A Simulation Approach for Predicting the Effect of Douglas-Fir Tussock Moth Defoliation on Juvenile Tree Growth and Stand Dynamics

A spatially dependent tree growth projection model was developed and used to simulate juvenile grand fir, Abies grandis (Dougl.) Lindl., Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, and ponderosa pine, Pinus ponderosa Laws., in northern Idaho. Simulated experiments using the model examined the effects of Douglas-fir tussock moth, Orgyia pseudotsugata McDunnough, defoliation on stand dynamics during the first 40 years of stand development. The sequence and intensity of the defoliation episodes were varied for different combinations of age, stand density, and tree species composition at the time of the first defoliation. The results are summarized in statistical models which predict the long-term effects of tussock moth defoliationon stand dynamics. Results indicate that the insect regulates primary forest production by reducing stand biomass and by redistributing growth energy from host to nonhost trees by altering intertree competitive relationships to the advantage of nonhosts