Abstract
Burn severity as inferred from satellite‐derived differenced Normalized Burn Ratio (dNBR) is useful for evaluating fire impacts on ecosystems but the environmental controls on burn severity across large forest fires are both poorly understood and likely to be different than those influencing fire extent. We related dNBR to environmental variables including vegetation, topography, fire danger indices, and daily weather for daily areas burned on 42 large forest fires in central Idaho and western Montana. The 353 fire days we analyzed burned 111,200 ha as part of large fires in 2005, 2006, 2007, and 2011. We expected that local “bottom‐up” variables like topography and vegetation would influence burn severity, but that our use of daily dNBR and weather data would uncover stronger relationships between the two than previous studies have shown. We found that percent existing vegetation cover had the largest influence on burn severity, while weather variables like fine fuel moisture, relative humidity, and wind speed were also influential but somewhat less important. Our results could reflect contrasting scales of predictor variables, as many topography and vegetation variables (30‐m spatial resolution) accounted for more of the variability in burn severity (also 30‐m spatial resolution) than did fire danger indices and many daily weather variables (4‐km spatial resolution). However, we posit that, in contrast to the strong influence of climate and weather on fire extent, “bottom‐up” factors such as topography and vegetation have the most influence on burn severity. While climate and weather certainly interact with the landscape to affect burn severity, pre‐fire vegetation conditions due to prior disturbance and management strongly affect vegetation response even when large areas burn quickly.
Highlights
Fires are globally important disturbances that affect ecosystems (Bond et al 2004, Bowman et al 2009)
The differenced Normalized Burn Ratio spectral index of burn severity calculated from pre-fire and one year post-fire satellite imagery has been correlated with field-based assessments of burn severity (Van Wagtendonk et al 2004, Cocke et al 2005, De Santis and Chuvieco 2009, Jones et al 2009), and with percent tree mortality in forested areas (Cocke et al 2005, Hudak et al 2007, Lentile et al 2007, Smith et al 2010)
We expected that (1) that ‘‘bottom-up’’ variables such as topography and vegetation will still have a large effect on burn severity and post-fire vegetation response, (2) that using burn severity and weather data matched to the same time period will result in stronger relationships between the two and advance our understanding of how weather influences severity, and (3) topography and vegetation will influence burn severity even when we focus only on the largest daily areas burned in the largest fires even as these fires burned under extreme weather
Summary
Fires are globally important disturbances that affect ecosystems (Bond et al 2004, Bowman et al 2009). Burn severity as inferred from remotely sensed imagery has been widely used to evaluate fire effects on ecosystems (Smith et al 2005, 2010, French et al 2008, Soverel et al 2010, Morgan et al 2014). The differenced Normalized Burn Ratio (dNBR) spectral index of burn severity calculated from pre-fire and one year post-fire satellite imagery has been correlated with field-based assessments of burn severity (Van Wagtendonk et al 2004, Cocke et al 2005, De Santis and Chuvieco 2009, Jones et al 2009), and with percent tree mortality in forested areas (Cocke et al 2005, Hudak et al 2007, Lentile et al 2007, Smith et al 2010)
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