Adapting western North American forests to climate change and wildfires: 10 common questions

A bi-objective mixed-integer linear programming model to optimize thinning schedules in wildfire-prone Pinus canariensis forests

Multi-objective programming is frequently used in forest management research to reconcile conflicting policy objectives. Given the frequent occurrence of fire in Pinus canariensis forests in the Canary Islands, forest managers have explored various silvicultural strategies, including thinning scheduling and intensity, to mitigate the adverse effects of fire on wood supply and improve fire suppression. This study presents an integrated modelling framework for computing Pareto frontiers. The framework results from maximizing economic returns and minimizing fire suppression difficulty and was achieved using thinning scenarios and assessing fire extinction risk. Thus, the response of Pinus canariensis forests to varying thinning schedules was evaluated across a geographic gradient in Tenerife (Canary Islands, Spain), using a bi-objective mixed-integer linear programming modelling (ɛ-constrained) approach. Multiple prospective alternatives for mitigating fire risk were formulated, considering specific parameters. For different levels of the relative tolerance parameter (β), the Pareto frontier revealed limited impacts on economic return (lower than 1.275) when the suppression difficulty index (SDI) was reduced. However, the total SDI and the number of stands thinned across the planning horizon were quite sensitive to variations in the β parameter. The latter results also had significant effects on average SDI, area of stands thinned, wood flows (roundwood and energy wood), thinning cost, and total return from wood production. Given that all the points along the Pareto frontier are optimal and dominant, the decision maker must operationally select the solution to be implemented. Regardless of the solution to be chosen, our findings highlight that an effective thinning schedule in fire-prone forests substantially enhances environmental outcomes, even with relatively modest reductions in the net benefits obtained from wood harvesting.

Unpacking the Taxonomy of Wildland Fire Collaboratives in the United States West: Impact of Response Diversity on Social-Ecological Resilience

We offer the first study unpacking the taxonomy of collaboratives that undertake wildland fire management and how that taxonomy relates to resilience. We developed a comprehensive inventory totaling 133 collaboratives across twelve states in the western United States. We extracted each collaborative's vision, mission, program goals, actions, and stakeholder composition. Based on this data we summarize temporal and spatial trends in collaborative formation and discuss formation drivers. Furthermore, we developed a cluster map of collaboratives based on patterns of co-occurrence of collaborative vision, mission, and goals. We identify distinct co-occurrence patterns of themes emerging from qualitative coding of collaborative missions, visions, and objectives, and define three distinct collaborative archetypes based on these. Finally, using theory-supported actions linked to basic, adaptive, and transformative social and ecological resilience, we code for presence or absence of these outcomes for each collaborative. We present the resilience outcomes by state and discuss how various collaborative typologies differentially impact levels of social and ecological resilience. Our study concludes that fire management actions for adaptive resilience such as fuels reduction, tree thinning, and revegetation are most numerous but that there is an emergent phenomenon of collaboratives engaging in transformative resilience that are mostly citizen-led networked organizations reshaping the social and ecological landscapes to include prescribed burning on a larger scale than present.

Integrating the physical harvesting of dead wood into fuel treatments to reduce wildfire hazards and enhance carbon benefits

A century-long fire suppression policy, coupled with global warming and severe drought, has led to increased incidence of devastating wildfires in the forests of the western United States that deviate from historical fire regimes. Forest managers commonly use mechanical treatments combined with prescribed burning (Rx burn) to reduce fuels, but the intensity of treatment efforts and efficacy of Rx burning in particular for reducing extreme fire risks and emissions, restoring historic fire regimes, and enhancing ecosystem services such as carbon sequestration is increasingly questioned in the context of global warming and wildland-urban interface enhancements of extreme fire risk. Here, we aim to examine the effectiveness of integrating physical harvesting - a surface fuel removal approach, minus subsequent combustion, equivalent to historic firewood collection practices by pre-Columbian Indigenous Peoples - as a fuel treatment alternative to minimize wildfire hazard and enhance carbon benefits from reduced emission and increased sequestration potentials. We utilized field plots data from Sierra Nevada range, California, USA, mixed conifer forests to simulate the outcomes of various forest management alternatives involving one or a combination of fuel reduction treatments on wildfire behavior and carbon pool using the Fire and Fuel Extension of Forest Vegetation Simulator (FFE-FVS). Carbon benefits were computed using standard conversion and substitution factors from published literature. Forest management alternatives involving a combination of fuel reduction treatments such as thinning, physical harvesting, and Rx burning (ThPyRx) and thinning and physical harvesting (ThPy) were effective in terms of; a) lowered probability of torching; b) reduced wildfire risk levels; c) lowered tree crowning percentage and crown fire potential; and d) minimum tree basal area killed. Although the average stand level carbon pool was not significantly different among treatments, forest managements involving physical harvesting (Py), alone and in combination with thinning (ThPy), were the two lowest pyrogenic carbon emitters. Moreover, the ThPy alternative not only had the highest live vs dead carbon ratio but also resulted in the greatest carbon sequestration prospects via biochar conversion and innovative wood use. Overall, integrating physical harvesting with existing fuel treatments shows promise for reducing wildfire hazards and improving carbon credits, offering a non-intrusive, carbon-beneficial alternative approach to dead fuel reduction. However, longer-term research, both simulation modeling and field experiments, will be helpful for testing its efficacy temporally with repeated treatments and in multiple forest types, especially its potential in restoring historic wildfire regimes for healthy and resilient forests in the western US.

A fire deficit persists across diverse North American forests despite recent increases in area burned

Rapid increases in wildfire area burned across North American forests pose novel challenges for managers and society. Increasing area burned raises questions about whether, and to what degree, contemporary fire regimes (1984-2022) are still departed from historical fire regimes (pre-1880). We use the North American tree-ring fire-scar network (NAFSN), a multi-century record comprising >1800 fire-scar sites spanning diverse forest types, and contemporary fire perimeters to ask whether there is a contemporary fire surplus or fire deficit, and whether recent fire years are unprecedented relative to historical fire regimes. Our results indicate, despite increasing area burned in recent decades, that a widespread fire deficit persists across a range of forest types and recent years with exceptionally high area burned are not unprecedented when considering the multi-century perspective offered by fire-scarred trees. For example, 'record' contemporary fire years such as 2020 burned 6% of NAFSN sites-the historical average-well below the historical maximum of 29% sites that burned in 1748. Although contemporary fire extent is not unprecedented across many North American forests, there is abundant evidence that unprecedented contemporary fire severity is driving forest loss in many ecosystems and adversely impacting human lives, infrastructure, and water supplies.

