The carbon balance of reducing wildfire risk and restoring process: an analysis of 10-year post-treatment carbon dynamics in a mixed-conifer forest

Forests sequester carbon from the atmosphere, helping mitigate climate change. In fire-prone forests, burn events result in direct and indirect emissions of carbon. High fire-induced tree mortality can cause a transition from a carbon sink to source, but thinning and prescribed burning can reduce fire severity and carbon loss when wildfire occurs. However, treatment implementation requires carbon removal and emissions to reduce high-severity fire risk. The carbon removed and emitted during treatment may be resequestered by subsequent tree growth, although there is much uncertainty regarding the length of time required. To assess the long-term carbon dynamics of thinning and burning treatments, we quantified the 10-year post-treatment carbon stocks and 10-year net biome productivity (NBP) from a full-factorial experiment involving three levels of thinning and two levels of burning in a mixed-conifer forest in California’s Sierra Nevada. Our results indicate that (1) the understory thin treatment, that retained large trees, quickly recovered the initial carbon emissions (NBP = 31.4 ± 4.2 Mg C ha−1), (2) the carbon emitted from prescribed fire in the burn-only treatment was resequestered within the historical fire return interval (NBP = 32.8 ± 3.5 Mg C ha−1), and (3) the most effective treatment for reducing fire risk, understory thin and burn, had negative NBP (−6.0 ± 4.5 Mg C ha−1) because of post-fire large tree mortality. Understory thinning and prescribed burning can help stabilize forest carbon and restore ecosystem resilience, but this requires additional emissions beyond only thinning or only burning. Retaining additional mid-sized trees may reduce the carbon impacts of understory thinning and burning.