Mid‐21st‐century climate changes increase predicted fire occurrence and fire season length, Northern Rocky Mountains, United States

Karin L Riley,Rachel A Loehman

doi:10.1002/ecs2.1543

Abstract

AbstractClimate changes are expected to increase fire frequency, fire season length, and cumulative area burned in the western United States. We focus on the potential impact of mid‐21st‐century climate changes on annual burn probability, fire season length, and large fire characteristics including number and size for a study area in the Northern Rocky Mountains. Although large fires are rare they account for most of the area burned in western North America, burn under extreme weather conditions, and exhibit behaviors that preclude methods of direct control. Allocation of resources, development of management plans, and assessment of fire effects on ecosystems all require an understanding of when and where fires are likely to burn, particularly under altered climate regimes that may increase large fire occurrence. We used the large fire simulation model FSim to model ignition, growth, and containment of wildfires under two climate scenarios: contemporary (based on instrumental weather) and mid‐century (based on an ensemble average of global climate models driven by the A1B SRES emissions scenario). Modeled changes in fire patterns include increased annual burn probability, particularly in areas of the study region with relatively short contemporary fire return intervals; increased individual fire size and annual area burned; and fewer years without large fires. High fire danger days, represented by threshold values of Energy Release Component (ERC), are projected to increase in number, especially in spring and fall, lengthening the climatic fire season. For fire managers, ERC is an indicator of fire intensity potential and fire economics, with higher ERC thresholds often associated with larger, more expensive fires. Longer periods of elevated ERC may significantly increase the cost and complexity of fire management activities, requiring new strategies to maintain desired ecological conditions and limit fire risk. Increased fire activity (within the historical range of frequency and severity, and depending on the extent to which ecosystems are adapted) may maintain or restore ecosystem functionality; however, in areas that are highly departed from historical fire regimes or where there is disequilibrium between climate and vegetation, ecosystems may be rapidly and persistently altered by wildfires, especially those that burn under extreme conditions.

Highlights

Climate changes are projected to profoundly influence wildfire regimes in forests of the western United States, in mountainous ecosystems such as those found within the Northern Rocky Mountain region (“Northern Rockies”; Fagre and Peterson 2000, McKenzie v www.esajournals.orgNovember 2016 v Volume 7(11) v Article e01543RILEy AND LoEHMAN et al 2004, Higuera et al 2015)
Burn probability Modeled annual burn probabilities for the contemporary period varied across the study area by geography, vegetation type, and fire regime group (FRG) from a low of zero to a high of 0.0385, equivalent to about a 4% annual occurrence probability of a large fire (Fig. 7a)
Burn probabilities were highest in the southern part of the study area, where grasses and shrubs are the dominant vegetation and FRG indicates short fire return intervals (Figs. 4 and 7a, Table 1; LANDFIRE 2008b)

Summary

Introduction

Climate changes are projected to profoundly influence wildfire regimes in forests of the western United States, in mountainous ecosystems such as those found within the Northern Rocky Mountain region (“Northern Rockies”; Fagre and Peterson 2000, McKenzie v www.esajournals.orgNovember 2016 v Volume 7(11) v Article e01543RILEy AND LoEHMAN et al 2004, Higuera et al 2015). Lengthening of the climatic fire season impacts fire patterns by increasing the likelihood that ignitions—natural or human-caused—will occur during conditions conducive to fire spread; more spreading ignitions and longer periods of burning are likely to result in larger fires and increased annual area burned relative to contemporary recorded fire activity (McKenzie et al 2004, Miller et al 2011) This shift may be especially pronounced in middle- to highelevation forested systems where fuels are abundant (Westerling et al 2006, Littell et al 2009)

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Discussion

Conclusion