In forests adapted to frequent fire, fuel treatments aim to restore resilience by disrupting the horizontal and vertical fuel continuity that drives catastrophic crown fires. Although foundational, traditional plot-scale measurements cannot capture the continuous structural patterns that influence fire behavior at stand or landscape spatial scales. Airborne laser scanning (ALS) can be a useful tool for characterizing these patterns beyond the plot scale. In this study, we evaluated the stand-scale effects of mechanical thinning (Mech), prescribed fire (Fire), and their combined application (Mech + Fire) at the Blodgett Forest Fire and Fire Surrogate (FFS) site in the Sierra Nevada, California, USA. Using wall-to-wall ALS metrics, we quantified vertical fuel continuity, canopy height distributions, spatial tree clustering, canopy gap extent, and internal gap structure (regeneration height and vertical complexity). Relative to untreated controls, all active treatment regimes reduced vertical fuel continuity. Fire and Mech + Fire treatments produced broad reductions, while Mech reduced overall load but retained localized high-continuity pockets. Mech + Fire uniquely expanded canopy gap extent and reduced gap height while preserving dominant overstory height. Fire increased within-gap heterogeneity but reduced maximum canopy height by > 3 m. Mech did not significantly increase gap extent or meaningfully alter canopy conditions. Tree clustering patterns remained similar to Control across all treatments. These results provide new stand-scale information that extends decades of plot-based FFS findings, demonstrating that combining fire with mechanical harvest treatments most effectively disrupts vertical and horizontal continuity.

Frequent-fire forests had fewer, larger trees and experienced less competition for resources prior to widespread fire suppression and logging in the early 20th century, yet landscape-scale assessments of historical forest structure, composition, and competition remain limited. We compiled five systematic timber inventories sampled from 1911 to 1936 to assess the historical structure and composition of ponderosa pine and mixed-conifer forests of the Sierra Nevada of California and the Cascades of Oregon and California. We summarized measures of tree size, density, species, and basal area for each dataset. We calculated the relative Stand Density Index (SDI) to quantify degrees of stand competition and utilized Principal Component Analysis and K-means clustering to assess patterns of forest structure across a wide latitudinal gradient. On average, historical frequent-fire forests had low tree densities, larger trees, high representation of fire-tolerant pines, and minimal competition, with variability within and among sites. Clustering analysis identified two distinct groups of stand conditions, but each site had membership within both groups, revealing landscape-level similarities despite our extensive study area. Our synthesis of historical timber inventories illustrates a spectrum of frequent-fire forest characteristics across broad spatial scales, providing managers with reference conditions that can inform contemporary forest restoration and support aspirations of operational resilience.

Networks of linear fuel treatments (Fuel Break Networks) are widely implemented in California, USA, fuel types to improve firefighter safety and facilitate fire containment. Despite frequent construction, landscape scale evaluations of their effectiveness with fire modeling remain limited in this region. This study presents a framework to assess how fuel break configuration, arrangement, and firefighter tactics influence fire control opportunities using a customized spatial metric for Uncontrollable Wildfire Risk (UWR). UWR combines outputs from fire modeling software widely available to fire and land management practitioners with suppression difficulty weights derived from previous literature. Fire spread simulations were conducted across four case study fuel break configurations in Southern California: Single Segment, Branching Network, Enclosed Network, and Multiple Segment Network. Three leverage scenarios (unstaffed, firebreak, and firing operations) were applied to each landscape. Linear mixed effects models and spatial analysis quantified the effects of distance from treatment, wind alignment, topography, treatment width, length, sinuosity, and proximity to other treatments on UWR. Results showed that increased leverage intensity consistently reduced UWR, while treatment geometry and spatial arrangement influenced risk reduction in some models. Notably, in some instances unstaffed fuel breaks increased burned area due to changes in fuel characteristics and subsequent fire behavior. This research highlights the importance of selecting appropriate outcomes for wildfire modeling evaluations of fuel break placement and operational utilization.

Giant sequoias are among the oldest and most massive trees in the world, and they are also among the most fire resilient. Historically, they experienced frequent, predominantly low- to moderate-severity fire that kept fuel loads low. However, over a century of fire exclusion has greatly increased fuel loads, which is interacting with the warming climate to increase wildfire activity and severity in the giant sequoia range. While the existing literature has documented mortality rates and explored the linkage between tree-level characteristics and delayed mortality, this study is the first to explore the role of forest structure in individual sequoia mortality. We sampled three giant sequoia groves that burned in the 2020 Castle Fire to explore the drivers of both individual large sequoia mortality and stand-level fire severity. For individual large sequoia mortality, we examined individual tree characteristics, topography, and the role of forest structure at two neighborhood sizes (7.5 m and 15 m surrounding the focal tree). We also explored how forest structure and topography influence stand-level basal area mortality. We surveyed 620 large sequoias (> 1 m diameter), 41

