Mountain pine beetle attack faster growing lodgepole pine at low elevations in western Montana, USA

Global change has impacted forests through altered disturbance regimes. In the western US, climate change has resulted in extensive and severe mountain pine beetle outbreaks. These outbreaks have the potential to impact forest function through the selection of certain phenotypes. We investigated the potential for bark beetle-induced selection by way of measuring growth and climate response in mountain pine beetle-killed and surviving lodgepole pine in the Northern Rockies. We had three objectives: (1) investigate differences in growth between beetle-killed and surviving lodgepole pine prior to a recent outbreak, (2) compare the climate-growth relationships for beetle-killed and surviving lodgepole pine and how those relationships explain observed growth differences and predict mortality risk, and (3) investigate growth differences and growth-climate relationships across north- and south-facing aspects and over an elevation range representing local climate gradients. Significantly higher growth rates were observed in beetle-killed trees at low-elevation sites, but not at mid or high elevations. While aspect influenced overall growth, it did not have a significant influence on the difference in growth between beetle-killed and surviving trees. Growth showed significant relationships with several climate variables (i.e., previous-year August temperatures, October temperatures, annual precipitation, and summertime climatic water deficit), with slight differences in those relationships between beetle-killed and surviving trees. Mixed effects models demonstrated that higher growth rates and age increased the probability of mortality during the outbreak at all elevations, and also that climatic water deficit and previous-year August maximum temperatures were related to the magnitude of growth differences between beetle-killed and surviving trees. Overall, mountain pine beetles tended to attack large, fast-growing, lodgepole trees, especially at lower elevations where trees may be more susceptible to seasonal water stress.