Native bark beetles are capable of causing widespread mortality during outbreak events in the forests of western North America. These disturbances can have vast effects on forest structure and there is concern that such changes could influence subsequent wildfire behavior and its impact on ecosystems. New research has considered the recent mountain pine beetle outbreak (~1996-2006), but few studies have analyzed the ecological legacies of historic events. Northern Rocky Mountain forests were impacted by a widespread mountain pine beetle outbreak in the late 1970s through the early 1980s. In this study, we evaluated the effect of the historic beetle outbreak and other biophysical variables on the burn severity (derived from Landsat data) of 11 different fires in Glacier National Park over an 18-year period (1988-2006). The extent and arrangement of these disturbances presented a unique opportunity to evaluate the influence of an earlier disturbance on ensuing wildfires. We used sequential autoregression to obtain accurate estimates of model parameters given the high level of spatial autocorrelation associated with wildfire. Models included additional explanatory variables known to influence burn severity such as topography and fuel moisture and variables were evaluated at increasing spatial scales.
The models explained between 54% and 80% of the variability in burn severity of each fire. Historic mountain pine beetle severity was a significant predictor of burn severity in 10 of the 11 fires. The relationship between beetle severity and burn severity was positive across all models and we found no relationship between the effect size of beetle severity and time since the beetle outbreak. However, our results suggest the influence of the beetle outbreak on burn severity is scale-dependent on the pattern of beetle intensity. Fires where beetle severity was the best predictor at broad scales tended to have a larger beetle effect size compared to fires where local beetle severity was the best predictor. A number of factors, including topography and weather, are responsible for the burn severity associated with a given fire. However, our work shows an historic high severity beetle outbreak can influence the burn severity of wildfire for many years after the initial disturbance event. Furthermore, our analysis of historic events can shed light on future compounded disturbances as much of the landscape impacted by the recent beetle outbreak moves beyond the gray phase.