Background

Dry mixed-conifer forests of the southwestern United States are experiencing rapid, anthropogenically driven fire regime change. Prior to the Euro-American settlement, most of these forests experienced frequent surface fires but are now vulnerable to uncharacteristically large, high-severity fires. Fire directly influences the structure and composition of these forests and, in turn, the wildlife that inhabit them. Changing fire regimes result in a certain decline of some species and uncertain consequences for others. The Mexican spotted owl (Strix occidentalis lucida) is a federally listed threatened species of particular note in southwestern mixed-conifer forests. High-severity fire is cited as the owl’s primary threat in the revised species recovery plan, but uncertainties surround the impacts of high-severity fire on the habitat of the threatened owl, particularly across a timeframe longer than a few years. Our objective was to explore the long-term (100-year) effects of fire severity on elements of forest structure vital for Mexican spotted owl nesting. We quantified structural attributes for nest/roost habitat across mixed-conifer forests that burned at varying severity levels and time periods in the last century. We then examined the drivers of structural attributes by detecting statistical differences between severity classes and time periods through permutational multivariate analysis of variance.

Results

High-severity fire has the strongest deleterious impact on elements of forest structure (total basal area, percent medium tree basal area, percent large tree basal area, large tree density, and canopy cover) vital to Mexican spotted owl nesting, and although the structural differences between severity classes diminish with time, it took ≥ 80–100 years to reach the structural conditions desired for Mexican spotted owl nesting after stand-replacing fires. The most important attribute measured, canopy cover, required 90–100 years after high-severity fires to reach levels most suitable for Mexican spotted owls in the Lincoln National Forest.

Conclusions

As fires increase in frequency, severity, and size compared to the last century, the Lincoln National Forest is projected to face an overall decrease in the structural conditions needed for Mexican spotted owl nesting habitat in this region. Short intervals between uncharacteristically high-severity fires in particular pose an imminent threat to nesting habitat.