Exacerbating risk in human-ignited large fires over western United States due to lower flammability thresholds and greenhouse gas emissions

Large fires in the western United States become highly probable when dry conditions surpass critical thresholds of vapor pressure deficit (VPDt). VPDt likely differs between human- and lightning-ignited fires, potentially leading to ignition-type varied responses of fire weather risk to natural variability and various anthropogenic forcings, yet a comprehensive quantification remains lacking. Here, through fire observations with ignition types and a machine learning method, we found that human-ignited large fires had consistently lower thresholds (VPDt) across western US ecoregions. Consequently, the annual number of flammable days (when VPD > VPDt) for human-caused large fires was 93% higher on average and increased 21% more rapidly than those caused by lightning during 1979-2020. Through robust statistical detection and attribution of Earth System Models, we found that the anthropogenic greenhouse gas (GHG) emissions predominantly (81%) controlled the human-related flammable day increases, which was 18% greater than the effect of GHGs on the increases in lightning-related flammable days. Such ignition-type varied fire weather risk indicates more large fire-prone conditions for human-regulated fire regimes when GHG emissions are enhancing and ignitions are not limited by fuels.

Reducing Fire Severity and Extent Bolsters Subalpine Forest Resilience to Global Change Through Key Demographic Pathways

High-elevation subalpine forests are experiencing rapid changes in climatic conditions, biological disturbances, and wildfire regimes. Despite this, evidence is mixed as to whether they will undergo major ecological transformation or be resilient to a confluence of global change drivers. Here we use subalpine fir (Abies lasiocarpa) and Englemann spruce (Picea engelmannii), which form co-dominant forests through much of the western United States, to investigate how species' demographic responses to global change influence forest community-wide resilience. We do this by adapting and building on an existing framework for post-disturbance ecological reorganization. With forest inventory data from the United States Forest Service Forest Inventory and Analysis (FIA) program, we quantify population trends for subalpine fir and Engelmann spruce across their joint distribution and organize them in a new conceptual framework for categorizing forest community trajectories. We then build hierarchical Bayesian demographic models of subalpine fir and Engelmann spruce mortality, regeneration, and recruitment as functions of climate, disturbance extent and severity, and forest structural predictors. We bring demographic predictions together in a multinomial classification model to quantify how combinations of demographic rates influence overall forest community trajectories. Finally, we apply future climate and disturbance scenarios to our demographic models to explore how subalpine forest resilience may change in the future. We found strong negative relationships between the demography of both species and disturbance extent and severity, and climatic responses in line with an energy-limited forest system. Future scenario model predictions indicate that reducing wildfire extent and severity can greatly bolster overall subalpine forest resilience; the preferred way to do this will vary according to fire history, forest type, biophysical setting, and land tenure. Opportunities for high-impact management interventions are concentrated in the northern Rocky Mountains, with centers of ongoing resilience in parts of the Oregon and Washington Cascades.

Temporal and spatial pattern analysis of escaped prescribed fires in California from 1991 to 2020

Background

Prescribed fires play a critical role in reducing the intensity and severity of future wildfires by systematically and widely consuming accumulated vegetation fuel. While the current probability of prescribed fire escape in the United States stands very low, their consequential impact, particularly the large wildfires they cause, raises substantial concerns. The most direct way of understanding this trade-off between wildfire risk reduction and prescribed fire escapes is to explore patterns in the historical prescribed fire records. This study investigates the spatiotemporal patterns of escaped prescribed fires in California from 1991 to 2020, offering insights for resource managers in developing effective forest management and fuel treatment strategies.

Results

The results reveal that the months close to the beginning and end of the wildfire season, namely May, June, September, and November, have the highest frequency of escaped fires. Under similar environmental conditions, areas with more records of prescribed fire implementation tend to experience fewer escapes. The findings revealed the vegetation types most susceptible to escaped prescribed fires. Areas with tree cover ranging from 20 to 60% exhibited the highest incidence of escapes compared to shrubs and grasslands. Among all the environmental conditions analyzed, wind speed stands out as the predominant factor that affects the risk of prescribed fire escaping.

Conclusions

These findings mark an initial step in identifying high-risk areas and periods for prescribed fire escapes. Understanding these patterns and the challenges of quantifying escape rates can inform more effective landscape management practices.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42408-024-00342-3.

Disentangling the drivers of wildfires

The risk of wildfires varies across regions with different vegetation.

Repeated fuel treatments fall short of fire-adapted regeneration objectives in a Sierra Nevada mixed conifer forest, USA

Fire exclusion over the last two centuries has driven a significant fire deficit in the forests of western North America, leading to widespread changes in the composition and structure of these historically fire-adapted ecosystems. Fuel treatments have been increasingly applied over the last few decades to mitigate fire hazard, yet it is unclear whether these fuel-focused treatments restore the fire-adapted conditions and species that will allow forests to persist into the future. A vital prerequisite of restoring fire-adaptedness is ongoing establishment of fire-tolerant tree species, and both the type and reoccurrence of fuel treatments are likely to strongly influence stand trajectories. Here, we leveraged a long-term study of repeated fuel treatments in a Sierra Nevada mixed-conifer forest to examine the regeneration response of six native tree species to the repeated application of common fuel treatments: prescribed fire, mechanical, mechanical plus fire, and untreated controls. Our objectives were to (1) quantify differences in forest structure and composition following the repeated application of alternative fuel treatments that may influence the establishment environment and then (2) identify the stand structure and climate conditions influencing seedling dynamics. We found that both treatment type and intensity are highly influential in shifting forests toward more fire-adapted conditions and determining species-specific regeneration dynamics. Specifically, the conifer species tracked here increased in either colonization or persistence potential following repeated applications of fire, indicating fire may be most effective for restoring regeneration conditions broadly across species. Fire alone, however, was not enough to promote fire-adapted composition, with concurrent mechanical treatments creating more favorable conditions for promoting colonization and increasing abundances of fire-tolerant ponderosa pine. Yet, even with repeated fuel treatment application, establishment of fire-intolerant species far exceeded that of fire-tolerant species over this 20-year study period. Moreover, increasing growing season water stress negatively impacted seedling dynamics across all species regardless of treatment type and intensity, an important consideration for ongoing management under heightened climatic stress. While repeated treatments are waypoints in restoring fire-adapted conditions, more intense treatments via gap-creation or hotter prescribed fires targeting removal of fire-intolerant species will be necessary to sustain recruitment of fire-tolerant species.

