SEVERE FIRE THREATENS AN ICONIC MATURE FOREST SPECIES, THE SPOTTED OWL, MORE THAN FUELS MANAGEMENT OR DROUGHT-RELATED TREE MORTALITY

Abstract

Seasonally dry forests are being transformed globally by intensifying fire regimes and drought-related tree mortality (DRTM), with profound implications for biodiversity. Reducing fuel densities is the primary tool available to forest managers striving to improve forest resilience, but implementation is constrained by concern over the effects on biodiversity – particularly older forest associated species, such as spotted owls. While the negative effects of severe fire on spotted owls are well documented, the influence of DRTM and fuels management on populations is uncertain, impeding fuels management. Here, we integrated a novel dataset of California disturbance history (Kramer et al. this issue.) with bioregional-scale passive acoustic monitoring to compare the relative effects of severe fire, severe DRTM, and fuels management on Sierra Nevada spotted owl occupancy. Spotted owls were less likely to occupy 400ha survey sites with a greater proportion of forest that burned at high severity over the past 13 years and sites with a greater proportion of “heavier” fuels management (>25% canopy reduction) but were insensitive to the proportion of “lighter” fuels management (<25% canopy reduction) at survey sites. However, the negative effect of severe fire on site occupancy was three times greater than that of heavier management, and severe fire resulted in the estimated loss of 509 occupied sites compared to only 65 lost from heavier fuels management. In contrast, spotted owls were more likely to occur at sites containing a greater proportion of severe DRTM, presumably because of foraging opportunities created by more open and heterogenous forest conditions. Our study represents the first to empirically assess the effects of the primary forest disturbance agents and fuels management on spotted owls. We found that severe fire is a greater threat to spotted owls than fuels management, adding to the growing body of literature underscoring the need to increase the pace and scale of fuels management to both promote forest resiliency and conserve older forest species.