Response of two oak species to extensive defoliation: Tree growth and vigor, phytochemistry, and herbivore suitability

This study was developed to experimentally determine whether the differential mortality of white oak versus black oak observed following defoliation events in the oak-dominated forests of the central hardwoods region of the eastern USA may be due to differences in carbon allocation between the two species. Black oak and white oak growing in a common garden were artificially defoliated (90%) using scissors in two consecutive years. Concurrent with the second defoliation event, herbivore performance and phytochemical characteristics were measured, followed immediately by measures of tree growth. In the dormant season following the second defoliation event, roots were sampled to assess tree vigor. Species-specific differences in foliar chemistry and herbivory were evident, regardless of defoliation. Defoliation-induced changes in above-ground biomass were evident in white oak, but not black oak. Defoliation-induced changes in foliar chemistry were more evident in black oak; these were reflected in greater differences in herbivore suitability. Herbivore consumption was correlated with depressed foliar C:N ratios and elevated foliar nitrogen. White oak root starch concentrations were markedly lower in defoliated trees, suggesting that white oak vigor is especially sensitive to resource limitations in the form of photosynthate loss. The implications of these results with respect to defoliation events and white oak mortality, as well as potential phylogenetic differences in response to severe defoliation between the red and white oak groups, are discussed.