an indirect correlated paternity approach. * Conclusions: Our findings indicate that extremely isolated stands of oaks are unlikely to be genetically and reproductively isolated, and remnant stands may contribute to maintaining genetic connectivity in highly modified landscapes.