Abstract
Numerous theoretical and empirical studies have shown that wildfire activity (e.g., area burned) at regional to global scales may be limited at the extremes of environmental gradients such as productivity or moisture. Fire activity, however, represents only one component of the fire regime, and no studies to date have characterized fire severity along such gradients. Given the importance of fire severity in dictating ecological response to fire, this is a considerable knowledge gap. For the western US, we quantify relationships between climate and the fire regime by empirically describing 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, respectively. We also concurrently summarize fire activity and severity among ecoregions, providing an empirically based description of the geographic distribution of fire regimes. Our results show that fire activity in the western US increases with fuel amount (represented by AET) but has a unimodal (i.e., humped) relationship with fuel moisture (represented by WD); fire severity increases with fuel amount and fuel moisture. The explicit links between fire regime components and physical environmental gradients suggest that multivariable statistical models can be generated to produce an empirically based fire regime map for the western US. Such models will potentially enable researchers to anticipate climate-mediated changes in fire recurrence and its impacts based on gridded spatial data representing future climate scenarios.
Highlights
Fire is a ubiquitous ecosystem process across the globe
Broad ecoregion-level biogeographic patterns are revealed in the biplot of actual evapotranspiration (AET) and water deficit (WD) (Fig. 3a); extreme differences in both AET and WD among contrasted ecoregions, such as the warm desert (WD) and Pacific Northwest (PNW), are evident
We explored how relativized burn ratio (RBR) [37] varied along gradients of AET and WD; shapes of the relationships were nearly identical to those we reported for delta normalized burn ratio (dNBR), but the strengths of the relationships were weaker
Summary
Fire is a ubiquitous ecosystem process across the globe. The concept of the fire regime has been used to describe the role of fire in an ecosystem in terms of its spatial-temporal patterns and ecosystem impacts [1]. Maps of fire regimes, in terms of fire frequency and severity, have been produced [2]. These maps allow geographic comparisons of fire regime components [3], but are a necessary first step for describing shifts in fire regimes resulting from factors such as climate change [4], fire suppression [5], and invasive species [6]. Given that fire severity – a measure of ecosystem change – can strongly dictate the response of biological communities to fire [21,22,23], an understanding of its environmental controls is a prerequisite for understanding the role of fire in ecosystems. Without an understanding of these controls, modeling and prediction of climate-mediated changes in fire regimes is tenuous
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