Proportion of forest area burned at high-severity increases with increasing forest cover and connectivity in western US watersheds

Context In western US forests, the increasing frequency of large high-severity fires presents challenges for society. Quantifying how fuel conditions influence high-severity area is important for managing risks of large high-severity fires and understanding how they are changing with climate change. Fuel availability and heterogeneity influence high-severity fire probability, but heterogeneity is insensitive to some aspects of forest connectivity that are important to potential high-severity fire transmission and thus high-severity area. Objectives To quantify the effects of fuel availability, heterogeneity, and connectivity on the proportion of forest area burned at high-severity (high-severity burn area). To use the extreme 2020 fire season to consider how climate change could affect high-severity burn area relationships. Methods We used datasets derived from remote sensing to quantify effects of forest fuel availability, heterogeneity, and connectivity on extreme (95th percentile) high-severity burn areas in western US coniferous watersheds from 2001 to 2020. We developed a connectivity metric to quantify potential high-severity fire transmission. Results High-severity burn area increased with increasing fuel availability and connectivity and decreased with increasing heterogeneity. In 2020, multiple large high-severity burn areas occurred in forests with high fuel availability, which only had small high-severity burn areas prior to 2020. Conclusions In forests with an annual fire season, management to limit forest connectivity and fuel accumulation and increase heterogeneity could mitigate the potential for large high-severity fires. In forests where climate usually limits fire, large high-severity fires may occur more frequently if climate change increases the frequency of years with inadequate climatic barriers to wildfire.