Effects of gypsy moth defoliation on tree growth preliminary models for effects of cumulative defoliation on individual host tree radial increment

Two hundred and one stands in the Ridge and Valley physiographic province of central Pennsylvania (USA) were followed from 1978 to 1985 and approximately one third of these stands were followed until 1995. Individual trees on three 0.10-acre (0.040 ha) plots per stand were visited each year. These stands experienced two major defoliation episodes by the gypsy moth (Lymantria dispar L.), first from 1981 to 1982 and again from 1986 to 1987; some trees and stands also experienced significant defoliation in the early 1990s In 1995, increment core samples were collected from plot trees based on a matrix of species, stand defoliation history, and crown class (dominance). In this paper, we consider only core samples from the red oak and white oak species groups. We first examine the relationship between the sample population and the earlier classification of the forested area where the research sites are located. Results indicated that using these data, models can be developed across the site classification scheme. We then consider the appropriateness of developing a single model for both species groups. Finally, we consider a cumulative effects model with the data organized by severity of defoliation to individual trees over three years (current and the past two years' defoliation). Results provide reasonable individual tree growth effects for the species under consideration. In a forest model that uses individual tree lists to simulate forest stands, these growth effects models can be linked to population dynamics models for the gypsy moth to obtain a dynamic model for an individual stand or larger forested areas.