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Background Trends of accelerating area burned in many regions worldwide are leading to increases in the amount of area experiencing short-interval reburns (i.e., fires occurring two or more times in the same place within 1–3 decades). Field and satellite indices of burn severity are well tested in forests experiencing a single recent fire, but the reliability of these indices in short-interval reburns is poorly understood. We tested how a commonly used field index (the Composite Burn Index, CBI) and satellite index (the Relative differenced Normalized Burn Ratio, RdNBR) compared to eight individual field measures of burn severity in reburns vs. areas burned in a single recent fire. We also tested if relationships in reburns depended on whether the first fire was stand replacing (fire that is lethal to most dominant trees). Results The correspondence between both CBI and RdNBR with individual burn severity measures differed in reburns compared to single fires for some metrics of burn severity. Divergence in the relationship between both CBI and RdNBR vs field measures was greatest when reburns occurred following a preceding stand-replacing fire, and measures were more comparable to single fires when the first fire was non-stand replacing (i.e., lower severity). When reburns occurred where a preceding fire was stand-replacing, CBI and RdNBR underestimated burn severity in the 2nd fire for tree-canopy metrics (e.g., canopy cover loss, tree mortality), as post-fire forests in early developmental stages are vulnerable to greater severity in a second fire. Conversely, when reburns followed a preceding fire that was not stand replacing, both CBI and RdNBR overestimated surface burn severity, as past low severity fires leave behind live fire resistant trees and can stimulate resprouting understory vegetation. Finally, neither CBI nor RdNBR were able to accurately detect the amount of deep wood charring that has emerged as an important product in reburns – particularly where both fires are stand-replacing. Conclusion Our findings inform interpretability of commonly used indices of burn severity in reburns, which is becoming an increasingly common application as fire activity increases. |
