Do Vegetation Fuel Reduction Treatments Alter Forest Fire Severity and Carbon Stability in California Forests?

Forest fire frequency, extent, and severity have rapidly increased in recent decades across the western United States (US) due to climate change and suppression-oriented wildfire management. Fuels reduction treatments are an increasingly popular management tool, as evidenced by California's plan to treat 1 million acres annually by 2050. However, the aggregate efficacy of fuels treatments in dry forests at regional and multi-decadal scales is unknown. We develop a novel fuels treatment module within a coupled dynamic vegetation and fire model to study the effects of dead biomass removal from forests in the Sierra Nevada region of California. We ask how annual treatment extent, stand-level treatment intensiveness, and spatial treatment placement alter fire severity and live carbon loss. We find that a similar to 30% reduction in stand-replacing fire was achieved under our baseline treatment scenario of 1,000 km2 year-1 after a 100-year treatment period. Prioritizing the most fuel-heavy stands based on precise fuel distributions yielded cumulative reductions in pyrogenic stand-replacement of up to 50%. Both removing constraints on treatment location due to remoteness, topography, and management jurisdiction and prioritizing the most fuel-heavy stands yielded the highest stand-replacement rate reduction of similar to 90%. Even treatments that succeeded in lowering aggregate fire severity often took multiple decades to yield measurable effects, and avoided live carbon loss remained negligible across scenarios. Our results suggest that strategically placed fuels treatments are a promising tool for controlling forest fire severity at regional, multi-decadal scales, but may be less effective for mitigating live carbon losses. California has seen a marked increase in forest fire activity in recent decades. This trend is expected to continue with climate change, endangering lives and altering ecosystems. Fuels reduction treatments, including controlled burning and mechanical removal of woody debris from fire-prone forests, have received increasing policy attention in recent years as a wildfire mitigation strategy. However, the impacts of regional-scale, multi-decadal fuels reduction programs are not well understood. We use a coupled dynamic vegetation and fire model to compare the impacts of different fuels treatment strategies to understand how intersecting technical and political constraints impact treatment efficacy over 100 years of treatments. We find that precise assignment of treatments to the most fuel-heavy stands in the region could decrease cumulative avoided fire-driven stand-replacement rates by similar to 50% compared with no-treatment simulations, and by similar to 30% when prioritization of fuel loading was lower. Opening remote, rugged, and multi-stakeholder lands for treatment in tandem with high prioritization of fuel loading could increase total avoided stand mortality events to similar to 90%. Overall, avoided live carbon loss rates are less sensitive to treatment, and remained relatively small in absolute terms. Importantly, we found that even effective treatments may take multiple decades to yield measurable results on a regional scale. Fuels treatments are increasingly legislated as a mitigation tool to adapt to the changing wildfire regime in the western United States Fuels reduction programs that identify high risk forests decrease fire severity, but treatment effects only emerge after several decades Prioritizing treatments of fuel-heavy stands minimizes the time to significant treatment effects and maximizes fire severity reductions