Patch‐Size Effects on Plant Phenolics in Successional Openings of the Southern Appalachians

Abstract

We examined the seasonal changes in plant chemical defenses of key tree species (Robinia pseudoacacia, Liriodendron tulipifera, Acer rubrum, and Cornus florida) regenerating during early succession in different patch sizes within Southern Appalachian forests. Trees of each species were sampled from five sizes of forest openings (0.016, 0.08, 0.4, 2.0, and 10 ha; two replicates each) and from the understory of surrounding forests. Leaves of rapidly growing, pioneer species (Robinia and Liriodendron) were slightly less tough and contained lower levels of hydrolyzable tannins and total phenolics, but higher condensed tannin concentrations than the slower growing, more shade—tolerant species (Acer and Cornus). These results generally support earlier findings indicating the evolutionary development of greater levels of constitutive phenolics in slow—growing than fast—growing species. The differential use of condensed and hydrolyzable tannins by slow— and fast—growing species may reflect adaptive trade—offs in the allocation of different types of phenolics. Regenerating trees often had tougher leaves and contained higher levels of phenolics in large than small openings by midsummer. The differences in phenolics were more pronounced for condensed tannins and when comparing the smallest canopy openings with larger patch sizes. Plant phenolics were also generally lower in understory trees than in conspecific sprouts in openings. These findings may reflect phenotypic carbon/nutrient adjustments as light availability changes across the patch—size gradient. The greater sunlight in large than small openings apparently promotes excess carbon production and a metabolic buildup of phenolic compounds. Independent measures of phenolic compounds peaked at different times during the growing season. Hydrolyzable tannins reached peak levels quite early whereas condensed tannins increased later in the growing season. Total phenolics peaked in midsummer. Herbivore damage was consistently high (5—9%) on Robinia sprouts in all openings. Insect herbivory on the remaining three species (1—5%) was significantly lower in large than small patch sizes. Tree species regenerating in the high sunlight of larger openings in Southern Appalachian forests thus tend to have tougher leaves, contain greater phenolics, and experience less herbivore damage than conspecifics in small openings.