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Prior studies have hypothesised that temperature is the single most important variable that influences wildland fire (e.g. Gillett et al. 2004). Antecedent temperature is hypothesised to advance the timing of snowmelt and green-up in forested systems, thereby potentially facilitating an earlier depletion of fuel moisture and extending the seasonal window during which fuels are receptive to fire (e.g. Westerling et al. 2006).We did not find strong evidence to support this hypothesis (particularly with respect to spring temperature) over the 1984–2010 period [in the western US], but rather found strong correlations between FSAB [fire season area burned] and summer temperature. Temperature is indirectly associated with fuel moisture stress and flammability; however, atmospheric variables more directly tied to moisture demand (i.e. ETo [reference evapotranspiration]) and the amount and timing of moisture supply as incorporated in biophysical variables may more accurately synthesise fuel flammability across the weather–climate continuum during the fire season.
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Forests and Ecosystems
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Wildfire Trends
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Geographic differences in climate–fire relationships and across predictor variables are apparent in both forested and non-forested areas. Several factors likely contribute to these differences including bi-modal fire regimes in a given region (e.g. autumn Santa Ana wind-driven wildfires v. summer wildfires in SO [Southern California]), sub-regional climate–fire relationships across different fuel classes or abundance, the length of the fire season, sub-regional differences in climate variability and human factors associated with ignition, landscape fire spread and fire suppression. The strongest relationships between in-season predictors and forested FSAB [fire season area burned] were found in the NR [Northern Rockies], EB [Eastern Great Basin], RM [Rocky Mountains] and NW [Pacific Northwest] GACCs [Geographic Area Coordination Center]. The NR, EB and NW have well defined fire seasons with over 95% of the area burned in forested lands occurring over a 3-month period (Fig. 2) that constrains the fire season to more limited temporal window and more common set of top-down climatic drivers of fire activity. These regions are also not fuel-limited; abundant moisture drives both consistent, abundant vegetation growth and serves as a barrier to large fire growth until conditions develop that drop moisture levels below critical thresholds. By contrast, the longer fire season in NO [Northern California] and SO [Southern California], and hence more diverse set of atmospheric drivers in addition to increased human ignitions likely dilutes climate– fire linkages.
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Strong correlations between FSAB [fire season area burned] and conditions realized during the fire season [in the western US] align with previous studies (e.g. Westerling et al. 2003; McKenzie et al. 2004; Gedalof et al. 2005; Morgan et al. 2008). The emergence of strong correlations during the fire season provides evidence of the importance of concurrent atmospheric conditions in fostering or inhibiting wildfire activity by compounding or superseding antecedent stressors (Fig. 7, and Bumbaco and Mote 2010). This highlights that long-term drought (.4 months) in forested areas is not a prerequisite for enabling large fire potential at macroscale levels. However, prolonged moisture deficits associated with subpar winter snowpack may be of more importance in enabling flammability at more localized scales in wetter forests with long fire-return intervals (e.g. forests west of the Cascade mountains in the Pacific Northwest or high elevation, northward facing aspects).
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Coherent antecedent climate–fire relationships at the macroscale levels analysed in this study [using area burned derived from the Monitoring Trends in Burn Severity dataset (1984–2010)] are generally consistent with previous analyses in forested and non-forested systems (e.g. Westerling et al. 2003; Littell et al. 2009). The lack of significant relationships between spring (March–May) temperature and FSAB [fire season area burned] differ from previous studies (e.g. Westerling et al. 2006) that encompassed a period of amplified springtime warming and snowpack recession in the western United States (Abatzoglou and Redmond 2007). We do, however, find midwinter SWE [snow water equivalent] was the strongest long-lead correlate to FSAB in several forested GACCs [Geographic Area Coordination Center]. Below-normal winter SWE may help precondition large-diameter dead fuels and contribute to the commencement of fuel availability in montane regions (e.g. Westerling et al. 2006; Gedalof 2011). This is further corroborated by the evolution of negative correlations with PDSI [Palmer Drought Severity Index] towards the end of spring that strengthen throughout the fire season in forested GACC areas.
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v3.2
