SummaryClimate change is expected to drive increased tree mortality through drought, heat stress, and insect attacks, with manifold impacts on forest ecosystems. Yet, climate-induced tree mortality and biotic disturbance agents are largely absent from process-based ecosystem models. Using data sets from the western USA and associated studies, we present a framework for determining the relative contribution of drought stress, insect attack, and their interactions, which is critical for modeling mortality in future climates. We outline a simple approach that identifies the mechanisms associated with two guilds of insects -bark beetles and defoliators -which are responsible for substantial tree mortality. We then discuss cross-biome patterns of insect-driven tree mortality and draw upon available evidence contrasting the prevalence of insect outbreaks in temperate and tropical regions. We conclude with an overview of tools and promising avenues to address major challenges. Ultimately, a multitrophic approach that captures tree physiology, insect populations, and tree-insect interactions will better inform projections of forest ecosystem responses to climate change.

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) were monitored for 4 years following three wildfires. Logistic regression analyses were used to develop models predicting the probability of attack by Douglas-fir beetle (Dendroctonus pseudotsugae Hopkins, 1905) and the probability of Douglas-fir mortality within 4 years following fire. Percent crown volume scorched (crown scorch), cambium injury, diameter at breast height (DBH), and stand density index for Douglas-fir were most important for predicting Douglas-fir beetle attacks. A nonlinear relationship between crown scorch and cambium injury was observed, suggesting that beetles did not preferentially attack trees with both maximum crown scorch and cambium injury, but rather at some intermediate level. Beetles were attracted to trees with high levels of crown scorch, but not cambium injury, 1 and 2 years following fire. Crown scorch, cambium injury, DBH, and presence/absence of beetle attack were the most important variables for predicting postfire Douglas-fir mortality. As DBH increased, the predicted probability of mortality decreased for unattacked trees but increased for attacked trees. Field sampling suggested that ocular estimates of bark char may not be a reliable predictor of cambium injury. Our results emphasize the important role of Douglas-fir beetle in tree mortality patterns following fire, and the models offer improved prediction of Douglas-fir mortality for use in areas with or without Douglas-fir beetle populations.Résumé : Des douglas de Menzies (Pseudotsuga menziesii (Mirb.) Franco) ont été suivis pendant 4 ans à la suite de trois incendies de forêt. Des analyses de régression logistique ont été utilisées pour élaborer des modèles de prédiction de la probabilité d'une attaque du dendroctone du douglas (Dendroctonus pseudotsugae Hopkins, 1905) et de la probabilité que le douglas de Menzies meure dans les quatre années suivant un feu. Le volume de cime roussi, les dommages au cambium, le diamètre à hauteur de poitrine (DHP) et l'indice de densité du peuplement de douglas de Menzies étaient les variables les plus importantes pour prédire les attaques du dendroctone. Une relation non linéaire entre le roussissement de la cime et les dommages au cambium a été observée, ce qui signifie que les dendroctones n'attaquent pas de préférence les arbres qui ont à la fois le maximum de dommages à la cime et au cambium mais plutôt un niveau intermédiaire quelconque. Les dendroctones étaient attirés par les arbres avec un degré élevé de roussissement de la cime mais sans dommages au cambium, un et 2 ans après un feu. Le roussissement de la cime, les dommages au cambium, le DHP et la présence ou l'absence d'attaques du dendroctone étaient les variables les plus importantes pour prédire la probabilité que le douglas de Menzies meure après un feu. La probabilité estimée de mortalité diminuait avec l'augmentation du DHP chez les arbres qui n'avaient pas été attaqués alors que la probabilité qu'un arbre meure augmentait avec l'augmentation du DHP chez...

Background Mountain pine beetle (MPB) is a native disturbance agent across most pine forests in the western US. Climate changes will directly and indirectly impact frequencies and severities of MPB outbreaks, which can then alter fuel characteristics and wildland fire dynamics via changes in stand structure and composition. To investigate the importance of MPB to past and future landscape dynamics, we used the mechanistic, spatially explicit ecosystem process model FireBGCv2 to quantify interactions among climate, MPB, wildfire, fire suppression, and fuel management under historical and projected future climates for three western US landscapes. We compared simulated FireBGCv2 output from three MPB modules (none, simple empirical, and complex mechanistic) using three focus variables and six exploratory variables to evaluate the importance of MPB to landscape dynamics. Results We found that inclusion of MPB (empirical or mechanistic) in the simulations significantly changed past and future landscape dynamics and that the mechanistic MPB module had more cross-scale interactions that increased variability, and perhaps realism, of simulation results. We also evaluated impacts of fire and fuel management on MPB dynamics and found that fire suppression influenced fuel loadings more than MPB disturbance, but at a landscape scale, most fuel treatment programs did little to change fuel loadings, MPB dynamics, and burned area, except under high fire suppression. Conclusions Synergistic interactions of climate, MPB, and wildfire catalyzed landscape-scale changes in vegetation distributions, fuels, and fire regimes in FireBGCv2 simulations. Models that simulate climate change on pine-dominated landscapes may be improved by including mechanistic MPB simulations to account for potentially important ecological interactions.