Does Wildfire Increase the Risk of Moutain Pine Beetle Outbreaks in Lodgepole Pine Forests of the Greater Yellowstone Ecosystem?

Abstract

We examined whether wildfire injury increased lodgepole pine, Pinus contorta, susceptibility to mountain pine beetle, Dendroctonus ponderosae, how it affects beetle reproduction, whether this interaction differs between endemic and epidemic populations, and how wildfire influences tree defense physiology. Wildfire predisposed trees to mountain pine beetle attack. In particular, fire-injured trees had a lower ability in synthesized monoterpenes in response to simulated attacks than did non-injured trees. However, beetles responded in a non-linear fashion; moderately-injured trees were most preferred. This interaction was influenced by beetle population size. Healthy and fire-injured trees were attacked when populations were high, but no healthy trees and no severely-injured trees were killed when populations were low. Beetle brood production per female was also curvilinear being highest in moderately-injured trees. This reflected a trade-off between high intraspecific competition arising from the large number of beetles needed to overcome defenses in healthy trees, and high interspecific competition and low substrate quality in severely injured trees. These results suggest that fire-injured trees can provide a resource for mountain pine beetles during the extended periods when populations are not high enough to overcome defenses of vigorous trees. But the likelihood that populations could transition from endemic to epidemic levels due to increased tree susceptibility from wildfire is constrained by the opposing factors of lower nutritional quality and more competition load in severely-injured trees, and the relatively low incidence of moderately-injured trees. Wildfire may cause some reproductive increases in populations that are already in outbreak mode.