Multi-criteria decision analysis for monitoring and evaluating soil erosion risk in forest fire-affected areas

Wildfires lead to socio-economic and environmental impacts. These impacts include hydrological instability, which can cause severe damage, especially where infrastructures are present. Post-rehabilitation measures can be useful in reducing or preventing erosion or hydrogeological risks. Decision-makers are called on to prioritize post-fire intervention areas and allocate public funds for this purpose. This work focuses on the assessment of erosion and hydrological risk potential in forested slope areas affected by wildfire using a Multi-Criteria Decision Analysis (MCDA) approach integrated with a GIS environment on a regional scale. Expert perception was considered using the pairwise comparison method as part of the Analytical Hierarchy Process (AHP). This allows expert stakeholders to rank relevant criteria, providing a quantitative metric (weight) for qualitative data. Two MCDA methods are used and compared: Weighted Linear Combination (WLC) and Ordered Weighted Averaging (OWA). Fire frequency, slope (gradient and length), and proximity to infrastructures were found to be the most important factors by the stakeholders. The WLC method provides evidence classified into high and moderate suitability class areas characterized by high values for fire frequency or slope gradient. Conversely, the OWA method, ranging from low to high risks, makes it possible to adapt the method and obtain a range of suitability maps. Novelties of the MCDA-GIS combined methodology adopted in this work are its application on a regional scale and the combination of vulnerability and driving-force factors (namely presence of grey infrastructures, fire frequency). The MCDA-GIS methodology can be suitable for public administrations in that it allows for mapping a regional area more quickly and thus facilitates sector planning.

The national Fire and Fire Surrogate study: Effects of fuel treatments in the Western and Eastern United States after 20 years

The national Fire and Fire Surrogate (FFS) study was initiated more than two decades ago with the goal of evaluating the ecological impacts of mechanical treatments and prescribed fire in different ecosystems across the United States. Since then, 4 of the original 12 sites remain active in managing and monitoring the original FFS study which provides a unique opportunity to look at the long-term effects of these treatments in different regions. These sites include California (Blodgett Forest Research Station), Montana (Lubrecht Experimental Forest), North Carolina (Green River Game Land), and Ohio (Ohio Hills). Although regions differed in ecosystem type (e.g., conifer- vs. hardwood-dominated), the overall goals of the FFS study were to promote desirable, fire-adapted species, reduce fire hazard, and improve understory diversity. Our study uses multivariate techniques to compare how these desired outcomes were maintained over the last 20 years and discusses whether we would modify the original treatments given what we know now. Our findings indicate that mechanical treatments and prescribed fire can promote desired tree species, mitigate potential fire behavior by reducing fuels and retaining larger-sized trees, decrease tree mortality, and stimulate regeneration-effects that are still apparent even after 20 years. However, we also found that maintaining desired outcomes was regionally specific with western sites (California and Montana) showing more desirable characteristics under mechanical treatments, while the eastern sites (North Carolina and Ohio) showed more desirable characteristics after prescribed burning. The beneficial effects of treatment were also more apparent in the long term when sites followed up with repeated treatments, which can be adapted to meet new objectives and conditions. These findings highlight the FFS study as an invaluable resource for research and provide evidence for meeting long-term restoration goals if treatments can be adapted to ecosystem type, be maintained by repeated treatments, and accommodate new goals by adapting treatments to changing conditions.

Fire management now and in the future: Will today's solutions still apply tomorrow?

Climate change and fire management actions are the two key drivers of fire regime changes now and into the future. The predicted effects of these drivers vary between regions and global climate projections; however, it is expected that fire regimes globally are likely to intensify. Increased wildfire extent, frequency and severity mean impacts to people, property, infrastructure, production and the environment are also likely to increase under worsening climate conditions. Fire management programs aim to reduce the influence of worsening climatic conditions on wildfires and risk to assets now and into the future However, given the pace of changes to fire regimes, trade-offs between assets are increasingly likely. Therefore, understanding the cost-effectiveness of fire management in the form of both fuel management and suppression is critical for managers to make informed decisions regarding resource allocation. We develop and test a Bayesian Decision Network (BDN) incorporating data from ~1200 fire regime simulations capturing 16 management strategies across six regions and six climate models. We quantify the effects of management and climate on fire size and risk to environmental, infrastructure, and production assets, as well as people and property. We calculate the overall cost-effectiveness of the management scenario based on the cost of implementing the program and the subsequent cost of impacts caused by wildfires. We found that costs increased under future climate conditions for all management scenarios in most regions. Despite some regional variation in the cost-effectiveness of management scenarios we were able to identify key scenarios which consistently had high cost-effectiveness. These were combinations of prescribed burning and suppression. Importantly, the model clearly demonstrates the risk of a do-nothing approach and highlights that action is needed to prevent high impacts now and into the future and to reduce the overall costs of wildfires.

A review of optimization and decision models of prescribed burning for wildfire management

Prescribed burning is an essential forest management tool that requires strategic planning to effectively address its multidimensional impacts, particularly given the influence of global climate change on fire behavior. Despite the inherent complexity in planning prescribed burns, limited efforts have been made to comprehensively identify the critical elements necessary for formulating effective models. In this work, we present a systematic review of the literature on optimization and decision models for prescribed burning, analyzing 471 academic papers published in the last 25 years. Our study identifies four main types of models: spatial-allocation, spatial-extent, temporal-only, and spatial-temporal. We observe a growing number of studies on modeling prescribed burning, primarily due to the expansion in spatial-allocation and spatial-temporal models. There is also an increase in complexity as the models consider more elements affecting prescribed burning effectiveness. We identify the essential components for optimization models, including stakeholders, decision variables, objectives, and influential factors, to enhance model practicality. The review also examines solution techniques, such as integer programming in spatial allocation, stochastic dynamic programming in probabilistic models, and multiobjective programming in balancing trade-offs. These techniques' strengths and limitations are discussed to help researchers adapt methods to specific challenges in prescribed burning optimization. In addition, we investigate general assumptions in the models and challenges in relaxation to enhance practicality. Lastly, we propose future research to develop more comprehensive models incorporating dynamic fire behaviors, stakeholder preferences, and long-term impacts. Enhancing these models' accuracy and applicability will enable decision-makers to better manage wildfire treatment outcomes.

Ladder fuels rather than canopy volumes consistently predict wildfire severity even in extreme topographic-weather conditions

Drivers of forest wildfire severity include fuels, topography and weather. However, because only fuels can be actively managed, quantifying their effects on severity has become an urgent research priority. Here we employed GEDI spaceborne lidar to consistently assess how pre-fire forest fuel structure affected wildfire severity across 42 California wildfires between 2019-2021. Using a spatial-hierarchical modeling framework, we found a positive concave-down relationship between GEDI-derived fuel structure and wildfire severity, marked by increasing severity with greater fuel loads until a decline in severity in the tallest and most voluminous forest canopies. Critically, indicators of canopy fuel volumes (like biomass and height) became decoupled from severity patterns in extreme topographic and weather conditions (slopes >20°; winds > 9.3 m/s). On the other hand, vertical continuity metrics like layering and ladder fuels more consistently predicted severity in extreme conditions - especially ladder fuels, where sparse understories were uniformly associated with lower severity levels. These results confirm that GEDI-derived fuel estimates can overcome limitations of optical imagery and airborne lidar for quantifying the interactive drivers of wildfire severity. Furthermore, these findings have direct implications for designing treatment interventions that target ladder fuels versus entire canopies and for delineating wildfire risk across topographic and weather conditions.

