Evidence for local adaptation in closely adjacent subpopulations of Northern red oak (Quercus rubra L.) expressed as resistance to leaf herbivores.

Many studies of herbaceous plant populations have illustrated the potential of adjacent subpopulations to adapt to local ecological conditions. However, the extent to which local adaptation on a small geographical scale can occur in outcrossing tree populations is not well understood. In this study, we reciprocally transplanted acorns from adjacent subpopulations of northern red oak (Quercus rubra L.) occupying north-, southwest-, and west-facing slopes within a 4-ha plot in a Missouri oak-hickory forest. The quantitative character we measured was leaf damage by herbivores on first-year seedlings, because it reflects resistance to insect herbivores-a quantitative trait that could be under different selective pressures in dissimilar microhabitats. We found that seedlings showed the least damage when planted at the site of the maternal plant. This finding provides initial but strong evidence of local adaptation and illustrates that selection associated with leaf harbivory may have a strong impact on the genetic structure of local tree populations. Such a result is unexpected for a widely outcrossing species on such a small geographical scale but indicates that genetic structuring is possible within other plant populations occupying heterogeneous environments.