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Southwestern ponderosa pine forests are vulnerable to fire‐driven conversion in a warming and drying climate, yet little is known about what kinds of ecological communities may replace them. To characterize postfire vegetation trajectories and their environmental determinants, plant assemblages (361 sample plots including 229 vascular plant species, surveyed in 2017) were sampled within eight burns that occurred between 2000 and 2003. I used nonmetric multidimensional scaling, k‐means clustering, principal component analysis, and random forest models to assess relationships between vegetation pattern, topographic and landscape factors, and gridded climate data. I describe seven postfire community types, including regenerating forests of ponderosa pine, aspen, and mixed conifers, shrub‐dominated communities of Gambel oak and mixed species, and herb‐dominated communities of native bunchgrasses and mixtures of ruderal, native, and nonnative species. Forest recovery was generally associated with cooler, mesic sites in proximity to forested refugia; shifts toward scrub and grassland types were most common in warmer, dryer locations distant from forested refugia. Under future climate scenarios, models project decreases in postfire forest recovery and increases in nonforest vegetation. However, forest to nonforest conversion was partially offset under a scenario of reduced burn severity and increased retention of forested refugia, highlighting important management opportunities. Burning trends in the southwestern United States suggest that postfire vegetation will occupy a growing landscape fraction, compelling renewed management focus on these areas and paradigm shifts that accommodate ecological change. I illustrate how management decisions around resisting, accepting, or directing change could be informed by an understanding of processes and patterns of postfire community variation and likely future trajectories. [ABSTRACT FROM AUTHOR]

In 2009, new guidance for wildland fire management in the United States expanded the range of strategic options for managers working to reduce the threat of high-severity wildland fire, improve forest health and respond to a changing climate. Markedly, the new guidance provided greater flexibility to manage wildland fires to meet multiple resource objectives. We use Incident Status Summary reports to understand how wildland fire management strategies have differed across the western US in recent years and how management has changed since the 2009 Guidance for Implementation of Federal Wildland Fire Management Policy. When controlling for confounding variation, we found the 2009 Policy Guidance along with other concurrent advances in fire management motivated an estimated 27 to 73% increase in the number of fires managed with expanded strategic options, with only limited evidence of an increase in size or annual area burned. Fire weather captured a manager's intent and allocation of fire management resources relative to burning conditions, where a manager's desire and ability to suppress is either complemented by fire weather, at odds with fire weather, or put aside due to other priorities. We highlight opportunities to expand the use of strategic options in fire-adapted forests to improve fuel heterogeneity. In 2009, new federal guidance in the United States expanded options available to wildland fire managers. We explored this guidance in a context informed by concurrent weather and resource conditions, finding it helped expand the use of strategic responses beyond suppression. We also found a manager's desire and ability to suppress is either complemented by fire weather, at odds with fire weather, or put aside due to other priorities. We highlight opportunities to improve fuel heterogeneity in fire-adapted forests. [ABSTRACT FROM AUTHOR]

Wildland fire emissions are routinely estimated in the US Environmental Protection Agency's National Emissions Inventory, specifically for fine particulate matter (PM2.5) and precursors to ozone (O3); however, there is a large amount of uncertainty in this sector. We employ a brute-force zero-out sensitivity method to estimate the impact of wildland fire emissions on air quality across the contiguous US using the Community Multiscale Air Quality (CMAQ) modelling system. These simulations are designed to assess the importance of wildland fire emissions on CMAQ model performance and are not intended for regulatory assessments. CMAQ ver. 5.0.1 estimated that fires contributed 11% to the mean PM2.5 and less than 1% to the mean O3 concentrations during 2008–2012. Adding fires to CMAQ increases the number of 'grid-cell days' with PM2.5 above 35 µg m−3 by a factor of 4 and the number of grid-cell days with maximum daily 8-h average O3 above 70 ppb by 14%. Although CMAQ simulations of specific fires have improved with the latest model version (e.g. for the 2008 California wildfire episode, the correlation r = 0.82 with CMAQ ver. 5.0.1 v. r = 0.68 for CMAQ ver. 4.7.1), the model still exhibits a low bias at higher observed concentrations and a high bias at lower observed concentrations. Given the large impact of wildland fire emissions on simulated concentrations of elevated PM2.5 and O3, improvements are recommended on how these emissions are characterised and distributed vertically in the model. Adding wildland fire emissions to the CMAQ model increases the number of 'grid-cell days' with daily fine particulate matter (PM2.5) concentrations above 35 µg m−3 by a factor of 4. Model results suggest the need to better characterise wildland fires. [ABSTRACT FROM AUTHOR]

