Wildland fire deficit and surplus in the western United States, 1984–2012

Sean A Parks,Marc-André Parisien,John Abatzoglou,Lisa M Holsinger,Solomon Z Dobrowski,Carol Miller

doi:10.1890/es15-00294.1

Abstract

Wildland fire is an important disturbance agent in the western US and globally. However, the natural role of fire has been disrupted in many regions due to the influence of human activities, which have the potential to either exclude or promote fire, resulting in a “fire deficit” or “fire surplus”, respectively. In this study, we developed a model of expected area burned for the western US as a function of climate from 1984 to 2012. We then quantified departures from expected area burned to identify geographic regions with fire deficit or surplus. We developed our model of area burned as a function of several climatic variables from reference areas with low human influence; the relationship between climate and fire is strong in these areas. We then quantified the degree of fire deficit or surplus for all areas of the western US as the difference between expected (as predicted with the model) and observed area burned from 1984 to 2012. Results indicate that many forested areas in the western US experienced a fire deficit from 1984 to 2012, likely due to fire exclusion by human activities. We also found that large expanses of non‐forested regions experienced a fire surplus, presumably due to introduced annual grasses and the prevalence of anthropogenic ignitions. The heterogeneity in patterns of fire deficit and surplus among ecoregions emphasizes fundamentally different ecosystem sensitivities to human influences and suggests that large‐scale adaptation and mitigation strategies will be necessary in order to restore and maintain resilient, healthy, and naturally functioning ecosystems.

Highlights

The arrival of Euro-American settlers in the late-1800s disrupted the natural ecological role of fire in the western US (Keane et al 2002), thereby resulting in departures in fire activity compared to earlier periods (Safford and Van de Water2014)
Many of these studies definitively demonstrated that fire activity today is much different in the western US compared to historical conditions, some have questioned the relevance of a past time period in defining reference conditions given that contemporary climate change is altering the biophysical environment (Harris et al 2006)
Large geographic expanses of fire surplus were apparent in some non-forested regions, a fire deficit was observed in two non-forested ecoregions (i.e., Central Shortgrass Prairie and Southern Shortgrass Prairie; Fig. 2, Table 2)

Summary

Introduction

The arrival of Euro-American settlers in the late-1800s disrupted the natural ecological role of fire in the western US (Keane et al 2002), thereby resulting in departures in fire activity compared to earlier periods (Safford and Van de Water2014). Increases in fire activity have been documented in certain ecosystems or localized areas due to factors such as invasive species that facilitate fire ignition and spread as well as high rates of human-caused ignitions (D’Antonio and Vitousek 1992, Syphard et al 2007, Balch et al 2013) Many of these studies definitively demonstrated that fire activity today is much different in the western US compared to historical conditions (e.g., pre EuroAmerican settlement), some have questioned the relevance of a past time period in defining reference conditions given that contemporary climate change is altering the biophysical environment (Harris et al 2006)

Methods

Results

Discussion

Conclusion