OOS 26-1 - Fire, northern spotted owls, and old forests in the Pacific Northwest

The northern spotted owl (NSO) is a critically imperiled species that has likely been extirpated from large portions of its former range. While barred owls are the most important threat currently facing NSO, recent increases in fire activity throughout much of western North America has raised concern that increasing severe wildfire within the range of the NSO could further impact this species. To assess the potential impact of wildfire on NSO, we analyzed over 3 decades of mark-recapture and wildfire data from throughout the frequent fire portion of the owl’s range to examine population- and individual-level impacts of wildfire on this old forest specialist. Our retrospective analysis found no measurable population-level impacts from wildfire, but important individual-level impacts that indicated wildfire effects occurred along a continuum that is dependent on patterns of burn severity. Most fires included in our analysis had patterns of burn severity consistent with the historic mixed-severity fire regime of the region; these patterns did not negatively impact NSO. However, wildfires with greater proportions of higher severity fire effects resulted in negative impacts to NSO apparent survival and territory displacement. We suggest this continuum of fire effects stems from patterns of NSO habitat use that are driven by fire. Throughout the frequent fire portion of the Pacific Northwest, NSO habitat consists of a core use area composed of dense, multi-layered old forests near drainage bottoms that NSO use for nesting and roosting, and more variable forest and non-forest conditions outside their core use area for foraging. Recent research on fire refugia, which are areas that burn less severely or frequently, as well as observed patterns of burn severity, suggest that conditions associated with these core use areas are more likely to burn at lower severity and thus be maintained by fire as old forest (i.e, fire-tended old growth). Areas outside their core use area burn more variably and are used by NSO for foraging, where frequent fire creates and maintains early seral stages where NSO prey becomes superabundant. Thus, fire creates and maintains both the old forest conditions required for nesting and roosting, and early seral stands that provide ample prey. While these results are encouraging and indicate fire has not had a measurable impact at the population level, future wildfire effects on NSO will depend upon future wildfire activity and severity in the region, which is predicted to change in response to climate change.