COS 175-5 - Proportion of forest area burned at high-severity increases with increasing forest cover and connectivity in western US watersheds

Abstract: In western US forests, climate change and fire suppression have driven an increase in the frequency of large high-severity fires. Quantifying how fuel conditions influence the size of high-severity fires is important for managing their risks and understanding how they are changing with climate change. Forest connectivity is important to the potential transmission of high-severity fire, and thus high-severity fire size. Previous studies conducted over large scales have found that fuel availability and heterogeneity (variability) influence the probability of high-severity fire. The goals of this study were to quantify the roles of forest fuel availability, heterogeneity, and connectivity in the extreme (95^th percentile) proportion of forest area burned at high-severity (high-severity burn area) in western US coniferous forest watersheds from 2001-2020. We used datasets of forest cover derived from remote sensing to quantify fuel availability, heterogeneity, and connectivity, and used satellite-derived spectral reflectance indices to model the composite burn index (CBI) in large fires that occurred in western US coniferous forest watersheds from 2001-2020. We developed a forest connectivity metric designed to quantify the proportion of the forest area susceptible to high-severity fire, if high-severity fire was present in any single forest location in the watershed. We found that in western US coniferous forest watersheds from 2001-2020, the proportion of forest area burned at high-severity increased with increasing fuel availability, decreased with increasing fuel heterogeneity, and increased weakly but robustly with increasing forest connectivity. The inclusion of the extreme 2020 fire season had an outsized influence on the estimated effect of fuel availability on high-severity burn area, shifting the relationship from hump-shaped to linear. Notably, the 2020 season included many large fires in the US Pacific Northwest (PNW), resulting in an increase in high-severity burn area in relatively wet forests with low long-term mean climatic water deficit. In forested watersheds with a consistent annual fire season, high fuel availability, low fuel heterogeneity, and high forest connectivity increase the potential size of large high-severity fires. In watersheds where climate is usually limiting to fire (e.g., the PNW), the risk of large high-severity fires will depend, in part, on whether climate change increases the frequency of years with inadequate climatic barriers to wildfire.