Optimizing fuel treatments for community wildfire mitigation planning

Fuel management is undertaken to mitigate the adverse consequences of wildfire. Finite mitigation budgets demand selective prioritization of forest stands for targeted fuel reduction treatments. A range of modeling methods have been used to identifiy optimal fuel treatment plans at various spatial and temporal scales of investigation; however, strategic analysis of fuel management alternatives can involve a range of limitations and challenges, including the prevalence of one-time solutions, static models lacking dynamic adaptability, and challenges in accounting for the stochastic nature of fire behaviour. To navigate these complexities, our study combines remote sensing-based analysis with a random search optimization algorithm to inform strategic fuel management and wildfire mitigation planning. For two communities in Alberta, Whitecourt and Hinton, we assessed landscape fire exposure within and around the built environment and rated hazardous fuels by the number of buildings they exposed (i.e., Building Exposure load, BEL). Through the assessment of BEL and the outcomes of the optimization algorithm, our model identified key areas for intervention, enabling a more informed allocation of mitigation resources. We found good alignment between expert-derived fuel treatment areas and our model-derived fuel reduction areas, PFRs, confirming the utility and relevance of our findings. The methodology is adaptable to diverse regional fuel characteristics and it also offers a phased implementation to assisting communities with financial constraints. The suggested systematic approach aids communities that lack local expertise in developing proactive fuel treatment strategies. Additionally, this study emphasizes the need to combine fuel treatment prioritization with community involvement, acknowledgment of potential local limitations, and financial planning to enhance its effectiveness and adaptability.

Wildland Fire-Related Smoke PM 2.5 and Cardiovascular Disease ED Visits in the Western United States

Background

The impact of short-term exposure to fine particulate matter (PM 2.5 ) due to wildland fire smoke on the risk of cardiovascular disease (CVD) remains unclear. We investigated the association between short-term exposure to wildfire smoke PM 2.5 and Emergency Department (ED) visits for acute CVD in the Western United States from 2007 to 2018.

Methods

ED visits for primary or secondary diagnoses of atrial fibrillation (AF), acute myocardial infarction (AMI), heart failure (HF), stroke, and total CVD were obtained from hospital associations or state health departments in California, Arizona, Nevada, Oregon, and Utah. ED visits included those that were subsequently hospitalized. Daily smoke, non-smoke, and total PM 2.5 were estimated using a satellite-driven multi-stage model with a high resolution of 1 km. The data were aggregated to the zip code level and a case-crossover study design was employed. Temperature, relative humidity, and day of the year were included as covariates.

Results

We analyzed 49,759,958 ED visits for primary or secondary CVD diagnoses, which included 6,808,839 (13.7%) AFs, 1,222,053 (2.5%) AMIs, 7,194,474 (14.5%) HFs, and 808,396 (1.6%) strokes. Over the study period from 2007-01-01 to 2018-12-31, the mean smoke PM 2.5 was 1.27 (Q1: 0, Q3: 1.29) µg/m 3 . A 10 µg/m 3 increase in smoke PM 2.5 was associated with a minuscule decreased risk for AF (OR 0.994, 95% CI 0.991-0.997), HF (OR 0.995, 95% CI 0.992-0.998), and CVD (OR 0.9997, 95% CI 0.996-0.998), but not for AMI and stroke. Adjusting for non-smoke PM 2.5 did not alter these associations. A 10 µg/m 3 increase in total PM 2.5 was linked to a small increased risk for all outcomes except stroke (OR for CVD 1.006, 95% CI 1.006-1.007). Associations were similar across sex and age groups.

Conclusion

We identified an unexpected slight lower risk of CVD ED visits associated with short-term wildfire smoke PM 2.5 exposure. Whether these findings are due to methodological issues, behavioral changes, or other factors requires further investigation.

Long-term frequent fire and cattle grazing alter dry forest understory vegetation

Understanding fire and large herbivore interactions in interior western forests is critical, owing to the extensive and widespread co-occurrence of these two disturbance types and multiple present and future implications for forest resilience, conservation and restoration. However, manipulative studies focused on interactions and outcomes associated with these two disturbances are rare in forested rangelands. We investigated understory vegetation response to 5-year spring and fall prescribed fire and domestic cattle grazing exclusion in ponderosa pine stands and reported long-term responses, almost two decades after the first entry fires. In fall burn areas open to cattle grazing, total understory cover prior to utilization was about 12% lower compared with fall burn areas where cattle were experimentally excluded. This response was not strongly driven by a particular palatable or unpalatable plant functional group. Fire and grazing are likely interacting in a numerically mediated process, as we found little evidence to support a functionally moderated pathway. Post-fire green-up may equalize forage to a certain extent and concentrate herbivores in the smaller burned areas within pastures, constraining a positive understory response to burning. Fall fire and grazing also increased annual forbs and resprouting shrubs. The effects of spring burning were relatively minor, and we found no interaction with grazing. The nonnative annual grass Bromus tectorum (cheatgrass) remains a problematic invader linked to fall burning but not grazing in stands that had higher propagule pressure when the experiment was initiated. At these sites, exotic grass was a major component of the vegetation by 2015, and invasion was also increasing in spring burn and unburned areas. Information from our study suggests that frequent fall fires and cattle grazing combined may reduce understory resilience in similar dry ponderosa pine forests. Consideration of longer fire return intervals, resting areas after fire, virtual fencing, or burning entire pastures may help to mitigate the effects noted in this study.

The effect of forest composition on outdoor recreation

Climate change will shift the composition of northern Minnesota forests from boreal to temperate by the end of the century. This shift in forest composition will likely affect outdoor recreation, a valuable ecosystem service and a key economic driver for the region. In this context, the objective of our paper is to empirically examine the relationship between forest composition and recreation. We analyze the effect of changes in forest composition for seven forest types on seven types of recreation using a lognormal pooled panel regression model for Minnesota's Laurentian Mixed Forest Province. Earlier research showed forest composition affected recreation at the level of broad groups of broadleaved or coniferous species. We find a statistically significant empirical association between forest composition and recreation at the forest type level (forest types within those broad groups). This relationship varies across forest types and recreation categories. For example, big game hunting is positively related to elm-ash-cottonwood and white-red-jack pine and negatively associated with aspen-birch. We find individual forest types within broad groups of broadleaved or coniferous forests, have different relationships with recreation, so that these broad groups are not sufficient in capturing the effect of forest composition on recreation. Our results are of interest in the context of current shifts in forest composition caused by climate change, which could also affect recreation. Our findings suggest adding a forest composition lens to existing policies could facilitate strategies for more effective recreation management and climate change adaptation.

