Rainfall effects on plant-herbivore processes in an upland oak forest

We examined the patterns of foliar chemistry and insect herbivory in white oak (Quercus alba), chestnut oak (Q. prinus), and red maple (Acer rubrum) trees occupying an upland oak forest on the Walker Branch Watershed in Tennessee. Different size classes of each species were sampled to evaluate life cycle changes in foliar N and phenolic chemistry and their corresponding influence on insect herbivory. Oak trees had consistently higher foliar N and hydrolyzable tannin concentrations, and they experienced greater herbivore damage than did red maple trees; maple trees had higher condensed tannin levels. Foliar chemistry and herbivore damage were usually quite similar for all size classes. However, condensed tannins, and occasionally total phenolics, were higher in well-insolated mature trees than in shaded understory conspecifics, fitting the pattern expected through plant carbon/nutrient balance relationships. Rainfall fluctuations during 1992-1994 significantly influenced plant-herbivore processes. Foliar N and phenolic concentrations were reduced in all species during the much drier summer of 1993, relative to 1992. The summer drought in 1993 was apparently severe enough to restrict carbon allocation to plant phenolics. In contrast, foliar N levels in oaks were somewhat elevated by the end of the wet 1994 growing season. Total phenol and hydrolyzable tannin levels remained quite low in 1994, when carbon may have been preferentially allocated to growth rather than differentiation, as N uptake increased during the wet summer following drought. Nevertheless, carbon allocation to condensed tannins was quite high for all species during the wet summer. Insect herbivore damage was directly correlated with rainfall totals over the 3-yr period. Herbivore damage on oaks was reduced during the 1993 drought, principally because of less impact by leaf skeletonizers. Insect feeding on oaks and maples was elevated during the wet 1994 summer, when strip feeding by lepidoptera larvae was at its highest. Our findings suggest that herbivore damage at Walker Branch appears to be largely dependent on the response of specific insect feeding guilds to moisture-related changes in foliar N and plant phenolics.