Changing fire regimes have important implications for biodiversity and challenge traditional conservation approaches that rely on historical conditions as proxies for ecological integrity. This historical-centric approach becomes increasingly tenuous under climate change, necessitating direct tests of environmental impacts on biodiversity. At the same time, widespread departures from historical fire regimes have limited the ability to sample diverse fire histories. We examined 2 areas in California's Sierra Nevada (USA) with active fire regimes to study the responses of bird, plant, and bat communities to a broad spectrum of temporal, spatial, and severity patterns of fire. Bird and plant species richness peaked in the first decade following fire. Species richness was highest with moderate burn severity for birds and with low burn severity for plants. Bat richness increased with longer mean fire-return intervals and was greatest in landscapes that included predominantly unburned areas or moderate to high burn severity patches. All taxa responded positively to pyrodiversity, with effect sizes varying with the metric used to assess variation in fire patterns. Our results suggest that restoring historical fire regimes would benefit biodiversity relative to most contemporary dry forests in California, but that total species richness would be highest under somewhat more frequent and varied severity fires than historical targets would indicate. Given the variable optima among taxa, managing for a range of complementary conditions that create local and landscape heterogeneity would best accommodate diverse flora and fauna and other forest conservation objectives.

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.

Wild and intentionally ignited fires are not new to North American landscapes or to the Indigenous cultures whose ancestral places encompass them. For millennia, Indigenous fire stewardship has been regionally and locally distributed across North American ecosystems. These practices reshaped fire regimes to provide safe living and foraging conditions and reduced wildfires and their emissions prior to Euro-American colonization. Euro-American colonization impacts initially included introduction of foreign diseases and widespread genocide, which broadly diminished the extent of Indigenous fire stewardship. Colonial policies and practices thereafter effectively altered vegetation and fuel patterns, fire regimes, and the once far-reaching effects of Indigenous fire stewardship. These influences have contributed to the current state of wildfires and their climate effects. Prior to colonization, Indigenous stewardship rights had been passed down through generations for millennia of active stewardship, and those rights were and continue to be protected under Indigenous law. However, US federal laws do not recognize these fundamental rights despite their legal standing in international law. Re-instating these rights would provide many advantages to addressing the modern wildfire and climate crisis. Re-instatement could be accelerated through linked land access, policy reform, and learning opportunities.

The active use of wildfire to meet forest management objectives is an important tool to increase the scale of forest restoration in dry, historically frequent-fire forests. While there are many benefits of reintroducing fire to these forests, the impact of wildland fire use policies in frequent-fire forests on aboveground carbon stocks has not yet been studied. In this study, we begin to fill this knowledge gap by assessing how fire frequency and severity affected aboveground carbon dynamics in two basins in the Sierra Nevada with a history of wildfire use over the past 20 to 50 years, compared to a nearby basin that has remained largely unburned. Across two spatial and temporal scales, live carbon stocks in wildfire use areas decreased by on average 22–48

An increasing wildfire problem in western North America has created a policy space for Indigenous fire stewardship (IFS) to mitigate wildfire. We compare how British Columbia and California have supported IFS—two jurisdictions with distinct ecosystems but similar histories of colonialism and its socio-ecological consequences. We examine how IFS is incorporated into each jurisdiction’s institutional framework, and the barriers to, and opportunities for implementation. Each jurisdiction’s approach to recognizing IFS is shaped by different constitutional frameworks and legal relationships with Indigenous Peoples. California recently developed policies and planning documents to support IFS and enable co-stewardship and contracting agreements similar to the policies of some federal agencies. However, barriers related to land tenure constrain IFS practitioners and inhibit meaningful implementation across broader landscapes. Compared to California, British Columbia has not shown as much openness to supporting independent IFS practitioners, but instead has begun a project to integrate aspects of IFS into the existing provincial wildfire service. While British Columbia has expressed interest in working toward a shared decision-making approach with First Nations, the present framework restricts IFS to Indigenous land tenures (which comprises only 0.4

Giant sequoias are the most massive individual trees on earth, and among the longest-lived. They also have a limited distribution, covering 10,000 ha across 70 distinct groves. Severe wildfires in 2020 and 2021 impacted 82

Despite widespread concern over increases in wildfire severity, the mechanisms underlying this trend remain unclear, hampering our ability to mitigate the severity of future fires. There is substantial uncertainty regarding the relative roles of extreme weather conditions, which are exacerbated by climate change, and forest management, in particular differences between private industrial timber companies and public land agencies. To investigate the effects of extreme weather and forest management on fire severity, we used light detection and ranging (LiDAR) data to characterize pre-fire forest structure across five large wildfires which burned 460,000 ha in the northern Sierra Nevada, California, USA. We found that the odds of high severity fire occurrence in these fires were 1.45 times higher on private industrial land than in publicly owned forests, an effect equivalent to a three standard deviation decrease in fuel moisture. Next, we quantified the relationships between key forest structure metrics and the probability of high severity fire, as well as how these relationships were modified by extreme weather. We found that dense, spatially homogeneous forests with high ladder fuels were more likely to burn at high severity. Extreme weather magnified the effect of density, suggesting that treatments which remove overstory trees are especially important in extreme conditions. Forests managed by private industry were more likely to be dense, spatially homogeneous, and contain high ladder fuel loads than publicly owned forests, offering a potential explanation for the increase in high-severity fire occurrence on private industrial land. Overall, these results illustrate the need for comprehensive forest management to mitigate fire severity in a warmer future.

