Assessing fuel treatments and burn severity using global and local analyses

Background

Wildfires in western U.S. dry forest ecosystems have increased in size and severity during recent decades due primarily to more than a century of fire suppression, exclusion of Indigenous fire, and a rapidly warming climate. Fuel treatments have been employed to restore historical forest conditions and mitigate burn severity. However, their influence on burn severity in the context of other environmental variables and firefighting operations has not been extensively explored. The 2021 Bootleg Fire in south-central Oregon provided an opportunity to evaluate the effectiveness of mechanical thinning (Tx), broadcast burning (Rx), and both treatments combined (TxRx) near the Sycan Marsh Preserve, where pre-fire LiDAR data were also available.

Results

We assessed burn severity 1 year after the Bootleg Fire accounting for the local variability of top environmental drivers, fuel treatments, and firefighting operations. We modeled the influence of burn severity drivers using Random Forest and examined mean predictor effects (global scale) and their spatially explicit variability across observations (local scale) using SHapley Additive exPlanations (SHAP) analysis. Within units treated with broadcast burning, the percentage of area burned at low severity was over 80%. In contrast, units treated with thinning-only and untreated forests were dominated by area burned at moderate (45%) and high (42%) severity, respectively. All treatment types facilitated firefighting operations. Broadcast burning units, in which suppression activities occurred during the Bootleg Fire, showed a marginal decrease in predicted burn severity. Under consistent severe weather conditions, our results underscored the central role of fuel characteristics, including fuel treatments, and their local variability in influencing burn severity. The most important determinant of burn severity was Rx, followed by top drivers representing fuel structure and accumulation.

Conclusions

Our study highlights that fuel characteristics and broadcast burning disproportionally impacted burn severity, with Rx being the most effective and economical treatment. By creating a reproducible framework to explain burn severity, at both global and local scales, we gained nuanced insights about the drivers of burn severity that could inform and enhance fire and fuel management practices across multi-ownership landscapes.