Blending Indigenous and western science: Quantifying cultural burning impacts in Karuk Aboriginal Territory

The combined effects of Indigenous fire stewardship and lightning ignitions shaped historical fire regimes, landscape patterns, and available resources in many ecosystems globally. The resulting fire regimes created complex fire-vegetation dynamics that were further influenced by biophysical setting, disturbance history, and climate. While there is increasing recognition of Indigenous fire stewardship among western scientists and managers, the extent and purpose of cultural burning is generally absent from the landscape-fire modeling literature and our understanding of ecosystem processes and development. In collaboration with the Karuk Tribe Department of Natural Resources, we developed a transdisciplinary Monte Carlo simulation model of cultural ignition location, frequency, and timing to simulate spatially explicit cultural ignitions across a 264,399-ha landscape within Karuk Aboriginal Territory in northern California. Estimates of cultural ignition parameters were developed with Tribal members and knowledge holders using existing interviews, historical maps, ethnographies, recent ecological studies, contemporary maps, and generational knowledge. Spatial and temporal attributes of cultural burning were explicitly tied to the ecology of specific cultural resources, fuel receptivity, seasonal movement patterns, and spiritual practices. Prior to colonization, cultural burning practices were extensive across the study landscape with an estimated 6972 annual ignitions, averaging approximately 6.5 ignitions per Indigenous fire steward per year. The ignition characteristics we document align closely with data on historical fire regimes and vegetation but differ substantially from the location and timing of contemporary ignitions. This work demonstrates the importance of cultural burning for developing and maintaining the ecosystems present at the time of colonization and underscores the need to work collaboratively with Indigenous communities to restore ecocultural processes in these systems.

Fire suppression makes wildfires more severe and accentuates impacts of climate change and fuel accumulation

Fire suppression is the primary management response to wildfires in many areas globally. By removing less-extreme wildfires, this approach ensures that remaining wildfires burn under more extreme conditions. Here, we term this the "suppression bias" and use a simulation model to highlight how this bias fundamentally impacts wildfire activity, independent of fuel accumulation and climate change. We illustrate how attempting to suppress all wildfires necessarily means that fires will burn with more severe and less diverse ecological impacts, with burned area increasing at faster rates than expected from fuel accumulation or climate change. Over a human lifespan, the modeled impacts of the suppression bias exceed those from fuel accumulation or climate change alone, suggesting that suppression may exert a significant and underappreciated influence on patterns of fire globally. Managing wildfires to safely burn under low and moderate conditions is thus a critical tool to address the growing wildfire crisis.

Assessing silvopasture management as a strategy to reduce fuel loads and mitigate wildfire risk

Managing private forests for wildfire resilience is challenging due to conflicting social, economic, and ecological decisions that may result in an increase of surface fuel loads leading to greater fire risk. Due to fire suppression and a changing climate, land managers in fire-prone regions face an increasing threat of high severity fires. Thus, land managers need fuel treatment options that match their forest types and management objectives. One potential option for producers that graze livestock is silvopasture management, where livestock, forages, and overstory vegetation are carefully managed for co-benefits on the same unit of land. This study compared forest composition and structure, fuel types, and vegetative biomass between silvopasture and non-grazed managed forests in Washington, U.S. We show that silvopasture management results in reductions in grass biomass, litter, and duff depth when compared to non-grazed managed forest. These findings point to the integrated nature of silvopasture, where management of overstory composition and structure, understory vegetation, and grazing can reduce fuel loads and potential wildfire risk.

Long-term efficacy of fuel reduction and restoration treatments in Northern Rockies dry forests

Fuel and restoration treatments seeking to mitigate the likelihood of uncharacteristic high-severity wildfires in forests with historically frequent, low-severity fire regimes are increasingly common, but long-term treatment effects on fuels, aboveground carbon, plant community structure, ecosystem resilience, and other ecosystem attributes are understudied. We present 20-year responses to thinning and prescribed burning treatments commonly used in dry, low-elevation forests of the western United States from a long-term study site in the Northern Rockies that is part of the National Fire and Fire Surrogate Study. We provide a comprehensive synthesis of short-term (<4 years) and mid-term (<14 years) results from previous findings. We then place these results in the context of a mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak that impacted the site 5-10 years post-treatment and describe 20-year responses to assess the longevity of restoration and fuel reduction treatments in light of the MPB outbreak. Thinning treatments had persistently lower forest density and higher tree growth, but effects were more pronounced when thinning was combined with prescribed fire. The thinning+prescribed fire treatment had the additional benefit of maintaining the highest proportion of ponderosa pine (Pinus ponderosa) for overstory and regeneration. No differences in understory native plant cover and richness or exotic species cover remained after 20 years, but exotic species richness, while low relative to native species, was still higher in the thinning+prescribed fire treatment than the control. Aboveground live carbon stocks in thinning treatments recovered to near control and prescribed fire treatment levels by 20 years. The prescribed fire treatment and control had higher fuel loads than thinning treatments due to interactions with the MPB outbreak. The MPB-induced changes to forest structure and fuels increased the fire hazard 20 years post-treatment in the control and prescribed fire treatment. Should a wildfire occur now, the thinning+prescribed fire treatment would likely have the lowest intensity fire and highest tree survival and stable carbon stocks. Our findings show broad support that thinning and prescribed fire increase ponderosa pine forest resilience to both wildfire and bark beetles for up to 20 years, but efficacy is waning and additional fuel treatments are needed to maintain resilience.

Water utility engagement in wildfire mitigation in watersheds in the western United States

Scaling up climate-adaptation in wildfire-prone watersheds requires innovative partnerships and funding. Water utilities are one stakeholder group that could play a role in these efforts. The overarching purpose of this study was to understand water utility engagement in wildfire mitigation efforts in the western United States. We conducted an online survey of water utilities in nine states and received 173 useable responses. While most (68%) respondents were concerned or very concerned about future wildfire events and the impact of wildfire on their operations, only 39% perceived their organization as responsible for mitigating wildfire risk. Federal land ownership decreased feeling responsible for wildfire mitigation, while concern for and information on wildfire increased feeling responsible for mitigation. The perception of response efficacy of mitigation actions for the 68 water utilities engaged in wildfire risk mitigation activities was very high, with most agreeing that mitigation actions are effective. Self-efficacy to implement mitigation actions, however, was mixed, with most utilities wanting more information on wildfire risk and impacts to watershed services. The most reported wildfire mitigation actions were forest thinning and stream restoration. Water utilities engaging in these actions typically partnered with government agencies or other water utilities to complete the work and funded these activities through water user fees and grants. Our findings suggest that water utility engagement in wildfire mitigation for water security could be increased through providing more assessments of wildfire risk to water utilities and through more outreach and engagement with water utilities operating on federal lands.

