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.

The ecological effects of forest fires burning with high severity are long-lived and have the greatest impact on vegetation successional trajectories, as compared to low-to-moderate severity fires. The primary drivers of high severity fire are unclear, but it has been hypothesized that wind-driven, large fire-growth days play a significant role, particularly on large fires in forested ecosystems. Here, we examined the relative proportion of classified burn severity for individual daily areas burned that occurred during 42 large forest fires in central Idaho and western Montana from 2005 to 2007 and 2011. Using infrared perimeter data for wildfires with five or more consecutive days of mapped perimeters, we delineated 2697 individual daily areas burned from which we calculated the proportions of each of three burn severity classes (high, moderate, and low) using the differenced normalized burn ratio as mapped for large fires by the Monitoring Trends in Burn Severity project. We found that the proportion of high burn severity was weakly correlated (Kendall τ = 0.299) with size of daily area burned (DAB). Burn severity was highly variable, even for the largest (95th percentile) in DAB, suggesting that other variables than fire extent influence the ecological effects of fires. We suggest that these results do not support the prioritization of large runs during fire rehabilitation efforts, since the underlying assumption in this prioritization is a positive relationship between severity and area burned in a day.

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