Increasing Synchronous Fire Danger in Forests of the Western United States

Abatzoglou, John T.; Juang, Caroline S.; Williams, A. Park; Kolden, Crystal A.; Westerling, A. L.

doi:10.1029/2020gl091377

Cited by 88 publications

(63 citation statements)

References 61 publications

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“…While there is value in spatially refined models, efforts to parameterize empirical relationships at localized scales can be limited by the stochastic nature of ignitions and fire weather-particularly in locations with long fire return intervals with zero-inflated distributions of annual burned area. Strong interannual relationships between fuel aridity and strain on national fire suppression resources shared across the region highlight the implicit value in considering larger spatial scales 54 . The macroscale approach is further justified because the leading mode of variability in fuel aridity across forested land is a commonly signed regionwide pattern that is strongly correlated (r 2 = 0.79) to the logarithm of forest-fire area (Supplementary Fig.…”

Section: Methodsmentioning

confidence: 99%

Projected increases in western US forest fire despite growing fuel constraints

Abatzoglou

Battisti

Williams

et al. 2021

Commun Earth Environ

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168

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Escalating burned area in western US forests punctuated by the 2020 fire season has heightened the need to explore near-term macroscale forest-fire area trajectories. As fires remove fuels for subsequent fires, feedbacks may impose constraints on the otherwise climate-driven trend of increasing forest-fire area. Here, we test how fire-fuel feedbacks moderate near-term (2021–2050) climate-driven increases in forest-fire area across the western US. Assuming constant fuels, climate–fire models project a doubling of forest-fire area compared to 1991–2020. Fire-fuel feedbacks only modestly attenuate the projected increase in forest-fire area. Even models with strong feedbacks project increasing interannual variability in forest-fire area and more than a two-fold increase in the likelihood of years exceeding the 2020 fire season. Fuel limitations from fire-fuel feedbacks are unlikely to strongly constrain the profound climate-driven broad-scale increases in forest-fire area by the mid-21st century, highlighting the need for proactive adaptation to increased western US forest-fire impacts.

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Section: Methodsmentioning

confidence: 99%

Projected increases in western US forest fire despite growing fuel constraints

Abatzoglou

Battisti

Williams

et al. 2021

Commun Earth Environ

Self Cite

168

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“…Our findings that drivers of fire severity shift as landscapes transition to an active fire regime are relevant to resource agencies that seek to maintain and restore fire resilience to fireexcluded forest landscapes under a changing climate (Long et al 2014, USDA Forest Service 2015, Board of Forestry 2018. Climate warming is expected to increase the length of the fire season (Westerling 2016), increase fuel aridity (Williams et al 2019), and will likely increase the number of days that experience high and extreme fire weather (Goss et al 2020, Abatzoglou et al 2021. These climate factors and the high fuel loads characteristic of fire-excluded forests are strong contributors to the increase in forest area burned and forest area burned at high severity during the last three decades across the western United States (Abatzoglou and Williams 2016, Westerling 2016, Parks and Abatzoglou 2020 and in the Klamath Mountains.…”

Section: Conclusion and Management Implicationsmentioning

confidence: 99%

Drivers of fire severity shift as landscapes transition to an active fire regime, Klamath Mountains, USA

2021

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Fire severity patterns are driven by interactions between fire, vegetation, and terrain, and they generate legacy effects that influence future fire severity. A century of fire exclusion and fuel buildup has eroded legacy effects, and contemporary fire severity patterns may diverge from historical patterns. In recent decades, area burned and area burned at high severity have increased and landscapes are transitioning back to an active fire regime where disturbance legacies will again play a strong role in determining fire severity. Understanding the drivers of fire severity is crucial for anticipating future fire severity patterns as active fire regimes are reestablished. We identified drivers of fire severity in the Klamath Mountains, a landscape with an active fire regime, using two machine learning statistical models: one model for nonreburns (n = 92) and one model for reburns (n = 61). Both models predicted low better than moderate or high-severity fire. Fire severity drivers contrasted sharply between non-reburns and reburns. Fire weather and fuels were dominant controls in non-reburns, while previous burn severity, fuel characteristics, and time since last fire were drivers for reburns. In reburns, areas initially burned at low (high) severity burned the same way again. This tendency was sufficiently strong that reburn fire severity could be predicted equally well with only severity of the previous fire in the model. Thus, reburn fire severity is more predictable than severity in non-reburns that are driven by the stochastic influences of fire weather. Reburn severity in aggregate was also higher than non-reburn severity suggesting a positive feedback effect that could contribute to an upward drift in fire severity as area burned increases. Terrain had low importance in both models. This indicates strong terrain controls in the past may not carry into the future. Low-and moderate-severity fire effects were prevalent in non-reburns under moderate fire weather and selfreinforcing behavior maintained these effects in reburns even under more extreme weather, particularly in reburns within 10 yr. Our findings suggest deliberate use of wildfire and prescribed fire under moderate conditions would increase fire resilience in landscapes transitioning to an active fire regime.

