Assessing risks to spotted owls from forest thinning in fire-adapted forests of the western United States
Concern for viable spotted owl (Strix occidentalis) populations has played prominently in the management of western forests in the United States. Historically, much of the debate has focused on the impacts of commercial timber harvest. Increasingly, the conflict is shifting to the habitat needs of owls versus the need for active management of fire-adapted forest ecosystems to reduce the occurrence of uncharacteristic wildland fire that threatens multiple resource values. While some authors have called for active fuels management within the range of spotted owls, concerns remain over potential effects of less intensive, but more extensive, fuel reduction activities (i.e., thinning and prescribed burning). Reconciling fire and fuels management with owl conservation requires rigorous analysis of both the short- and long-terms risks of action versus no action. We use published literature and data from the southern Sierra Nevada to examine the potential effects of landscape-level reductions in canopy cover (CC) on owl occupancy and reproduction. Using a combination of population data, canopy cover measurements, and forest simulation models, we show that modest fuels treatments in the Sierra Nevada would not be expected to reduce canopy cover sufficiently to have measurable effects on owl reproduction. Sixty-year simulations predict that mechanical thinning or mechanical thinning plus fuel-break construction treatments in combination with either no fire or mixed-lethal fire scenarios will not degrade canopy conditions in productive owl territories, nor impede improvement of non-productive territories. In contrast, lethal fire simulations produced a pronounced and lasting negative effect. Our analysis supports the hypothesis that habitat needs for owl reproduction can be incorporated in developing effective fire and fuels management strategies that lessen the chances of uncharacteristic wildfire. Projections of future population trends are tempered by the knowledge that non-habitat factors such as variations in weather profoundly affect population dynamics.