Radial Growth Response of Douglas-fir and Grand Fir to Larval Densities of the Douglas-fir Tussock Moth and the Western Spruce Budworm

Abstract

Abstract Statistical models were developed relating corrected tree-ring chronologies of grand fir (Abies grandis [Dougl. ex D. Don] Lindl.) and Douglas-fir (Pseudotsugata menziesii var. glauca [Beissn.] Franco) to population time series of Douglas-fir tussock moth (Orgyia pseudotsugata [McDunnough]) and western spruce budworm larvae (Choristoneura occidentalis Freeman) in the same stands. The study was conducted in northeastern Oregon and spanned 22 yr including 2 periods of defoliator outbreaks as well as an interval of no outbreaks. Changes in the radial increment of host trees, corrected for tree-ring age and weather, were inversely related to larval densities in the same year of growth and in each of the 4 preceding years. The strongest growth response was a delayed function of larval density with a 3 yr time delay. Regression models giving the best fit to pooled data were second-degree polynomials in which log larval density with a 3 yr delay accounted for 28.7% of the variation in rate of change in corrected tree-ring indices, and 46.4% of the variation in the absolute size of corrected indices. The results imply that reductions in radial growth are directly related to density of feeding larvae and that the cumulative maximum effect on increment in the lower bole lags defoliation by 3 yr. The models are recommended either for directly forecasting mean reductions in radial increment from monitored densities of larvae or for inversely predicting past mean densities of defoliating larvae from dendrochronology data. For. Sci. 43(2):194-205.