Biophysical drivers of mountain pine beetle outbreaks in lodgepole pine forests in the Intermountain West, U.S.

Mountain pine beetle, Dendroctonus ponderosae Hopkins, is among the most significant forest insect pests, particularly in lodgepole pine, Pinus contorta Dougl. ex Loud., forests in western North America. We established a network of 125, 0.081-ha circular plots in P. contorta forests in Colorado, Idaho, Montana, Utah, and Wyoming to document tree mortality and stand-level responses to D . ponderosae outbreaks. We found notable reductions in several key metrics but also observed significant variability in per-year (0–71%) and cumulative mortality (2–87%). We used this long-term monitoring dataset and employed generalized linear mixed effects modeling to determine the biophysical drivers of the variation in P . contorta mortality. Models were fitted to the entire (2005–2024) dataset, and to two truncated datasets: outbreak years only (2005–2012) and maximum per year P . contorta mortality per plot. Explanatory variables included tree, plot, and environmental factors (temperature and water availability). Our models highlighted several significant drivers of P. contorta mortality. The most significant and consistent were individual tree dbh and quadratic mean diameter (QMD). Larger individual tree dbh and larger QMD positively influenced the probability of P . contorta mortality. Tree dbh interacted significantly with water availability variables, particularly November-March snowpack. Increased snowpack and greater densities of nonhost tree species decreased mortality. These results suggest that management efforts should account for the frequency of large-diameter P. contorta and that density reduction and/or increasing tree species (nonhost) and structural diversity may provide mitigation strategies for reducing P . contorta mortality attributed to D. ponderosae in the Intermountain West.