Large and severe bushfires (wildfires) continue to burn and cause terrible damage in Australia and the US. Both countries have responded to this threat by implementing management strategies and policies with differing results. This paper examines solutions that each country is working to implement with the goal of achieving a more sustainable fire environment for ecosystems and people, focusing on prescribed burn programs. While there are similarities in responses, there are also differences that are highlighted. When comparing prescribed burn programs, there are major differences in program efficiency between the two countries. In Australia, you can plan and implement a burn in the same year, including incorporating specialist feedback, while in the US, this takes years on federal lands. While Australia and the US are similar in certain planning functions, Australia has streamlined the process to facilitate implementation which is a great advantage. Australia also has the Prescribed Burn Decision Support Tool to support fire managers in determining the risk of prescribed fire implementation and to document managers’ decisions. The US provides more comprehensive training for prescribed fires including producing lighting (ignition) and holding bosses and has better fire behaviour and modelling systems that can assist in planning prescribed fires. There are major differences between the two countries regarding implementing prescribed fires with Australia being more efficient. Australia has developed a streamlined process that facilitates action and a mechanism to ensure bushfire risk is reduced when identified. Sourcing and incorporating sound environmental guidance into prescribed burn plans in a reasonable timeframe is critical to the success and effectiveness of fire management programs. Allowing more Indigenous people to lead land stewardship and creating new workforces that focus on prescribed fire and ecosystem stewardship would be significant steps forward for both countries. While there are challenges, looking at what each county is doing successfully could enhance outcomes in both countries.

Restoring a low-intensity, frequent-fire regime in fire-prone forests offers a promising natural climate solution. Management interventions that include prescribed fire and/or mechanical treatments have effectively reduced fire hazards in the Western United States, yet concerns remain regarding their impact on forest carbon storage. This study used results from a long-term, replicated field experiment to assess the impacts of a restored disturbance regime on carbon dynamics in a Sierra Nevada, mixed conifer forest. The carbon consequences of the treatments were compared to a dynamic baseline of untreated controls (Control). After 19 years of wildfire mitigation, all treated stands stored less carbon than Control, but a larger proportion was sequestered in wildfire-resistant pools (i.e., large trees or fire-resistant species). Notably, only the most intensive treatment regime-thinning, mastication, and prescribed fire (Mech+Fire)-became a net carbon source by Year 20 (-60 MgC/ha). Annual average net ecosystem productivity (NEP) in Control and prescribed fire-only (Fire, 5.6-5.8 MgC/ha/year) more than doubled that of the mechanical treatments (2.0-2.1 MgC/ha/year). Moreover, temporal trends diverged. By the 3rd post-fire interval, the live vegetation carbon accumulation stalled in Control (0.9 ± 1.0 MgC/ha/year, mean ± SE) and accelerated in Fire (6.6 ± 1.2 MgC/ha/year). In contrast, surface fuel recovery was initially faster in Fire but slowed significantly by the 3rd interval, suggesting that the increased productivity under a frequent-fire regime does not necessarily lead to rapid surface fuel buildup once the regime is established. A simulated wildfire in Year 20 killed 6×-16× more live tree carbon in Control (46% mortality). Still, Control maintained the highest post-fire carbon storage. Despite the inherent carbon costs of wildfire mitigation, our 20-year study highlights management pathways that minimize the trade-off between wildfire hazard and carbon storage in Sierra Nevada mixed conifer forests.

