COS 99-4
Fire activity and severity vary along proxy gradients representing fuel amount and fuel moisture in the western US
Numerous theoretical and empirical studies have shown that wildfire activity (e.g., area burned) at regional to global scales is limited at the extremes of macro-scale environmental gradients such as productivity or moisture. Fire activity, however, represents only one component of the fire regime, and no studies to date have been conducted that characterize fire severity along such gradients. This is a considerable knowledge gap, especially given the importance of fire severity in dictating ecological response to fire. For the western US, we empirically describe both fire activity and severity along two climatic water balance gradients, actual evapotranspiration (AET) and water deficit (WD), that can be considered proxies for fuel amount and fuel moisture. This was accomplished by summarizing area burned, satellite-inferred fire severity, AET and WD for each 50,000 ha pixel, then plotting and modelling each of the relationships. We limited our study to pixels containing a high proportion (≥ 80%) of protected areas (wilderness and national park) in the western US, where anthropogenic influences (e.g., forest management) are minimal relative to unprotected lands.
Results/Conclusions
Our results show that fire activity in the western US increases with fuel amount. The unimodal relationship that previous theoretical and empirical studies have reported was not seen here; this is likely because our study was limited to the western US, and therefore, we evaluated a truncated range of AET. Fire activity does, however, exhibit the expected unimodal (i.e. humped) relationship with fuel moisture. Fire severity, on the other hand, increases with both fuel amount and fuel moisture. To our knowledge, this is the first broad-scale characterization of fire activity and severity along physical environmental gradients, and as such, provides an enhanced view of contemporary fire regimes that complements existing classifications of fire activity. The explicit links between fire regime components and physical environmental gradients provides an initial framework for generating an empirically based fire regime map for the western US, and as such, can be used with gridded climate-change predictions to anticipate climate-mediated changes in fire recurrence and impacts.