High-severity wildfire effects on carbon stocks and emissions in fuels treated and untreated forest

Forests contain the world's largest terrestrial carbon stocks, but in seasonally dry environments stock stability can be compromised if burned by wildfire, emitting carbon back to the atmosphere. Treatments to reduce wildfire severity can reduce emissions, but with an immediate cost of reducing carbon stocks. In this study we examine the tradeoffs in carbon stock reduction and wildfire emissions in 19 fuels-treated and -untreated forests burned in twelve wildfires. The fuels treatment, a commonly used thinning ‘from below’ and removal of activity fuels, removed an average of 50.3 Mg C ha−1 or 34% of live tree carbon stocks. Wildfire emissions averaged 29.7 and 67.8 Mg C ha−1 in fuels treated and untreated forests, respectively. The total carbon (fuels treatment plus wildfire emission) removed from treated sites was 119% of the carbon emitted from the untreated/burned sites. However, with only 3% tree survival following wildfire, untreated forests averaged only 7.8 Mg C ha−1 in live trees with an average quadratic mean tree diameter of 21 cm. In contrast, treated forest averaged 100.5 Mg C ha−1 with a live tree quadratic mean diameter of 44 cm. In untreated forests 70% of the remaining total ecosystem carbon shifted to decomposing stocks after the wildfire, compared to 19% in the fuels-treated forest. In wildfire burned forest, fuels treatments have a higher immediate carbon ‘cost’, but in the long-term may benefit from lower decomposition emissions and higher carbon storage.

Research highlights▶ Fuels treatment reduced wildfire emissions by 56%. ▶ Total C loss in fuels treated/burned was higher than in untreated/burned forest. ▶ Wildfire transitioned 70% of untreated forest's carbon into decomposing stocks. ▶ Fuels treatment substantially reduced tree mortality and retained large tree C stocks. ▶ With high mortality, burned/untreated forests will likely become long-term C sources.