Past and current forest management affects wildland fire smoke impacts on downwind human populations. However, mismatches between the scale of benefits and risks make it difficult to proactively manage wildland fires to promote both ecological and public health. Building on recent literature and advances in modeling smoke and health effects, we outline a framework to more directly quantify and compare smoke impacts based on emissions, dispersion, and the size and vulnerability of downwind populations across time and space. We apply the framework in a case study to demonstrate how different kinds of fires in California's Central Sierra Nevada have resulted in very different smoke impacts. Our results indicate that the 257,314-acre Rim Fire of 2013 probably resulted in 7 million person-days of smoke impact across California and Nevada, which was greater than 5 times the impact per burned unit area than two earlier wildfires, Grouse and Harden of 2009, that were intentionally managed for resource objectives within the same airshed. The framework and results suggest strategies and tactics for undertaking larger-scale burns that can minimize smoke impacts, restore forest ecosystems, and reduce the potential for more hazardous wildfire and smoke events. [ABSTRACT FROM AUTHOR]

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Recent growth in the frequency and severity of US wildfires has led to more wildfire smoke and increased public exposure to harmful air pollutants. Populations exposed to wildfire smoke experience a variety of negative health impacts, imposing economic costs on society. However, few estimates of smoke health costs exist and none for the entire Western US, in particular, which experiences some of the largest and most intense wildfires in the US. The lack of cost estimates is troublesome because smoke health impacts are an important consideration of the overall costs of wildfire. To address this gap, this study provides the first time series estimates of PM2.5 smoke costs across mortality and several morbidity measures for the Western US over 2005-2015. This time period includes smoke from several megafires and includes years of record-breaking acres burned. Smoke costs are estimated using a benefits transfer protocol developed for contexts when original health data are not available. The novelty of our protocol is that it synthesizes the literature on choices faced by researchers when conducting a smoke cost benefit transfer. On average, wildfire smoke in the Western US creates $165 million in annual morbidity and mortality health costs. [ABSTRACT FROM AUTHOR]

Characterising the impacts of wildland fire and fire suppression is critical information for fire management decision-making. Here, we focus on decisions related to the rare larger and longer-duration fire events, where the scope and scale of decision-making can be far broader than initial response efforts, and where determining and demonstrating efficiency of strategies and actions can be particularly troublesome. We organise our review around key decision factors such as context, complexity, alternatives, consequences and uncertainty, and for illustration contrast fire management in Andalusia, Spain, and Montana, USA. Two of the largest knowledge gaps relate to quantifying fire impacts to ecosystem services, and modelling relationships between fire management activities and avoided damages. The relative magnitude of these and other concerns varies with the complexity of the socioecological context in which fire management decisions are made. To conclude our review, we examine topics for future research, including expanded use of the economics toolkit to better characterise the productivity and effectiveness of suppression actions, integration of ecosystem modelling with economic principles, and stronger adoption of risk and decision analysis within fire management decision-making. [ABSTRACT FROM AUTHOR]

Interannual climate variations have been important drivers of wildfire occurrence in ponderosa pine forests across western North America for at least 400 years, but at finer scales of mountain ranges and landscapes human land uses sometimes over-rode climate influences. We reconstruct and analyse effects of high human population densities in forests of the Jemez Mountains, New Mexico from ca 1300 CE to Present. Prior to the 1680 Pueblo Revolt, human land uses reduced the occurrence of widespread fires while simultaneously adding more ignitions resulting in many small-extent fires. During the 18th and 19th centuries, wet/dry oscillations and their effects on fuels dynamics controlled widespread fire occurrence. In the late 19th century, intensive livestock grazing disrupted fuels continuity and fire spread and then active fire suppression maintained the absence of widespread surface fires during most of the 20th century. The abundance and continuity of fuels is the most important controlling variable in fire regimes of these semi-arid forests. Reduction of widespread fires owing to reduction of fuel continuity emerges as a hallmark of extensive human impacts on past forests and fire regimes. [ABSTRACT FROM AUTHOR]

