Association between environmental and genetic heterogeneity in forest tree populations

Within a plant species, environmental heterogeneity has the potential to influence the distribution of genetic variation among populations through several evolutionary processes, including natural selection, differential gene exchange, and chance associations caused by genetic drift or founder effects. We evaluated the relationship between environmental characteristics and the distribution of genotypes for three common woody species (Quercus alba, Carya tomentosa, and Sassafras albidum) in Missouri Ozark forests. We measured the correlation between multivariate genotypes and forest structure variables, and assessed the influence of physical landscape on multivariate genotypes. For all three species, we found significant relationships between genetic variation and environmental heterogeneity. We discovered that populations in local habitat patches with different forest structures also differed in combinations of multivariate genotypes. In contrast, we did not detect significant differences in multivariate genotypes among soil-type/aspect classes in any of the three study species, suggesting that genotypic differentiation is operating on a finer scale than soil or aspect differences. We conclude that natural selection, possibly interacting with founder events, has influenced the population differentiation of these three long-lived plant species. Such microgeographic variation in response to environmental heterogeneity is expected for a broad range of species, even when extensive gene flow is present.