Biomass stocks in California's fire-prone forests: mismatch in ecology and policy

Restoration of fire-prone forests can promote resiliency to disturbances, yet such activities may reduce biomass stocks to levels that conflict with climate mitigation goals. Using a set of large-scale historical inventories across the Sierra Nevada/southern Cascade region, we identified underlying climatic and biophysical drivers of historical forest characteristics and projected how restoration of these characteristics manifest under future climate. Historical forest conditions varied with climate and site moisture availability but were generally characterized by low tree density (similar to 53 trees ha(-1)), low live basal area (similar to 22 m(2) ha(-1)), low biomass (similar to 34 Mg ha(-1)), and high pine dominance. Our predictions reflected broad convergence in forest structure, frequent fire is the most likely explanation for this convergence. Under projected climate (2040-2069), hotter sites become more prevalent, nearly ubiquitously favoring low tree densities, low biomass, and high pine dominance. Based on these projections, this region may be unable to support aboveground biomass >40 Mg ha(-1) by 2069, a value approximately 25% of current average biomass stocks. Ultimately, restoring resilient forests will require adjusting carbon policy to match limited future aboveground carbon stocks in this region.

Automated summary

The restoration of fire-prone forests and their resilience to disturbances is important, but it may conflict with climate mitigation goals by reducing biomass stocks. This study analyzed historical inventories in the Sierra Nevada/southern Cascade region to identify drivers of historical forest characteristics and projected future changes under climate scenarios. The predictions showed convergence in forest structure, with frequent fire being the likely explanation. Under projected climate, hotter sites favor low tree densities, low biomass, and high pine dominance. The study suggests that by 2069, the region may not be able to support aboveground biomass exceeding 25% of current average biomass stocks. Carbon policy needs to be adjusted to match the limited future aboveground carbon stocks in this region.