Modeling the loss of climate protection from spruce beetles (Dendroctonus rufipennis) under changing conditions in Alaska’s forests

Alaska’s large areas of undisturbed habitat are globally significant, particularly in the context of rapid ongoing climate change in the far north. Linking climate and ecological models enables land managers and communities to adapt to future changes. Spruce beetles (Dendroctonus rufipennis) are endemic to Alaska, but the state has experienced recent increases in beetle outbreak extent and severity, resulting in large-scale forest mortality. Outbreaks depend on multiple factors, including forest age, health, and composition, but prior research also links outbreaks to weather and climate, via regulation of beetle development and mortality due to cold. However, because no comprehensive model previously linked climate impacts on both beetle survival and reproduction, it has been impossible to predict how cold-climate landscape-level protection from outbreaks may decline. We created a model to assess past, present, and future relationships between weather and beetle outbreaks in Alaska and an interactive online tool for land managers and the public. The model is based upon summer heat, which determines whether beetles mature in one year or two; fall cooling, which kills beetles if it occurs rapidly during a key timeframe; and extreme winter cold, which contributes to overwintering mortality, especially without insulating snowpack. Using gridded daily historical data (Daymet) and outputs from four earth system/global climate models (ESM/GCMs), each run under plausible greenhouse gas futures (RCP 4.5 and 8.5), we modeled historical and future protection from beetles. Our results show a dramatic projected shift across forested areas of Alaska from the past (1988-2017) to the present (2010-2039), near future (2040-2069) and distant future (2070-2099). The online tool categorizes climate protection levels as “low,” “medium,” and “high,” where areas of high climate protection are unlikely to suffer outbreaks even if forest conditions are otherwise susceptible. Historically, most of Interior Alaska was highly protected from landscape-scale outbreaks, with moderate protection in Southcentral and Southeast Alaska. By the end of the century, all model outputs show low protection in southern parts of the state. Scenarios with higher warming suggest that large portions of western and central Interior Alaska may also lose climatic protection, with only regions near the Yukon border still in the “high” protection category. This model does not directly predict beetle outbreaks, and some forested areas are likely to be protected by other ecological and biophysical factors. However, results suggest that forest managers should consider beetle management planning that no longer relies on the protection of Alaska’s cold climate.