Is proportion burned severely related to daily area burned?

Abstract

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.

Spatiotemporal patterns of unburned areas within fire perimeters in the northwestern United States from 1984 to 2014

“…This consistency of heterogeneous pattern regardless of size is congruent with Birch et al. (), who examined burn severity patterns for individual days of fire progression and found no relationship between days with large fire growth and more homogeneous and higher‐severity fire patterns.…”

“…Our results indicate that this is not the case and that fire refugia continue to form with consistent heterogeneity and pattern even as fires have grown larger over recent decades; this remains true even when stratifying by ecoregion within the northwest. This consistency of heterogeneous pattern regardless of size is congruent with Birch et al (2014), who examined burn severity patterns for individual days of fire progression and found no relationship between days with large fire growth and more homogeneous and higher-severity fire patterns.…”

Climate Contributors to Forest Mosaics: Ecological Persistence Following Wildfire

“…It is notable that the Burning Index, the only metric that incorporates wind, was only significantly positively correlated with persistent patch size and density in Glacier, where the fire regime is characterized by stand-replacing fire, but not the other two sites, which are characterized by more mixed-severity fire regimes. This contrasts studies identifying wind as significantly correlated to fire size in each of the three ecoregions (Abatzoglou and Kolden 2013), and suggests that wind may play a role in Glacier in producing fast-moving fire "runs" that skip over large patches, a hypothesis that is supported by Birch et al's (2014) findings that large fire "runs" in the Northern Rockies are not significantly higher severity than smaller runs. Because BI here was a summer average, we interpret the results as a windier summer being associated with fire behavior that produces fewer, but larger persistent patches.…”

The climate space of fire regimes in north‐western North America

“…Fire size also had a significant, unimodal relationship with burn severity. Large fires tend to have heterogeneous levels of burn severity and may contain larger, continuous areas of severely burned stands (related to higher values of dNBR), whereas smaller fires may be dominated by one level of burn severity (Turner et al, 1994;Birch et al, 2014). Climate domains with the largest fires were associated with moderate values of dNBR and those with smaller fires were associated with higher or lower than average burn severity.…”