Growing convergence research: Coproducing climate projections to inform proactive decisions for managing simultaneous wildfire risk

We apply a convergence research approach to the urgent need for proactive management of long-term risk associated with wildfire in the United States. In this work we define convergence research in accordance with the US National Science Foundation-as a means of addressing a specific and compelling societal problem for which solutions require deep integration across disciplines and engagement of stakeholders. Our research team brings expertise in climate science, fire science, landscape ecology, and decision science to address the risk from simultaneous and impactful fires that compete for management resources, and leverages climate projections for decision support. In order to make progress toward convergence our team bridges spatial and temporal scale divides arising from differences in disciplinary and practice-based norms. We partner with stakeholders representing US governmental, tribal, and local decision contexts to coproduce a robust information base for support of decision making about wildfire preparedness and proactive land/fire management. Our approach ensures that coproduced information will be directly integrated into existing tools for application in operations and policy making. Coproduced visualizations and decision support information provide projections of the change in expected number of fires that compete for resources, the number of fire danger days per year relative to prior norms, and changes in the length and overlap of fire season in multiple US regions. Continuing phases of this work have been initiated both by stakeholder communities and by our research team, a demonstration of impact and value.

A multi-benefit framework for funding forest management in fire-driven ecosystems across the Western U.S

Forests across the Western U.S. face unprecedented risk due to historic fire exclusion, environmental degradation, and climate change. Forest management activities like ecological thinning, prescribed burning, and meadow restoration can improve landscape resilience. Resilient forests are at a lower risk of high-intensity wildfires, drought, insects, and other disturbances and provide a wide range of benefits to ecosystems and communities. However, insufficient funding limits implementation of critically needed management. To address this challenge, we propose a multi-benefit framework that leverages the diverse benefits of forest management to engage a suite of stakeholders in sharing project costs. We take a three-pronged approach to develop our conceptual model: examining existing frameworks for environmental project implementation, conducting a literature review of forest management benefits, and evaluating case studies. Through our framework, we describe the steps to engage partners, starting by identifying benefits that could accrue to potential public and private beneficiaries, and moving through an iterative and collaborative process of valuing benefits, which can accrue over different spatial and temporal scales, in close consultation with potential beneficiaries themselves. The aim of this approach is to stack funding streams associated with each valued benefit to fully fund a given forest management project. The multi-benefit framework has the potential to unlock new sources of funding to meet the exceptional challenges of climate and wildfire disturbances. We apply the framework to dry forests of the Western U.S., but opportunities exist for expanding and modifying this approach to any geography or ecosystem where management provides multiple benefits.

Climate change is narrowing and shifting prescribed fire windows in western United States

Escalating wildfire activity in the western United States has accelerated adverse societal impacts. Observed increases in wildfire severity and impacts to communities have diverse anthropogenic causes-including the legacy of fire suppression policies, increased development in high-risk zones, and aridification by a warming climate. However, the intentional use of fire as a vegetation management tool, known as "prescribed fire," can reduce the risk of destructive fires and restore ecosystem resilience. Prescribed fire implementation is subject to multiple constraints, including the number of days characterized by weather and vegetation conditions conducive to achieving desired outcomes. Here, we quantify observed and projected trends in the frequency and seasonality of western United States prescribed fire days. We find that while ~2 C of global warming by 2060 will reduce such days overall (-17%), particularly during spring (-25%) and summer (-31%), winter (+4%) may increasingly emerge as a comparatively favorable window for prescribed fire especially in northern states.

Fire intensity impacts on physiological performance and mortality in Pinus monticola and Pseudotsuga menziesii saplings: a dose-response analysis

Fire is a major cause of tree injury and mortality worldwide, yet our current understanding of fire effects is largely based on ocular estimates of stem charring and foliage discoloration, which are error prone and provide little information on underlying tree function. Accurate quantification of physiological performance is a research and forest management need, given that declining performance could help identify mechanisms of-and serve as an early warning sign for-mortality. Many previous efforts have been hampered by the inability to quantify the heat flux that a tree experiences during a fire, given its highly variable nature in space and time. In this study, we used a dose-response approach to elucidate fire impacts by subjecting Pinus monticola var. minima Lemmon and Pseudotsuga menziesii (Mirb.) Franco var. glauca (Beissn.) Franco saplings to surface fires of varying intensity doses and measuring short-term post-fire physiological performance in photosynthetic rate and chlorophyll fluorescence. We also evaluated the ability of spectral reflectance indices to quantify change in physiological performance at the individual tree crown and stand scales. Although physiological performance in both P. monticola and P. menziesii declined with increasing fire intensity, P. monticola maintained a greater photosynthetic rate and higher chlorophyll fluorescence at higher doses, for longer after the fire. Pinus monticola also had complete survival at lower fire intensity doses, whereas P. menziesii had some mortality at all doses, implying higher fire resistance for P. monticola at this life stage. Generally, individual-scale spectral indices were more accurate at quantifying physiological performance than those acquired at the stand-scale. The Photochemical Reflectance Index outperformed other indices at quantifying photosynthesis and chlorophyll fluorescence, highlighting its potential use to quantify crown scale physiological performance. Spectral indices that incorporated near-infrared and shortwave infrared reflectance, such as the Normalized Burn Ratio, were accurate at characterizing stand-scale mortality. The results from this study were included in a conifer cross-comparison using physiology and mortality data from other dose-response studies. The comparison highlights the close evolutionary relationship between fire and species within the Pinus genus, assessed to date, given the high survivorship of Pinus species at lower fire intensities versus other conifers.

Valuing the benefits of forest restoration on enhancing hydropower and water supply in California's Sierra Nevada

Forest restoration through mechanical thinning, prescribed burning, and other management actions is vital to improving forest resilience to fire and drought across the Western United States, and yields benefits that can be monetized, including improvements in water supply and hydropower. Using California's Sierra Nevada as a study area, we assess the water and energy benefits of forest-restoration projects. By using a scalable top-down approach to track annual evapotranspiration following forest disturbance, coupled with hydropower simulations that include energy-price information, and marginal prices for water sales, we project the potential economic benefits of hydropower and water sales accruing to water-rights holders. The results found that water-related benefits from strategically planned fuels-reduction treatments now being carried out can be sufficient to offset costs of management actions aimed at forest restoration, especially in the face of climate change. Our findings justified investments in restoring forests and reinforce the central role of water and hydropower providers in partnerships for management of source-water watersheds. Results also highlighted the importance of accurate, scalable data and tools from the hydrology and water-resources community.

