Research and Publications
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Increasing presence of woody plants in dryland ecosystems, also known as “woody encroachment,” is commonly attributed to anthropogenic land-use changes such as livestock grazing and wildfire suppression. However, empirical evidence to support these external drivers has not uncovered a unifying mechanism. We test whether plant demographic processes could be responsible for woody encroachment using tree-ring data from pinyon and juniper woodland populations in the western United States. Our results indicate that woody encroachment patterns can largely be predicted by a null model based only on steady tree population growth. Modern increases in woodland density, which are typically viewed as a natural resource management problem, may therefore be a result of long-term population expansion and recovery.
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Using random forest modeling and Shapley local importance measures, we found that weather and fuels were both dominant drivers of fire severity, and past fuel treatments were successful at reducing severity—even during extreme fire progression days. First-entry fires were more typically driven by top-down climate and weather variables, while for reburns (i.e., overlapping fire footprints within the period of record), severity was largely mitigated by reduced fuels and a positive influence of topography (e.g., burning downslope). Likewise, reburns overall exhibited lower fire severity than first entry fires, suggesting strong negative feedbacks associated with past fire footprints. The normalized difference moisture index (NDMI)—an indicator of live fuel loading and moisture levels—was a leading predictor of fire severity for both first-entry fires and reburns. NDMI values < 0 (i.e., low biomass) were associated with reduced fire severity, while values > 0.25 (i.e., high biomass) were associated with increased severity. Forest management was effective across a variety of conditions, especially under low to moderate wind speeds (< 17 m·s−1), and where canopy base heights were ≥ 1.3 m.
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Data-driven decision support can help guide sustainable grazing management by providing an accurate estimate of grazing capacity, in coproduction with managers. Here, we described the development of a decision support model to estimate grazing capacity and illustrated its application on two sites in the western United States. For the Montgomery PassWild Horse Territory in California and Nevada, the upper limit estimated in the capacity assessment was 398 horses and the current population was 654 horses. For the Eagle Creek watershed of the Apache-Sitgreaves National Forest of eastern Arizona, the lower end of capacity was estimated at 1560 cattle annually, compared to the current average of 1090 cattle annually. In addition to being spatio-temporally comprehensive, the model provides a repeatable, cost-effective, and transparent process for establishing and adjusting capacity estimates and associated grazing plans that are supported by scientific information, in order to support livestock numbers at levels that are sustainable over time, including levels that are below average forage production during drought conditions. This modeling process acts as a decision support tool because it enables different assumptions to be used and explored to accommodate multiple viewpoints during the planning process.
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Through interviews with wildland fire and forest managers (e.g., Incident Commanders, Agency Administrators, Fire Management Officers, and Fuels Planners) on seven western wildfire incidents during 2020 and 2021, we investigated how forest fuel treatments were utilized during firefighting. We found that treatments were considered and used during incidents in various ways, including to conduct burnouts, for direct modification of fire behavior, as access points for firefighters or equipment, or as components of contingency plans. Most interviewees said treatments provided additional options and flexibility in decision-making, enhancing both firefighter and community safety. For instance, treatments were used to reduce overhead hazards to firefighters and, in some cases, were prepared to serve as safety zones.
Reliability of satellite-based vegetation maps for planning wildfire-fuel treatments in shrub steppe
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We compared commonly used, publicly available vegetation cover and fuels maps, specifically the Rangeland Analysis Platform (RAP) and LANDFIRE, with field-based assessments at two U.S. National Parks dominated by sagebrush steppe: City of Rocks National Reserve and Craters of the Moon National Monument and Preserve. Plant-community composition and fuels measured at ∼1700 field locations spanning ∼300,000 ha revealed that 1) RAP generally underestimated each vegetation cover type where the cover was actually abundant, and conversely overestimated cover types where they were actually scarce, and 2) there was considerable disagreement in fuel-bed maps derived from LANDFIRE compared to field observations. As a result, there were substantial discrepancies in the spatial patterning of wildfire behavior estimated from the fire-spread model FLAMMAP when parameterized with LANDFIRE compared to field-based fuel-bed maps created from Random Forests models. Reliable maps of vegetation cover and fuel conditions are needed to help guide fuels and invasive species management, especially given recent increases in pre- and post-fire treatments in arid and semiarid landscapes. The costs associated with poorly informed fuel reduction may greatly exceed the costs of field-based vegetation and fuels inventory to inform effective design of vegetative fuels treatments.
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With climate change causing more extreme weather events globally, climate scientists have argued that societies have three options: mitigation, adaptation or suffering. In recent years, devastating wildfires have caused significant suffering, yet the extent of this suffering has not been defined. To encapsulate this suffering, we determined impacts and effects of extreme wildfires through two systematic literature reviews. Six common themes of wildfire suffering emerged: environmental, social, physical, mental, cultural and resource suffering. These themes varied in scale: from local to regional; from individuals to communities; and from ecosystems to landscapes. We then applied these themes in the Las Maquinas (Chile) and Fort McMurray (Canada) wildfires. This highlighted several adaptation strategies that can reduce suffering, however our exploration indicates these strategies must address social and ecological factors. This analysis concludes that suffering from wildfires is diverse and widespread, and that significant engagement with adaptation strategies is needed if this is going to decrease.
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The population of the United States is aging as the Baby Boom generation grows older. In 2020, 23 percent of the U.S. population had reached age 60. The share of the population at older ages is forecast to increase to 26 percent in 2030 and 29 percent in 2050. Wildfire risks are also increasing, and older populations are especially vulnerable. This report found that most (87 percent) of the recent population growth in places with moderate-to-high wildfire risk has been among people over the age of 60. Already, the proportion of older people living in places with more wildfire risk is higher than in the population at large. In rural areas with the greatest wildfire risk, 35 percent of people living in those areas are over the age of 60. The number of older people exposed to wildfire risk is expected to increase as populations grow older and as wildfire increases in frequency and intensity.
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Our study aimed to document the expansion of PBAs and provide insight into their structure, function, and impacts. Leaders from 135 known PBAs across the United States were invited to participate in an online survey. Survey results demonstrate a widespread emergence of PBAs in the United States, successfully mobilizing thousands of volunteers to collectively burn more than 34,000 ha annually. PBAs demonstrated that they are reducing myriad barriers to prescribed burning while meeting their goals to broaden access to the use of fire using a neighbors-helping-neighbors model to provide training, pool resources, and reduce the costs of prescribed burning. By including volunteers with diverse levels of experience and backgrounds, PBAs are changing the narrative of who has access to the use of fire.
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Previous research has examined individual factors contributing to wildfire risk, but the compounding effects of these factors remain underexplored. Here, we introduce the “Integrated Human-centric Wildfire Risk Index (IHWRI)” to quantify the compounding effects of fire-weather intensification and anthropogenic factors—including ignitions and human settlement into wildland—on wildfire risk. While climatic trends increased the frequency of high-risk fire-weather by 2.5-fold, the combination of this trend with wildland-urban interface expansion led to a 4.1-fold increase in the frequency of conditions conducive to extreme-impact wildfires from 1990 to 2022 across California. More than three-quarters of extreme-impact wildfires—defined as the top 20 largest, most destructive, or deadliest events on record—originated within 1 km from the wildland-urban interface. The deadliest and most destructive wildfires—90% of which were human-caused—primarily occurred in the fall, while the largest wildfires—56% of which were human-caused—mostly took place in the summer. By integrating human activity and climate change impacts, we provide a holistic understanding of human-centric wildfire risk, crucial for policy development.
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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.