Evaluating the drivers of fire severity during the 2020 California wildfires in relation to management, vegetation, and time since last fire

Alison K. Paulson, Department of Environmental Science and Policy, University of California, Davis, Davis, CA, Shana E. Gross, Pacific Southwest Region, USDA Forest Service, South Lake Tahoe, CA, Derek J.N. Young, Dept. of Plant Sciences, University of California, Davis, Davis, CA and Hugh D. Safford, Pacific Southwest Region, USDA Forest Service, Vallejo, CA; Department of Environmental Science and Policy, University of California, Davis, CA

Background/Question/Methods
The 2020 fire season in California was unprecedented in terms of acres burned (>4,200,000 acres), producing six of the 20 largest wildfires on record in the state. The initial impacts of these fires on social-ecological communities have been drastic: more than 10,000 structures lost, communities exposed to harmful smoke emissions for extended periods of time, and lives of both civilians and firefighters lost. Many of the forests that burned in 2020 previously experienced more than 100 years of fire suppression, leading to the buildup of fuels that exacerbate fire spread and severity. Additionally, climate change has extended the length of the growing season and increased the prevalence of conditions that favor fire. During the peak of the 2020 fire season, there were calls for both increased forest management to reduce forest fuels and immediate action on climate change mitigation. Here, we used initial assessments of fire severity in four 2020 fires (August Complex, Castle, Creek, and North Complex) to evaluate how forest management activities (e.g., thinning and prescribed fire) affect fire severity. To do so, we compared fire severity within areas that had forest management during the past 20 years versus areas without any previous fire management activity.
Results/Conclusions
While results varied across fires and forest management activities, initial results indicate that fire severity (measured using basal area loss correlations with remotely sensed RdNBR) was reduced in areas where forest management actions occurred during the past 20 years, with lower proportions of high-severity fire (>90% basal area loss) inside management boundaries compared with outside. For instance, across all treatment types and former wildfire areas within the Castle fire, the proportion of the fire boundary that burned at low severity was 50% higher inside treated areas compared with untreated areas and the proportion that burned at moderate or high severity was 54% lower. These results provide evidence that forest management activities can reduce future fire severity on landscapes even under extreme fire weather conditions. But, of the four fires explored here, only 2-15% of the fire areas were treated prior to the 2020 fires. Thus, while forest management has the potential to reduce fire severity, it is unclear if the scope of current forest management activities is sufficient to offset the increase in fire activity resulting from past fire suppression and climate change.