Tumor heterogeneity: preclinical models, emerging technologies, and future applications

Heterogeneity describes the differences among cancer cells within and between tumors. It refers to cancer cells describing variations in morphology, transcriptional profiles, metabolism, and metastatic potential. More recently, the field has included the characterization of the tumor immune microenvironment and the depiction of the dynamics underlying the cellular interactions promoting the tumor ecosystem evolution. Heterogeneity has been found in most tumors representing one of the most challenging behaviors in cancer ecosystems. As one of the critical factors impairing the long-term efficacy of solid tumor therapy, heterogeneity leads to tumor resistance, more aggressive metastasizing, and recurrence. We review the role of the main models and the emerging single-cell and spatial genomic technologies in our understanding of tumor heterogeneity, its contribution to lethal cancer outcomes, and the physiological challenges to consider in designing cancer therapies. We highlight how tumor cells dynamically evolve because of the interactions within the tumor immune microenvironment and how to leverage this to unleash immune recognition through immunotherapy. A multidisciplinary approach grounded in novel bioinformatic and computational tools will allow reaching the integrated, multilayered knowledge of tumor heterogeneity required to implement personalized, more efficient therapies urgently required for cancer patients.

Denis LA. Shifting social-ecological fire regimes explain increasing structure loss from Western wildfires

Structure loss is an acute, costly impact of the wildfire crisis in the western conterminous United States ("West"), motivating the need to understand recent trends and causes. We document a 246% rise in West-wide structure loss from wildfires between 1999-2009 and 2010-2020, driven strongly by events in 2017, 2018, and 2020. Increased structure loss was not due to increased area burned alone. Wildfires became significantly more destructive, with a 160% higher structure-loss rate (loss/kha burned) over the past decade. Structure loss was driven primarily by wildfires from unplanned human-related ignitions (e.g. backyard burning, power lines, etc.), which accounted for 76% of all structure loss and resulted in 10 times more structures destroyed per unit area burned compared with lightning-ignited fires. Annual structure loss was well explained by area burned from human-related ignitions, while decadal structure loss was explained by state-level structure abundance in flammable vegetation. Both predictors increased over recent decades and likely interacted with increased fuel aridity to drive structure-loss trends. While states are diverse in patterns and trends, nearly all experienced more burning from human-related ignitions and/or higher structure-loss rates, particularly California, Washington, and Oregon. Our findings highlight how fire regimes-characteristics of fire over space and time-are fundamentally social-ecological phenomena. By resolving the diversity of Western fire regimes, our work informs regionally appropriate mitigation and adaptation strategies. With millions of structures with high fire risk, reducing human-related ignitions and rethinking how we build are critical for preventing future wildfire disasters.

Climate risk, natural resources, and climate change mitigation options in BRICS: implications for green recovery

The study tests the dynamic nexus between climate risk, natural resource exploration strategy in BRICS economies, and climate change mitigation. The study further extended the research aim and presented the recommendations for greening the globe by suggesting green recovery. Climate change protection and climate risk reduction may be efficiently funded using climate risks and natural resources. Still, it is vital to look at the carbon risk in BRICS countries as an example. The researchers used the GMM analysis technique to infer the study findings. According to the study's findings, environmental mitigation was significant at 17%, and financial strength and carbon risks were significant at 22.0%. In addition, the 20.5% association between climate risks and environmental drift in the BRICS nations highlights climate change concerns. A state's financial strength is essential to execute green economic recovery strategies, one of the most highly regarded measures to reducing climate change and guaranteeing long-term economic status at the national level. As a result of the study on green economic growth, decision-makers are provided with specific policy recommendations.

Postfire futures in southwestern forests: Climate and landscape influences on trajectories of recovery and conversion

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.

The contribution of Indigenous stewardship to an historical mixed-severity fire regime in British Columbia, Canada

Indigenous land stewardship and mixed-severity fire regimes both promote landscape heterogeneity, and the relationship between them is an emerging area of research. In our study, we reconstructed the historical fire regime of Ne Sextsine, a 5900-ha dry, Douglas fir-dominated forest in the traditional territory of the T'exelc (Williams Lake First Nation) in British Columbia, Canada. Between 1550 and 1982 CE, we found median fire intervals of 18 years at the plot level and 4 years at the study-site level. Ne Sextsine was characterized by an historical mixed-severity fire regime, dominated by frequent, low-severity fires as indicated by fire scars, with infrequent, mixed-severity fires indicated by cohorts. Differentiating low- from mixed-severity plots over time was key to understanding the drivers of the fire regime at Ne Sextsine. Low-severity plots were coincident with areas of highest use by the T'exelc, including winter village sites, summer fishing camps, and travel corridors. The high fire frequency in low-severity plots ceased in the 1870s, following the smallpox epidemic, the forced relocation of Indigenous peoples into small reserves, and the prohibition of Indigenous burning. In contrast, the mixed-severity plots were coincident with areas where forest resources, such as deer or certain berry species, were important. The high fire frequency in the mixed-severity plots continued until the 1920s when industrial-scale grazing and logging began, facilitated by the establishment of a nearby railway. T'exelc oral histories and archeological evidence at Ne Sextsine speak to varied land stewardship, reflected in the spatiotemporal complexity of low- and mixed-severity fire plots. Across Ne Sextsine, 63% of cohorts established and persisted in the absence of fire after colonial impacts beginning in the 1860s, resulting in a dense, homogeneous landscape that no longer supports T'exelc values and is more likely to burn at uncharacteristic high severities. This nuanced understanding of the Indigenous contribution to a mixed-severity fire regime is critical for advancing proactive fire mitigation that is ecoculturally relevant and guided by Indigenous expertise.

Wildfire, Smoke Exposure, Human Health, and Environmental Justice Need to be Integrated into Forest Restoration and Management

PURPOSE OF REVIEW

Increasing wildfire size and severity across the western United States has created an environmental and social crisis that must be approached from a transdisciplinary perspective. Climate change and more than a century of fire exclusion and wildfire suppression have led to contemporary wildfires with more severe environmental impacts and human smoke exposure. Wildfires increase smoke exposure for broad swaths of the US population, though outdoor workers and socially disadvantaged groups with limited adaptive capacity can be disproportionally exposed. Exposure to wildfire smoke is associated with a range of health impacts in children and adults, including exacerbation of existing respiratory diseases such as asthma and chronic obstructive pulmonary disease, worse birth outcomes, and cardiovascular events. Seasonally dry forests in Washington, Oregon, and California can benefit from ecological restoration as a way to adapt forests to climate change and reduce smoke impacts on affected communities.

