The Influence of Light and Nutrients on Foliar Phenolics and Insect Herbivory

We examined the changes in leaf phenolic chemistry and insect herbivory from saplings of two temperate deciduous species, Liriodendron tulipifera (tulip poplar) and Cornus florida (dogwood), planted in five microenvironments in Gilmer County, Georgia, USA. The experimental design permitted comparisons between saplings grown in an open field, under shade cloth within the field, on the edge between field and forest, in forest understory, and within canopy gaps established within the forest. Half of the trees in each microenvironment were fertilized. Leaves from each tree were sampled at the end of the growing season and 1989 and 1990 and analyzed for toughness, percent dry mass, total phenolics, hydrolyzable tannins, condensed tannins, and insect herbivory (percent leaf area damaged). The shade—tolerant dogwood saplings contained higher levels of total phenolics and hydrolyzable tannins than the shade—intolerant tulip poplar saplings. Dogwood generally had lower levels of herbivory. These results support earlier studies suggesting that slow—growing, shade—tolerant species tend to have higher levels of phenolics and experienced lower levels of herbivory than fast growing, shade—intolerant species. However, dogwood leaves contained lower levels of condensed tannins and were as tough as tulip poplar leaves. Sunlight availability had a significant positive influence on levels of phenolics in both species. Leaf phenolics generally increased with greater insolation from forest to field and when sunlight was greater within field for forest habitats. However, the levels of tannins in dogwood saplings only dropped significantly in the deep shade of the forest. The similar levels of dogwood phenolics in most microenvironments are indicative of the relatively high photosynthetic efficiency of this species in reduced light environments. Overall, these results are consistent with carbon/nutrient balance theory that predicts trade—offs in the allocation of photosynthate from defense to growth as light declines. Levels of insect herbivory and total phenolics were inversely related for dogwood. However, the relationship with tannins was less apparent. Herbivory on tulip poplar was unrelated to changes in phenolics, possibly reflecting the greater chemical diversity of that species. Fertilization increased the biomass of both species, but had no apparent influence on levels of leaf phenolics or insect hervivory. The lack of a fertilization effect was unexpected in light of previous suggestions that fertilization results in reduced phenolics and increased herbivory.