Developing stand density thresholds to address mountain pine beetle susceptibility in eastern Washington forests

This study examines the insect/host relationship between the Mountain Pine Beetle (MPB) (Dendroctonus ponderosae) and one of its major hosts, lodgepole pine (Pinus contorta) to discern how carrying capacity and climate influence this disturbance in eastern Washington forests. The research specifically focuses on host susceptibility as a function of site, stand, and climate variables.; The study finds that incorporating site carrying capacity as a predictor of lodgepole pine susceptibility to MPB attack improves our ability to estimate impacts. However, that even with the inclusion of a site carrying capacity variable, only 19.8-24.2% of the deviance in MPB attack is directly attributable to site and stand attributes. The remaining explainable deviance is attributable to climate parameters. Historically accurate predictors of MPB susceptibility such as basal area and stand density are no longer significant predictors of MPB attack after the year 2000. Carrying capacity is a significant predictor even during times of drought. Thinning to a basal area target that does not consider carrying capacity would not reduce susceptibility to MPB attack under current climate conditions.; Research on MPB epidemics elsewhere found that the impact of climate change is a result of improved MPB winter survival, but these results find that summer temperature and its impact on average Vapor Pressure Deficit is the primary driver of host susceptibility to MPB in eastern Washington. While pre-growing season moisture is a contributing factor in MPB attack, drought is not as significant a variable as the shift in temperature. Changes in average summer VPD appear to have reached a threshold which is fundamentally altering tree susceptibility to MPB populations. While increasing insect activity is certainly implicated in the dramatic increase in MPB levels in the recent past, the role of increasing host susceptibility because of climate change can no longer be ignored.