There is a growing professional and public perception that 'extreme' wildland fires are becoming more common due to changing climatic conditions. This concern is heightened in the wildland-urban interface where social and ecological effects converge. 'Mega-fires', 'conflagrations', 'extreme' and 'catastrophic' are descriptors interchangeably used increasingly to describe fires in recent decades in the US and globally. It is necessary to have consistent, meaningful and quantitative metrics to define these perceived 'extreme' fires, given studies predict an increased frequency of large and intense wildfires in many ecosystems as a response to climate change. Using the Monitoring Trends in Burn Severity dataset, we identified both widespread fire years and individual fires as potentially extreme during the period 1984-2009 across a 91.2 x 106-ha area in the north-western United States. The metrics included distributions of fire size, fire duration, burn severity and distance to the wildland-urban interface. Widespread fire years for the study region included 1988,2000, 2006 and 2007. When considering the intersection of all four metrics using distributions at the 90th percentile, less than 1.5% of all fires were identified as potentially extreme fires. At the more stringent 95th and 99th percentiles, the percentage reduced to <0.5% and 0.05%. Correlations between area burnt and climatic measures (Palmer drought severity index, temperature, energy release component, duff moisture code and potential evapotranspiration) were observed. We discuss additional biophysical and social metrics that could be included and recommend both the need for enhanced visualisation approaches and to weigh the relative strength or importance of each metric. [ABSTRACT FROM AUTHOR]

Wildfire regimes are changing across the globe with several ecosystems witnessing more frequent fires across longer fire seasons. The western United States is one such region. The NASA RECOVER Historic Fires Database (HFD) contains all documented wildfires across the western United States occurring between 1950 and 2019 (n = 55,566). This study analyzed the spatiotemporal patterns of these wildfires using ArcGIS Pro Geographic Information System (GIS) software to characterize changes in fire frequency, size, and severity over time. Analysis of annual fire frequency and acres burned reveals a near exponential growth in fire frequency (R2 = 0.71, P < 0.001) and size (R2 = 0.67, P < 0.001) since 1950. A comparison of mean and median acres burned annually suggests the occurrence of mega-fires (wildfires burning more than 100,000 acres) is also increasing. To illustrate this, this study found the mean size of fires occurring in the decade of the 1950s was 1204 acres while in the most recent decade (2010–2019) mean fire size has more than doubled, reaching an average of 3474 acres. The trend in fire severity between 2001 and 2017 used 365 Differenced Normalized Burn Ratio (dNBR) layers calculated using Landsat or Sentinel-2 satellite imagery. Results suggest fire severity has remained relatively stable in light of increasing fire frequency and size, however more research is required to more fully understand changes in fire severity. The results of this study and other related studies are important as they provide useful information to land managers and policy makers regarding the changing wildfire regime currently being witnessed across the western United States. [ABSTRACT FROM AUTHOR]

Climate changes and associated shifts in ecosystems and fire regimes present enormous challenges for the management of landscapes in the Southwestern US. A central question is whether management strategies can maintain or promote desired ecological conditions under projected future climates. We modeled wildfire and forest responses to climate changes and management activities using two ecosystem process models: FireBGCv2, simulated for the Jemez Mountains, New Mexico, and LANDIS-II, simulated for the Kaibab Plateau, Arizona. We modeled contemporary and two future climates—“Warm-Dry” (CCSM4 RCP 4.5) and “Hot-Arid” (HadGEM2ES RCP 8.5)—and four levels of management including fire suppression alone, a current treatment strategy, and two intensified treatment strategies. We found that Hot-Arid future climate resulted in a fundamental, persistent reorganization of ecosystems in both study areas, including biomass reduction, compositional shifts, and altered forest structure. Climate changes increased the potential for high-severity fire in the Jemez study area, but did not impact fire regime characteristics in the Kaibab. Intensified management treatments somewhat reduced wildfire frequency and severity; however, management strategies did not prevent the reorganization of forest ecosystems in either landscape. Our results suggest that novel approaches may be required to manage future forests for desired conditions. [ABSTRACT FROM AUTHOR]