Limited tree mortality in unburned areas linked to bark beetle spillover from wildfires

Increased fire activity in the western United States since 2000 has produced an abundance of fire-injured trees at risk to lethal attack by bark beetles. Large populations of bark beetles reproducing in fire-injured trees may disperse (or spillover) from inside the fire perimeter to adjacent, unburned forests, potentially causing extensive tree mortality. In the western United States and Canada, fire-injured Douglas-fir (DF; Pseudotsuga menziesii) are frequently colonized by Douglas-fir beetle (DFB; Dendroctonus pseudotsugae), prompting concern among land managers about elevated risk of spillover. We investigated spatiotemporal patterns of DF tree mortality from DFB in unburned areas surrounding 61 wildfires (2000-2017) with a high likelihood for spillover in the Northern Rocky Mountains, USA. We developed a multiple-scale analytical framework to examine tree mortality potentially associated with spillover following fire. Synchronous fluctuation in the amount of DF mortality within and beyond the flight distance of DFB in the region and surrounding individual fires (0-10 km) suggested that DFB activity primarily responded to a broader scale process, such as drought, rather than proximity to burned trees. Using shorter and longer range dispersal scenarios, we estimated that at <0.25 km from the fire perimeter, the dominant source of DFBs transitioned from burned to unburned sources due to the closer proximity of DFBs from unburned sources. Some fires (8%-15%; range of fires from sensitivity analysis) did exhibit evidence of DFB spillover, but spillover occurred <1 km from fires (based on our criteria) and DF tree mortality associated with spillover was 0.2%-0.3% of total DF damage area during the study period. Spillover was not associated with climate conditions that increase host tree stress, rather it was associated with greater DF mortality from DFB in the prior year in the same area (i.e., poorly linked to spillover). Site-specific monitoring of post-fire DFB populations in susceptible, unburned DF forests adjacent to fires by land managers may be necessary to determine the risk of DFB emigrating from burned areas. Our findings inform post-fire planning and the ecological implications of disturbance interactions that occurred in the early 21st century during a period of amplified wildfire and DFB activity.