Climate and Fire Interactions
How do temperature, relative humidity and vapor pressure deficit relate to fire severity?
The FireBGCv2 model in the Jemez mountains projected an increase in fire frequency and high-severity fire due to the predicted increases in temperature and moisture deficit.
The presence of live fuel was the most influential factor in predicting low-severity fire followed by year-of-fire climate. Low severity fire also increased as temperatures and the climatic moisture deficit decreased. Topography and longer-term climate factors were not important predictors of low-severity fire.
Average daytime temperature was the most important predictor of high severity fire in the reburn. When temperatures exceeded a threshold of >27.3°C (81°F), snag basal area and shrub cover were the strongest variable driving reburn severity patterns.
March through August VPD, a measure of the ability of the atmosphere to extract moisture from surface vegetation, is more strongly correlated with area burned at high severity than temperature alone for the Southwest region.
Large fire occurrence was highly synchronous across broad spatial scales and significantly correlated to short-term climate anomalies. Specifically, precipitation anomalies 90 days prior to the fire had the strongest influence on large fire occurrence and percent high severity fire more than temperature or relative humidity.
The optimized Random Forest model found that vegetation and topography were stronger predictors of burn severity than daily weather. Temperature and humidity were only marginally important. Wind speed percentile was, however, the strongest climate/weather predictor of fire severity (and third strongest predictor overall) for daily areas burned when considering only the largest daily areas burned (99.5% percentile or > 600 ha) while temperature and humidity were not important in this model at all. The authors suggest that the factors influencing fire severity are not necessarily the same as those that control fire extent.
The authors found that fire activity/area burned and fire severity across the western US increased with increasing actual evapotranspiration (AET) which represented fuel amount in the study. Fire severity decreased with increasing water deficit.
The authors found significant increases in the fire severity across all global climate model scenarios by the end of the century likely due to the role temperature plays in fire occurrence and severity. Global fire severity was expected to increases the greatest in the Northern Hemisphere by as much as three times the baseline Cumulative Severity Rating.
The authors found that topography had a stronger influence on burn severity than climate (temperature and precipitation), but that climate before and during the fire was still a significant predictor of burn severity. The authors suggest that climate conditions can overwhelm topographic limitations on area burned and fire severity during dry years with widespread fire.
For the Cerro Grande fire, which occurred during an extreme La Niña event, topography had a stronger influence on reducing severity in some areas than the regional effects of climate. Conversely, on several of the fires, topography conducive to fire spread did not produce increased severity, and temperature and wind had a stronger influence on fire effects.
The authors found that during historically cooler periods, forests burned frequently at low severity, which they suggest was driven by increases in understory vegetation growth. Historically warm periods were linked to severe drought and an increase in high severity fires that caused large debris-flow events and fire-related erosion.
The authors summarized findings on high-severity fire regimes in subalpine forests and found that climatic variation is the predominant influence on fire frequency and severity in this ecosystem type and suggest that fuel reduction treatments would move stand structure away from its historical range of variability. For low severity fire regimes in low-elevation ponderosa pine forest, the authors found that fire frequency and severity were driven by the spatial and temporal variation of fine fuels more so than climate.
The authors’ modeling effort found that seasonal severity rating (SSR) may increase 10-50% by 2060 likely increasing area burned and fire severity across much of the U.S.