Previous wildfires and management treatments moderate subsequent fire severity

We investigated the relative importance of daily fire weather, landscape position, climate, recent forest and fuels management, and fire history to explaining patterns of remotely-sensed burn severity - as measured by the Relativized Burn Ratio - in 150 fires occurring from 2001 to 2019, which burned conifer forests of northeastern Washington State, USA. Daily fire weather, annual precipitation anomalies, and species' fire resistance traits were important predictors of wildfire burn severity. In areas burned within the past two to three decades, prior fire decreased the severity of subsequent burns, particularly for the first 16 postfire years. In areas managed before a wildfire, thinning and prescribed burning treatments lowered burn severity relative to untreated controls. Prescribed burning was the most effective treatment at lowering subsequent burn severity, and prescribed burned areas were usually unburned or burned at low severity in subsequent wildfires. Patches that were harvested and planted <10 years before a wildfire burned with slightly higher severity. In areas managed within 5 years after an initial fire, postfire harvest and planting reduced prevalence of stand-replacing fire in reburns. However, overall, postfire management actions after a first wildfire only weakly influenced the severity of subsequent fires. The importance of fire-fire interactions to moderating burn severity establishes the importance of stabilizing feedbacks in active fire regimes, and our results demonstrate how silvicultural treatments can be combined with prescribed fire and wildfires to maintain resilient landscapes.

Automated summary

This study in northeastern Washington State, USA, analyzed 150 fires occurring from 2001 to 2019 in conifer forests, and found that daily fire weather, annual precipitation anomalies, and species' fire resistance traits were important predictors of wildfire burn severity. Previous fires within the past two to three decades reduced the severity of subsequent burns, while prescribed burning was the most effective treatment at lowering subsequent burn severity. Overall, postfire management actions had a weak influence on the severity of subsequent fires, highlighting the importance of stabilizing feedbacks in active fire regimes.