Research and Publications
Intensifying fire season aridity portends ongoing expansion of severe wildfire in western US forests
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Here, we quantified area burned and area burned severely in western US forests from 1985 to 2022 and evaluated trends through time. We also assessed key relationships between area burned, extent and proportion burned severely, and fire season climate aridity. Lastly, using the strong relationships between fire season aridity and both area burned and area burned severely, we predicted future fire activity under ongoing warming. While annual area burned increased 10-fold over our study period, area burned severely increased 15-fold. Disproportionate increases in severe fire occurred across a wide range of forest types from 1985 to 2022. Importantly, we found that the proportion of area burned severely increased with fire extent at the scale of individual fires and total annual area burned. The relationships between fire season aridity and fire were strong, and our models predicted further increases in fire activity, leading to 2.9- and 4-fold increases in area burned and area burned severely, respectively, under mid-21st century climate. Without a substantial expansion of management activities that effectively reduce fire severity (e.g., thinning of understory and fire-intolerant trees combined with prescribed fire), wildfires will increasingly drive forest loss and degrade ecosystem services including carbon storage, biodiversity conservation, and water yield, with major impacts to human communities.
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This review informs prevention decision-making by highlighting current best practices categorized under the four key approaches to fire prevention–education, enforcement, engineering, and administration–while simultaneously revealing themes and gaps that merit further attention. We focus on interventions that can reduce accidental or negligent ignitions within the purview of land management and fire prevention professionals. We conclude with a call to modernize the field of wildfire prevention social science that promotes the diversification of study locations, design, and prevention techniques studied. Improved research and documentation surrounding the outcomes of individual or combinations of strategies and the user groups they target can help transition anecdotal assessments of prevention effectiveness into empirically informed decision-making that supports more strategic administration.
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This Storymap is developed and maintained by the Wildfire Risk Management Science Team at the USDA Forest Service Rocky Mountain Research Station. Information presented here represents ongoing efforts of team members and their collaborators and partners at research universities (Oregon State University, Colorado State University), land managing agencies (The National Forest System, National Park Service, Bureau of Land Management, and multiple state partners), and independent land and resource management partners. Additional support is provided by Wildland Fire Management Research, Development & Application Risk Management Assistance.
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Here, we model relationships between satellite observations of fire radiative power (FRP) and contemporaneous fire weather index, and then we project how FRP is likely to change under near-term warming scenarios. The models project widespread growth in FRP, with increases expected across 88% of fire-prone areas worldwide under 1.5 °C warming. Projected increases in FRP were highest in the Mediterranean biome and Temperate Conifer Forest biome, and increases were twice as large under 2 °C warming compared to 1.5 °C. Disaster-prone areas of the wildland-urban interface saw an average of 3.6 times greater projected increases than non-disaster-prone areas, suggesting wildfire impacts will intensify most in regions already vulnerable to dangerous wildfires. These findings emphasise the urgent need to anticipate changes to fire behaviour and proactively manage wildland-urban ecosystems to reduce future fire intensity.
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Across plots that burned in multiple low- to moderate-severity fires, our findings indicated that post-fire outcomes in these systems are variable, resulting in a range of structural conditions following a first reburn (i.e., second fire). Areas with high levels of dead biomass burned at significantly higher severity in the third fire compared to those with higher shrub cover. Following a second fire, many plots exceeded historical estimates of stand structure metrics for yellow pine and mixed-conifer forests of the Sierra Nevada, particularly for coarse woody debris load, with some plots exceeding historical natural range of variation (NRV) estimates for live tree density. In plots with a history of varying fire severity in the initial and second fires, we found that snag basal area was associated with higher fire severity in the third fire.
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This study compares burned areas to climate and fuel conditions in three temperate regions: the desert, shrub, and forest ecoregions of western North America, west-central Europe, and southwestern South America. In each region the mean annual aridity index (AI, precipitation over potential evapotranspiration) spans arid to humid climates. We examined how the fraction of area burned from 2001 to 2021 varied with mean annual AI, mean aboveground biomass, and land cover type distributions. All three regions had low fractions of area burned for the driest climate zones (AI < 0.5), a sign of fuel limitation to burned area. Fraction of area burned increased with mean aboveground biomass for these dry zones. Fraction of area burned peaked at intermediate AI (0.7–1.5) for all regions and declined again in the wettest climate zones (AI > 1.5), a sign of climate limitation to burned area. Of the three regions, western North America had the highest burned area, fraction of area burned, and fire sizes. Fragmentation of vegetation patches by the high Andes Mountains in southwestern South America and by intensive land use changes in west-central Europe likely limited fire sizes. All three regions are at risk for future wildfires, particularly in areas where fire is currently climate limited.
