Assessing spatiotemporal relationships between wildfire and mountain pine beetle disturbances across multiple time lags

Native bark beetles and wildfire are among the most prominent natural disturbances in the conifer forests of western North America. Using multiple approaches and scales, researchers have sought to understand how each disturbance may, or may not, influence the subsequent likelihood or severity of the other disturbance. Our objectives were to complement existing research on the linkage of mountain pine beetle and wildfire disturbances by combining large-scale spatial data at variable temporal resolutions to characterize the discrepancy between observed and expected overlap areas of fire and beetle disturbances. Multiple time lags, differing by 1-yr intervals, spanning the 30-yr period 1984-2014 in the U.S. states of Washington and Oregon were examined for wildfire following bark beetle epidemics and for beetle epidemics following wildfire. We used existing spatial data sets of wildfires and insect pest disturbances to perform overlay analyses to characterize the two relationships. We also compared years of peak vs. non-peak fires, as well as the influence of several individual, large disturbances. We found a clear association of increased bark beetle epidemic shortly after wildfire. The evidence of an association between increased fire extent and severity following bark beetle epidemics was less straightforward, although it suggested that the nature of the observed patterns varied with fire severity. In some cases, individual disturbance seasons or events were highly influential on the overall trends. Variability in the results highlights the challenges of quantifying landscape-level relationships among disturbance types and supports the existing calls for further analyses with additional ecological variables.