Forest thinning and prescribed burning treatments reduce wildfire severity and buffer the impacts of severe fire weather

Abstract

Background: The capacity of forest fuel treatments to limit the behavior and severity of subsequent wildfires depends on weather and fuel conditions at the time of burning. We compared the impacts of five fuel treatments—including two thin-only, a thin-burn, a burn-only, and a control—on fire severity using a 1200 hectare randomized and replicated experiment that burned almost entirely in a subsequent wildfire. We evaluated four fire severity metrics (mortality, average bole char height, percent crown volume torched [PCVT], and percent crown volume affected [PCVA]) and leveraged pre-fire surface and canopy fuels data to better understand the mechanisms driving differences in wildfire severity among treatments and how they changed with fire weather. Results: We found strong mitigating effects of treatments on fire behavior and tree mortality, despite 20 years having elapsed since mechanical thinning and 10 years since the second entry of prescribed fire. The thin-burn treatment resulted in the lowest fire severity across all four metrics and the untreated control the highest. Prefire canopy and surface fuel loads were associated with all four fire severity metrics, with the exception that surface fuel loads were not associated with PCVT, a fire severity metric associated with crown fire behavior. The relationship between fuel treatment effectiveness and fire weather varied among fire severity metrics, with maximum fuel treatment benefit at intermediate burning index values for tree mortality, intermediate to high burning index values for PCVA, and high burning index for bole char height and PCVT. Conclusions: We conclude that reducing canopy bulk density via mechanical thinning treatments can help to limit crown fire behavior for 20 years or more. However, reducing surface fuels is necessary to limit scorching and the total crown impacts associated with tree mortality. Further, while fuel treatment effectiveness may decline at the most severe fire weather for some fire severity metrics (total crown impacts and mortality), it is maximized under severe fire weather conditions for others (bole charring and torching). Our results provide strong evidence for the use of fuel treatments to mitigate fire behavior and resulting fire severity even under extreme fire weather conditions.