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While substantial efforts have been made to conserve critical mule deer habitat, less attention has been given to mule deer habitat affected by invasive annual grasses (IAGs) and there is limited information about how mule deer respond to IAG invasions. We evaluated mule deer resource selection in a sagebrush grassland community impacted by IAGs in northeast Wyoming. We then used empirical model estimates to forecast how IAG management could impact mule deer habitat in the future following a strategic IAG framework focused on defending and growing sagebrush core areas most threatened by IAGs. We found that mule deer responded to IAGs in a nonlinear pattern across all seasons and strongly avoided areas once cover exceeded approximately 20%. When projecting results 20 yr into the future, we found that over half of the study area is expected to experience significant declines in mule deer habitat quality if IAGs continue to spread at the same rate observed over the past two decades. However, with targeted IAG treatments, we predicted widespread improvements in mule deer habitat, particularly in priority areas where ecological integrity can be restored with future IAG management. Our findings reinforce the emerging notion that ecosystem-based frameworks designed to defend and grow intact sagebrush steppe through strategic management efforts also have the potential to benefit species of conservation interest. As current conservation efforts to mitigate IAGs are not progressing fast enough to address the magnitude of the IAG problem in sagebrush across the west, strategic management efforts will be necessary to maintain important habitats for numerous sagebrush occurring wildlife.
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To assess relationships between fire spread rates and landscape burn severity patterns, we used satellite fire detections to create day-of-burning maps for 623 fires comprising 4267 single-day events within forested ecoregions of the southwestern United States. We related satellite-measured burn severity and a suite of high-severity patch metrics to daily area burned. Extreme fire spread events (defined here as burning > 4900 ha/day) exhibited higher mean burn severity, a greater proportion of area burned severely, and increased like adjacencies between high-severity pixels. Furthermore, increasing daily area burned also resulted in greater distances within high-severity patches to live tree seed sources. High-severity patch size and total high-severity core area were substantially higher for fires containing one or more extreme spread events than for fires without an extreme event. Larger and more homogenous high-severity patches produced during extreme events can limit tree regeneration and set the stage for protracted forest conversion. These landscape outcomes are expected to be magnified under future climate scenarios, accelerating fire-driven forest loss and long-term ecological change.
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To investigate the effects of extreme weather and forest management on fire severity, we used light detection and ranging (LiDAR) data to characterize pre-fire forest structure across five large wildfires which burned 460,000 ha in the northern Sierra Nevada, California, USA. We found that the odds of high severity fire occurrence in these fires were 1.45 times higher on private industrial land than in publicly owned forests, an effect equivalent to a three standard deviation decrease in fuel moisture. Next, we quantified the relationships between key forest structure metrics and the probability of high severity fire, as well as how these relationships were modified by extreme weather. We found that dense, spatially homogeneous forests with high ladder fuels were more likely to burn at high severity. Extreme weather magnified the effect of density, suggesting that treatments which remove overstory trees are especially important in extreme conditions. Forests managed by private industry were more likely to be dense, spatially homogeneous, and contain high ladder fuel loads than publicly owned forests, offering a potential explanation for the increase in high-severity fire occurrence on private industrial land. Overall, these results illustrate the need for comprehensive forest management to mitigate fire severity in a warmer future.
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Ventenata and medusahead are winter annuals that emerge in late fall, winter, and spring. These grasses mature early in the summer providing fine fuels for wildfires.Ventenata and medusahead are frequently found together where conditions allow. These invasive annual grasses increase wildfire danger in shrublands and woodlands of the American West.
- Fine, highly flammable fuel loads facilitate larger and more frequent fires.
- Relatively high silica content make these grasses less palatable for grazing (unlike cheatgrass which is palatable in its green phase), and creates a build-up of litter on the soil surface.
- These species can spread throughout areas that once acted as natural fire breaks.