RECENT FINDINGS

Each wildfire season, large smoke events, and their adverse impacts on human health receive considerable attention from both the public and policymakers. The severity of recent wildfire seasons has state and federal governments outlining budgets and prioritizing policies to combat the worsening crisis. This surging attention provides an opportunity to outline the actions needed now to advance research and practice on conservation, economic, environmental justice, and public health interests, as well as the trade-offs that must be considered. Scientists, planners, foresters and fire managers, fire safety, air quality, and public health practitioners must collaboratively work together. This article is the result of a series of transdisciplinary conversations to find common ground and subsequently provide a holistic view of how forest and fire management intersect with human health through the impacts of smoke and articulate the need for an integrated approach to both planning and practice.

Cumulative Tree Mortality from Commercial Thinning and a Large Wildfire in the Sierra Nevada, California

Debate remains about the effectiveness of commercial thinning as a wildfire management strategy, with some studies reporting somewhat lower severity in thinned forests, and some reporting higher severity, during wildfires. However, while vegetation severity is a measure of basal area tree mortality, research on this question generally omits tree mortality from thinning itself. We investigated whether cumulative tree mortality, or cumulative severity, from commercial thinning and wildfire was different between thinned and unthinned forests in the Caldor Fire of 2021 in the northern Sierra Nevada mountains of California, USA. We found significantly higher cumulative severity in commercial thinning areas compared to unthinned forests. More research is needed to determine whether cumulative severity is higher in commercially thinned forests in other large western US wildfires.

Adaptation Strategies and Approaches for Managing Fire in a Changing Climate

As the effects of climate change accumulate and intensify, resource managers juggle existing goals and new mandates to operationalize adaptation. Fire managers contend with the direct effects of climate change on resources in addition to climate-induced disruptions to fire regimes and subsequent ecosystem effects. In systems stressed by warming and drying, increased fire activity amplifies the pace of change and scale of severe disturbance events, heightening the urgency for management action. Fire managers are asked to integrate information on climate impacts with their professional expertise to determine how to achieve management objectives in a changing climate with altered fire regimes. This is a difficult task, and managers need support as they incorporate climate adaptation into planning and operations. We present a list of adaptation strategies and approaches specific to fire and climate based on co-produced knowledge from a science–management partnership and pilot-tested in a two-day workshop with natural resource managers and regional stakeholders. This “menu” is a flexible and useful tool for fire managers who need to connect the dots between fire ecology, climate science, adaptation intent, and management implementation. It was created and tested as part of an adaptation framework used widely across the United States and should be applicable and useful in many fire-prone forest ecosystems.

Evidence for widespread changes in the structure, composition, and fire regimes of western North American forests

Implementation of wildfire- and climate-adaptation strategies in seasonally dry forests of western North America is impeded by numerous constraints and uncertainties. After more than a century of resource and land use change, some question the need for proactive management, particularly given novel social, ecological, and climatic conditions. To address this question, we first provide a framework for assessing changes in landscape conditions and fire regimes. Using this framework, we then evaluate evidence of change in contemporary conditions relative to those maintained by active fire regimes, i.e., those uninterrupted by a century or more of human-induced fire exclusion. The cumulative results of more than a century of research document a persistent and substantial fire deficit and widespread alterations to ecological structures and functions. These changes are not necessarily apparent at all spatial scales or in all dimensions of fire regimes and forest and nonforest conditions. Nonetheless, loss of the once abundant influence of low- and moderate-severity fires suggests that even the least fire-prone ecosystems may be affected by alteration of the surrounding landscape and, consequently, ecosystem functions. Vegetation spatial patterns in fire-excluded forested landscapes no longer reflect the heterogeneity maintained by interacting fires of active fire regimes. Live and dead vegetation (surface and canopy fuels) is generally more abundant and continuous than before European colonization. As a result, current conditions are more vulnerable to the direct and indirect effects of seasonal and episodic increases in drought and fire, especially under a rapidly warming climate. Long-term fire exclusion and contemporaneous social-ecological influences continue to extensively modify seasonally dry forested landscapes. Management that realigns or adapts fire-excluded conditions to seasonal and episodic increases in drought and fire can moderate ecosystem transitions as forests and human communities adapt to changing climatic and disturbance regimes. As adaptation strategies are developed, evaluated, and implemented, objective scientific evaluation of ongoing research and monitoring can aid differentiation of warranted and unwarranted uncertainties.

Wildfire and climate change adaptation of western North American forests: a case for intentional management

Forest landscapes across western North America (wNA) have experienced extensive changes over the last two centuries, while climatic warming has become a global reality over the last four decades. Resulting interactions between historical increases in forested area and density and recent rapid warming, increasing insect mortality, and wildfire burned areas, are now leading to substantial abrupt landscape alterations. These outcomes are forcing forest planners and managers to identify strategies that can modify future outcomes that are ecologically and/or socially undesirable. Past forest management, including widespread harvest of fire- and climate-tolerant large old trees and old forests, fire exclusion (both Indigenous and lightning ignitions), and highly effective wildfire suppression have contributed to the current state of wNA forests. These practices were successful at meeting short-term demands, but they match poorly to modern realities. Hagmann et al. review a century of observations and multi-scale, multi-proxy, research evidence that details widespread changes in forested landscapes and wildfire regimes since the influx of European colonists. Over the preceding 10 millennia, large areas of wNA were already settled and proactively managed with intentional burning by Indigenous tribes. Prichard et al. then review the research on management practices historically applied by Indigenous tribes and currently applied by some managers to intentionally manage forests for resilient conditions. They address 10 questions surrounding the application and relevance of these management practices. Here, we highlight the main findings of both papers and offer recommendations for management. We discuss progress paralysis that often occurs with strict adherence to the precautionary principle; offer insights for dealing with the common problem of irreducible uncertainty and suggestions for reframing management and policy direction; and identify key knowledge gaps and research needs.

Disturbance shapes the US forest governance frontier: A review and conceptual framework for understanding governance change

Conflict in US forest management for decades centered around balancing demands from forested ecosystems, with a rise in place-based collaborative governance at the end of the twentieth century. By the early 2000s, it was becoming apparent that not only had the mix of players involved in forest management changed, but so had the playing field, as climate-driven disturbances such as wildfire and insect and disease outbreaks were becoming more extensive and severe. In this conceptual review paper, we argue that disturbance has become the most prominent driver of governance change on US national forests, but we also recognize that the governance responses to disturbance are shaped by variables such as discourses, institutional history and path dependence, and institutional innovation operating at different system levels. We review the governance changes in response to disturbance that constitute a new frontier in US federal forest governance and offer a conceptual framework to examine how these governance responses are shaped by multi-level factors.