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“…Although increased fuel aridity under climate change may prime western Cascade forests to burn more frequently and at higher severities than historically observed (94), and especially among early-seral stands (86), the difference in re behavior between the two meteorological periods indicate that highseverity mega res are unlikely to occur without coinciding extreme wind events. Although little work has been conducted on extreme summer wind events in the PNW, increases in annual downslope wind activity have been observed in the Cascades during the 1979-2018 period (95) and global warming has been linked to severe storms and shifts in storm tracks (e.g., (96)).…”

Section: Discussionmentioning

confidence: 96%

Extreme Winds Flip Influence of Fuels and Topography on Megafire Burn Severity in Mesic Conifer Forests Under Record Fuel Aridity

Evers¹,

Busby²,

Holz³

2021

Preprint

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The coupling of unusually hot and dry weather have led to global increases in the occurrence of megafires. Despite the conventional wisdom that extreme heat and aridity overwhelm the controls on burn severity patterns (i.e., vegetation mortality), we hypothesize that wind is the main driver of megafire events in temperate mesic forests with climate-restricted fire regimes, yet that fuels and topography remain important influences on burn severity patterns. The infrequent occurrence of large high-severity wildfire in these forests means that contemporary empirical data (e.g., remote sensing) from past megafires are largely missing. During the extraordinary 2020 fire season, ca. 0.8 million ha burned in the North American Pacific Northwest (PNW) over two weeks under record-breaking fuel aridity and winds, representing the first modern example of megafires that characterize disturbance regimes west of the region’s Cascade Mountains. Considering increasing concern and uncertainty surrounding the drivers of megafire events in temperate mesic forests, our objective was to understand the relative influence of, and potential interactions between, weather, fuels, and topography on high-severity (> 75% tree mortality) fire probability among five synchronous megafires in the western Cascade Mountains. To assess the influence of several potential drivers of high-severity fire and whether these relationships varied with land use and ownership, we developed remotely sensed fire extent and burn severity maps for two periods of the explosive 2020 PNW fire season: (1) during extreme winds and (2) after the extreme winds subsided. The area burned during the windstorm accounted for 90% of the total fire sizes and saw a 2.5-times greater proportion of high-severity fire than during the period without winds. Our results suggest that wind is the major driver of megafires in forests with climate-limited fire regimes, yet that fuels and topography shape burn severity patterns even under extreme fuel aridity and winds. The relative influence of topography on burn severity outweighed fuels during the windstorm, while fuels outweighed the influence of topography after winds subsided. Early-seral forests primarily concentrated on private lands, burned more severely than their older and taller counterparts, regardless of topography, over the entire megafire event. Meanwhile, mature stands burned severely only under extreme winds and especially on steeper slopes. Although climate change and land-use legacies may prime mesic temperate forests to burn more frequently and at higher severities than historically observed, and especially among early-seral forests, our work suggests that future high-severity megafires are only likely to occur during coinciding periods of heat, fuel aridity, and extreme winds.

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Increasing Synchronous Fire Danger in Forests of the Western United States

Cited by 88 publications

References 61 publications

Projected increases in western US forest fire despite growing fuel constraints

Projected increases in western US forest fire despite growing fuel constraints

Drivers of fire severity shift as landscapes transition to an active fire regime, Klamath Mountains, USA

Extreme Winds Flip Influence of Fuels and Topography on Megafire Burn Severity in Mesic Conifer Forests Under Record Fuel Aridity

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