RAINFALL EFFECTS ON PLANT-HERBIVORE PROCESSES IN AN UPLAND OAK FOREST

We examined the patterns of foliar chemistry and insect herbivory in w hite oak (Quercus alba), chestnut oak (e. prinus), and red maple (Acer rubrum) trees occupying an upland oak forest on the Walker Branch Wat ershed in Tennessee. Different size classes of each species were sampl ed to evaluate life cycle changes in foliar N and phenolic chemistry a nd their corresponding influence on insect herbivory. Oak trees had co nsistently higher foliar N and hydrolyzable tannin concentrations, and they experienced greater herbivore damage than did red maple trees; m aple trees had higher condensed tannin levels. Foliar chemistry and he rbivore damage were usually quite similar for all size classes. Howeve r, condensed tannins, and occasionally total phenolics, were higher in well-insolated mature trees than in shaded understory conspecifics, f itting the pattern expected through plant carbon/nutrient balance rela tionships. Rainfall fluctuations during 1992-1994 significantly influe nced plant-herbivore processes. Foliar N and phenolic concentrations w ere reduced in all species during the much drier summer of 1993, relat ive to 1992. The summer drought in 1993 was apparently severe enough t o restrict carbon allocation to plant phenolics. In contrast, foliar N levels in oaks were somewhat elevated by the end of the wet 1994 grow ing season. Total phenol and hydrolyzable tannin levels remained quite low in 1994, when carbon may have been preferentially allocated to gr owth rather than differentiation, as N uptake increased during the wet summer following drought. Nevertheless, carbon allocation to condense d tannins was quite high for all species during the wet summer. Insect herbivore damage was directly correlated with rainfall totals over th e 3-yr period. Herbivore damage on oaks was reduced during the 1993 dr ought, principally because of less impact by leaf skeletonizers. Insec t feeding on oaks and maples was elevated during the wet 1994 summer, when strip feeding by lepidoptera larvae was at its highest. Our findi ngs suggest that herbivore damage at Walker Branch appears to be large ly dependent on the response of specific insect feeding guilds to mois ture-related changes in foliar N and plant phenolics.