Landscape genetic structure of bur oak (Quercus macrocarpa) savannas in Illinois

We examined population genetic structure among 14 stands of Quercus macrocarpa Mich. (Fagaceae), bur oak, in northeastern and central Illinois using five microsatellite loci. We chose Q. macrocarpa because its scattered, pre-settlement distribution in the prairie/savanna landscape of this region allows us to examine the effects of long-term population fragmentation on a wind-pollinated tree species. A pairwise comparison of Wright's FST among all pairs of stands showed small but significant differentiation among most stand pairs. However, a Bayesian clustering approach showed no underlying genetic structure. An analysis of molecular variance (AMOVA) showed that over 97% of genetic variation occurred within the stands, while less than 3% occurred among stands. A Mantel test showed no evidence for isolation-by-distance. Therefore, levels of population differentiation among the study stands are low and are not correlated to distance. Despite a dispersed and fragmented distribution that persisted for several thousand years, the savanna bur oaks of this region appear to comprise a single genetic population. The lack of population genetic structure is most likely due to high levels of pollen-mediated gene flow in this species. Our findings suggest that wind-pollinated trees may be quite resilient to the negative genetic consequences of habitat fragmentation.