Increasing area burned in forested regions around the world is leading to many areas burning more than once in short-interval reburns (areas that have experienced two fires within 1–3 decades, Prichard et al. 2017). Reburns are expected in any fire-prone region, though their consequences can differ based on ecological context. For example, in forests adapted to historically frequent fire, short-interval reburns can represent a return to historical fire-return intervals and foster resilience to fire (Larson et al. 2013). In contrast, in forests with historically longer fire-return intervals, short-interval reburns can produce novel extreme levels of burn severity and catalyze ecological shifts (Donato et al. 2009, Turner et al. 2019). Building an understanding of how to quantify burn severity in reburns and accurately tracking trends in such metrics is important for characterizing how fire regimes are changing.
Reliable and widely used indices exist to measure burn severity in the field, with satellites, and linking field- and satellite data. The Composite Burn Index, or CBI, is a unitless semi-quantitative ocular estimation of burn severity across five forest strata developed to validate remote sensing measures based on the normalized burn ratio (NBR) which is sensitive to fire-caused vegetation changes (Key and Benson 2006). CBI is useful in that it is a relatively quick (requiring less than one hour per plot) field protocol which corresponds well with field measures based on plant injury, fuel consumption, and tree mortality (Miller et al. 2009). Individual field measures such as fire-killed basal area and char height have also been established as metrics of burn severity (Harvey et al. 2014), and CBI generally corresponds well with many independently measured metrics (Saberi et al. 2022). Plot-level measures of burn severity, such as CBI, have been used to calibrate satellite-derived burn severity indices—allowing for the wall-to-wall mapping of burn severity across broad spatial extents. For example, RdNBR (relative differenced normalized burn ratio, Miller and Thode, 2007) is one of the most commonly used indices (Lentile et al. 2006) and provides an index of burn severity by calculating the ratio of the difference between the near infrared and shortwave infrared bands from pre-and post-fire Landsat imagery. Despite the widespread use of these field and satellite indices to measure burn severity in both single burns and reburns (e.g., Parks et al. 2014a, Harvey et al. 2016a), how well they perform in short-interval reburns has not been widely tested.
A short-interval reburn may affect how these indices record burn severity in several ways. First, the severity (the magnitude of fire impacts on vegetation, Keeley 2009) of the first fire can affect forest structure in ways that lead to different post-fire vegetation that a second fire encounters. For example, a first fire that is non-stand replacing (i.e., burns at low severity) is unlikely to drastically alter forest structure, leaving behind biological legacies such as live thick-barked, fire resistant trees if they were present pre-fire. This is particularly likely in low-severity and frequent-fire regimes dominated by trees with adaptations to survive low-intensity fires, and understory vegetation that can either resprout or regenerate from seed (Agee 1996). Conversely, a first fire that is stand-replacing (high severity) produces greater effects on stand structure and leaves fewer post-fire live biological legacies. This outcome is more likely to occur in high-severity fire regimes dominated by trees with fewer adaptations to survive fire, but instead possessing adaptations such as aerial seedbanks or wind-dispersed seed (Agee 1996). As such forests are in an early-seral stage following one fire, this can lead to reburns where the 2nd fire encounters young fire-sensitive tree seedlings and saplings with tree crowns close to the forest floor. In such cases, burn severity in the 2nd fire can be so extreme that most above-ground live and downed woody material is consumed (e.g., Turner et al. 2019, Fig. 1, bottom right panel). Thus, it is possible that burn severity indices well-calibrated to single-fires may relate differently to burn severity in reburns depending on the severity of—and structural legacies left behind by—the first fire.
In this study, we address these gaps by asking the following research questions. First, how does the relationship between eight independent field measures of burn severity and CBI (Q1) and RdNBR (Q2) vary between single fires and areas that have experienced short-interval reburns? Further, in each question, we tested whether the relationship depended on if the first fire in a reburn was stand-replacing. We expect the relationships between individual field measures of burn severity and both CBI and RdNBR to differ depending on the severity of the previous burn and the magnitude of initial fire-caused changes to forest structure (i.e., what biological legacies remain after one fire). For RdNBR, we expected differences to depend on the ability of the top-down perspective of the satellite to capture spectral signatures from different forest strata or different stand densities. Finally, we were interested in testing how well either CBI or RdNBR captured indices of extreme burn severity (e.g., deep charring and combustion of woody material) that has been observed in recent short-interval fires (Turner et al. 2019).