While the reintroduction of recurring fire restores a key process in frequent-fire adapted forests, the ability to significantly shift the structure and composition of departed contemporary forests has not been clearly demonstrated. Our study utilized an extensive network of field plots across three short-interval successive fires occurring in the northern Sierra Nevada, California. We evaluated the influence of plot-level forest structure and composition, topography, and weather on fire severity in a third successive fire (i.e., second reburn). Additionally, we assessed the range of forest structural conditions that emerge following multiple low- to moderate-severity fires, whether these conditions were associated with fire severity in a third fire, and how they compare to historical estimates for these forests. Across plots that burned in multiple low- to moderate-severity fires, our findings indicated that post-fire outcomes in these systems are variable, resulting in a range of structural conditions following a first reburn (i.e., second fire). Areas with high levels of dead biomass burned at significantly higher severity in the third fire compared to those with higher shrub cover. Following a second fire, many plots exceeded historical estimates of stand structure metrics for yellow pine and mixed-conifer forests of the Sierra Nevada, particularly for coarse woody debris load, with some plots exceeding historical natural range of variation (NRV) estimates for live tree density. In plots with a history of varying fire severity in the initial and second fires, we found that snag basal area was associated with higher fire severity in the third fire. Low- to moderate-severity fire has the ability to restore ecosystem processes and reduce future fire severity in the long term, but our results suggest that it can also create fuel conditions that drive higher fire severity in successive fires. Our study demonstrates that vegetation and fuel conditions existing prior to the initial first-entry fire can largely influence post-reburn outcomes.

Forest die-backs linked to extreme droughts are expected to increase as the climate dries and warms. An example is the 2012-2016 hotter drought in California that induced widespread tree mortality in the Sierra Nevada, California. The sudden increase in snags (i.e., standing dead trees) raised immediate concerns about their impact on wildfire hazard and longer-term questions about their effect on ecosystem structure and function. We quantified the likely progression of snag fall and fuel succession following the recent extensive mortality event in the southern Sierra Nevada mixed conifer forest. Our results used data from a long-term demography study to project trends in surface fuel loads at three study sites in Yosemite, Sequoia and Kings Canyon national parks. In the short term (2017-2021), fine woody debris and litter + duff significantly increased across all three sites (>145 % and >55 %, respectively); coarse woody debris increased significantly at one site (48.6 %); and total fuel loads increased significantly at two of the three sites (38 % and 69 %). Snag longevity increased with size, with the relationship varying by species. Yellow pine was a notable outlier: size played a small role in influencing its fall rates. Overall, species-specific snag fall rates in the southern Sierra Nevada were 20 % to 40 % slower than previously reported. By 2040, projected median cumulative inputs of biomass from future snag fall range from 49.4 Mg ha(-1) to 136.1 Mg ha(-1)across our three sites, which exceeds the amounts currently present (47.17-89.97 Mg ha-1) and is well above estimates of historical coarse woody debris amounts in the Sierra Nevada (17.7 Mg ha -1). These results provide a robust empirical basis to refine the snag fall algorithm in vegetation simulation models. Options to manage the impact of extreme number of snags and their large surface combustible biomass include salvage operations and prescribed burning, with both methods having operational, financial, and legal limitations that need to be considered.

Large and downed woody fuels remaining behind a wildfire’s flame front tend to burn in a smoldering regime, producing large quantities of toxic gases and particulate emissions, which deteriorates air quality and compromises human health. Smoldering burning rates are affected by fuel type and size, the amount of oxygen reaching the surface, and heat losses to the surroundings. An external wind has the dual effects of bringing fresh oxidizer to the fuel surface and porous interior, while at the same time enhancing convective cooling. In this work, a series of experiments were conducted on single and adjacent poplar dowels to investigate the effect of fuel geometry and wind speed on smoldering of woody fuels, including its burning rate and combustion products. Dowels had variable thickness (19.1 and 25.4 mm), aspect ratios, and arrangement (number of dowels and spacing between them). Using measurement of mass loss, CO, and HC production as indicators of the smoldering intensity, the results indicate that the arrangement of smoldering objects significantly affects burning rates and emissions. Specifically, spacings of 1/8 and 1/4 of the dowel thickness enhanced the smoldering process. The smoldering intensity was also enhanced by increased external wind (ranging between 0.3 m/s and 1.5 m/s), but its effect was dependent upon the spacing between the dowels. The convective losses associated with the spacing were further investigated with a simplified computational model. The simulations show that the wind significantly increases convective losses from the smoldering surfaces, which in turn may offset the increase in smoldering intensity related to the higher oxygen flux at higher wind speeds.

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.

Over the last four decades, wildfires in forests of the continental western United States have significantly increased in both size and severity after more than a century of fire suppression and exclusion. Many of these forests historically experienced frequent fire and were fuel limited. To date, fuel reduction treatments have been small and too widely dispersed to have impacted this trend. Currently new land management plans are being developed on most of the 154 National Forests that will guide and support on the ground management practices for the next 15–20 years. During plan development, we recommend that Strategic Fire Zones (SFZs) be identified in large blocks (≥ 2,000 ha) of Federal forest lands, buffered (≥ 1–2.4 km) from the wildland-urban interface for the reintroduction of beneficial fire. In SFZs, lightning ignitions, as well as prescribed and cultural burns, would be used to reduce fuels and restore ecosystem services. Although such Zones have been successfully established in a limited number of western National Parks and Wilderness Areas, we identify extensive remote areas in the western US (8.3–12.7 million ha), most outside of wilderness (85–88