Recent and future climate suitability for whitebark pine mortality from mountain pine beetles varies across the western US

Abstract

Recent mountain pine beetle outbreaks in whitebark pine forests have been extensive and severe. Understanding the climate influences on these outbreaks is essential for developing management plans that account for potential future mountain pine beetle outbreaks, among other threats, and informing listing decisions under the Endangered Species Act. Prior research has focused on one geographic region, but geographic variability in beetle and tree physiological responses to climate conditions have been documented. Here we evaluate geographic variability in climate influences on recent beetle outbreaks in whitebark pine and estimate future climate suitability for outbreaks across much of the range of whitebark pine in the western US. To accomplish these objectives, we developed and analyzed statistical models for three different geographic regions as well as a Westwide model, then applied the Westwide model to a suite of climate projections. The general patterns of climate-tree mortality relationships were similar across the three regions of our study. However, the relative importance of individual climate metrics preceding and during the recent outbreaks varied geographically because of the different climates in the regions. Winter minimum temperatures appeared to be limiting prior to outbreaks in the colder regions. All regions experienced low summer precipitation prior to or during outbreak initiation. Future climate suitability for beetle outbreaks is estimated to increase or remain stable in the coldest regions and decline slightly in the warmest region by the end of this century. Across the study area, projections of higher winter temperatures and decreased summer precipitation (with lower confidence than for temperatures) contribute to increased climate suitability for outbreaks, while projected higher fall/spring/summer temperatures contribute to decreased suitability. Some regional variability exists; in particular, the effect of winter warming is muted in the warmest region (Cascades) where winter temperatures appear to be less limiting. However, all regions are projected to experience fewer years with very low suitability, which commonly occurred prior to the recent outbreaks and may have limited beetle populations. Given the inherent uncertainty in climate projections and ecological responses to novel climates, management plans that incorporate sites that are expected to experience a range of expected future climate conditions might increase the chances of whitebark pine persistence in a warmer future.