Prescribed fire affects white oak seedling phytochemistry: implications for insect herbivory

Abstract

Fire suppresses vegetative competition, increases light penetration and soil nutrients, and is an integral part of the disturbance regime forming the oak ( Quercus)-dominated forests of eastern North America. Fire can affect plant foliar chemistry through changes in nutrient availability and light intensity, which can influence herbivore feeding patterns. We considered the interactions between fire and oak seedling herbivory by assessing the effects of prescribed fire on white oak ( Quercus alba L.) foliar chemistry. We hypothesized that fire would provide increased light penetration and added nutrients that would enhance the nutritional status of foliage, thereby leading to greater herbivore pressure. We compared white oak foliar chemistry and root characteristics of seedlings from once-burned (1998), twice-burned (1996 and 1998), and non-burned sites over 2 years. We also challenged seedlings with supplemental herbivory and measured foliar chemical responses to herbivore pressure. In two separate experiments, native, seasonal, host-adapted lepidopteran herbivores (eastern tent caterpillar, Malacosoma americana F., and fall webworm, Hyphantria cunea Drury) were caged and allowed to feed on seedlings from burned and non-burned sites on two dates during the 1999 growing season. We found little difference in plant chemistry among our burn treatments, but sample year strongly influenced our results, perhaps due to fluctuations in soil conditions caused by drought in the post-burn year (1999). Foliar calcium levels were moderately lower in seedlings from burned sites the year of the burn ( P=0.06), but this difference disappeared 1 year post-burn. Most phytochemical components were higher in seedlings sampled 1 year post-burn (1999) than in those sampled in the year of the burn (1998). Regardless of burn (site) treatment, caterpillar feeding resulted in foliage with lower concentrations of carbohydrates ( P=0.0001), nitrogen ( P=0.006), and tannins ( P=0.06). Foliar carbohydrates tended to increase within each year, and appeared inversely related to foliar nitrogen and tannins. Our results do not support the hypothesis that prescribed burning alters white oak seedling phytochemistry, or increases seedling susceptibility to herbivory through bottom-up changes in foliar chemistry, nor do they explain anecdotal accounts of increased herbivory on burned sites.