Evaluating potential trade-offs among fuel treatment strategies in mixed-conifer forests of the Sierra Nevada

Fuel treatments in fire-suppressed mixed-conifer forests are designed to moderate potential wildfire behavior and effects. However, the objectives for modifying potential fire effects can vary widely, from improving fire suppression efforts and protecting infrastructure, to reintroducing low-severity fire, to restoring and maintaining variable forest structure and wildlife habitat. In designing a fuel treatment, managers can alter the treatment's prescription, placement, and extent (collectively the "treatment strategy") to optimally meet one objective. However, the potential for trade-offs among different objectives is rarely tested systematically in fire-prone landscapes. To evaluate trade-offs in mechanical fuel treatment objectives related to fire severity, smoke production, forest heterogeneity, and avian wildlife habitat, we used a cross-platform modeling approach based on spatially explicit modifications of forest structure data for a 7820-ha landscape in the Lake Tahoe Basin, California. We examine whether (1) a more uniform treatment strategy aimed at fire hazard reduction (FHR) had negative effects on wildlife diversity, (2) a strategy focused on protecting the wildland-urban interface (WUI) left other portions of the landscape vulnerable to high-severity fire, and (3) increasing the extent of fuel treatments across the landscape led to greater reductions in fire severity and smoke emissions. When approximately 13% of the landscape was treated, the proportion of the landscape vulnerable to high-severity fire decreased by 13-44%, with the more uniform FHR strategy leading to greater reductions. Slight increases in predicted avian species richness that followed all treatment strategies were not closely linked to increases in canopy variability. The WUI protection strategy led to considerable reductions in fire severity at the landscape scale. Increasing the extent of treatments to 30% of the landscape did little to further reduce the area vulnerable to high-severity fire, with additional reductions of 4-7% depending on the prescription. However, increasing the extent of treatments reduced the extent of harmful downwind smoke impacts, primarily by reducing rate of fire spread. Treatment strategies will depend on specific management objectives, but we illustrate that trade-offs are not necessarily inherent in general outcomes of fuel treatments.