Climate and Fire Interactions
How does drought relate to fire severity?
The presence of live fuel was the most influential factor in predicting low-severity fire followed by year-of-fire climate. Low severity fire increased as temperatures and the climatic moisture deficit decreased. Topography and longer-term climate factors were not important predictors of low-severity fire.
Topography, pre-outbreak basal area, climate and short-term weather conditions during the fires all had a stronger influence on fire severity than spruce beetle infestation (< 5 years).
The authors found that fire severity was significantly related to drought severity. They suggest that projected increases in forest drought stress will likely lead to more intense and severe fires in the future, especially combined with high fuel densities, the legacy of a century of fire suppression.
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.
Despite occurring during time periods of similar short-term climate conditions, differences in fire severity and fire size were observed between areas of differing vegetation types. This is likely the result of differences in species composition and fuel amount and condition associated with longer-term climate effects.
The dendrochronology analysis in the study found that fairly large patches of high-severity fire were relatively common on the North Rim of the Grand Canyon. In dry mixed-conifer forests, historically, high-severity fire years were significantly drier than normal based on reconstructed PDSI, while years of low-severity fire were only slightly, though not significantly, drier than normal. They also found that for mixed-conifer ecosystems, prior wet years (typically needed to build up fuels in drier SW forests) were not necessary for severe fire to occur.
Both individual and phase combinations of ENSO, PDO, and AMO were strongly associated with moisture variability. The authors found that all stand-replacing fire years and 18 of 20 synchronous fire years occurred during periods of regional drought that were 2-4 times as dry as normal conditions. The authors suggest that ENSO strongly and consistently affects regional moisture conditions in the Southwest, thereby fire occurrence, including high severity fire occurrence, but that the effects can be tempered by phases of PDO.
The authors found that long term climatic stress, measured by climatic water deficit, predisposed trees to higher mortality from fire damage. The author suggest that warming temperatures increase fire severity, and ultimately tree mortality, independent of fire intensity.
Teleconnections between ENSO variability and fire activity are well documented across the Southwest. The authors suggest that shifts in the PDO may be a stronger gauge of the frequency of extreme fire weather than ENSO cycles for the Southwest, specifically. La Niña events typically bring dry winters to the Southwest and are related to a higher frequency of extreme fire weather events in May, especially when the PDO is in the negative phase causing consecutive drought years. Years associated with a positive PDO phase tend to result in wet winters, and consequently, rapid accumulation of fuels. A switch to a negative PDO phase may dry excess fuels leading to earlier and more extreme fire activity during these cycles.
High severity fire typically varied along elevation and moisture gradients, so that areas with increased dominance of Douglas fire typically burned at higher severities while xeric sites dominated by ponderosa pine typically burned at low-severities consistently. However, periods of drought or wind events can override fuel conditions leading to higher fire severities and variation from year to year.
Burn severity was significantly correlated with lack of precipitation represented by total number of days without rain and maximum consecutive number of days without rain. Snowpack was only marginally related to area burned and fire severity in the upper elevation forest types spruce-fir and mixed conifer. Finally, they also found that longer fire seasons measured by an increase in the total and consecutive number of days without rain may have increased area burned by providing longer periods of weather favorable for fire activity.
The Jasper Fire burned under extreme weather conditions and drought, however, the post-fire burn severity mosaic was highly variable due to the differences in pre-fire forest vegetation and topography. Despite summer drought conditions and abnormally low fuel moisture content of woody fuels, the fire was not dominated by high-severity fire. Instead, forest structure has the strongest influence on burn severity over topography or weather/climate conditions.
Beetle outbreaks followed by extreme drought, as occurred prior to the fire in the study, or wind are required to increase the flammability of the large, dead fuels, resulting in increased fire severity.
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.