Effect of increasing temperatures on the distribution of spruce beetle in Engelmann spruce forests of the Interior West, USA

The spruce beetle (Dendoctronus rufipennis) is a pervasive bark beetle indigenous to spruce (Picea spp.) forests of North America. In the last two decades outbreaks of spruce beetle have increased in severity and extent. Increasing temperatures have been implicated as they directly control beetle populations, potentially inciting endemic populations to build to epidemic (outbreak) proportions. However, stand structure and composition conditions will also influence beetle populations. We tested the effect of temperature variables (minimum cool season temperature and maximum warm season temperature), and habitat controls (structure and composition) on the prediction of spruce beetle presence/absence for 4198 Engelmann spruce (Picea engelmannii Parry ex. Engelm.) plots in the Interior West, USA. Predictions were applied to three global climate models (GCMs) for three time periods. While both temperature variables were important, results suggested habitat variables (spruce basal area and spruce composition) were more influential for the prediction of current beetle presence. Future beetle prevalence varied from 6.1% to 24.2% across GCMs and time periods. While both temperature variables increased over time, in some cases the increases were not proportional, which led to differential predictions of beetle population prevalence in space and time among GCMs. Habitat variables that characterized current spruce beetle susceptibility changed as future temperatures increased. Application of results to forest management will include adjusting monitoring programs to reflect the potential increased overall prevalence of the beetle, and modifying the characterization of high hazard spruce stands to reflect increasing beetle presence in stands with lower basal area and spruce composition than currently observed.