Wildfires are doing necessary work for western US forests that have evolved with fire. Here we define "good fire" as areas that burn in an ecologically beneficial way, with a severity that is analogous to their historical fire regimes prior to European settlement. We focus on low-moderate severity wildfire as this type of burning can promote regeneration, even in the context of a warming climate (Davis et al., 2023). We quantify the amount of forested area (i.e., deciduous, conifer, or mixed forest types) burned in low-moderate severity good fire and compare that to the amount of forested area that received prescribed burns (2010-2020). Our results show that 50% of the forested area burned by wildfires in the western US burned as low-moderate severity good fire. Moreover, the total area of this type of good fire was 3.3 million ha for wildfires (N = 18,126 events), compared with 2.7 million ha for forested prescribed burning (N = 25,761 events) over this same period. Lower severity wildfires are doing important work to promote healthy forests, 23% more by area than accomplished through prescribed burning. We also quantified high severity good fire in systems that historically experienced stand replacement, which represents an additional 10% of the forest area burned. Knowing that fires are likely going to increase in frequency and area with warming (Stephens et al., 2023), our key challenge in the future will be promoting good fire while still protecting lives and property. ### Competing Interest Statement The authors have declared no competing interest.

Abstract Wildfires devastated communities in Oregon and Washington in September 2020, burning almost as much forest west of the Cascade Mountain crest (“the westside”) in 2 weeks (~340,000 ha) as in the previous five decades (~406,00 ha). Unlike dry forests of the interior western United States, temperate rain forests of the Pacific Northwest have experienced limited recent fire activity, and debates surrounding what drove the 2020 fires, and management strategies to adapt to similar future events, necessitate a scientific evaluation of the fires. We evaluate five questions regarding the 2020 Labor Day fires: (1) How do the 2020 fires compare with historical fires? (2) How did the roles of weather and antecedent climate differ geographically and from the recent past (1979–2019)? (3) How do fire size and severity compare to other recent fires (1985–2019), and how did forest management and prefire forest structure influence burn severity? (4) What impact will these fires have on westside landscapes? and (5) How can we adapt to similar fires in the future? Although 5 of the 2020 fires were much larger than any others in the recent past and burned ~10 times the area in high‐severity patches >10,000 ha, the 2020 fires were remarkably consistent with historical fires. Reports from the early 1900s, along with paleo‐ and dendro‐ecological records, indicate similar and potentially even larger wildfires over the past millennium, many of which shared similar seasonality (late August/early September), weather conditions, and even geographic locations. Consistent with the largest historical fires, strong east winds and anomalously dry conditions drove the rapid spread of high‐severity wildfire in 2020. We found minimal difference in burn severity among stand structural types related to previous management in the 2020 fires. Adaptation strategies for similar fires in the future could benefit by focusing on ignition prevention, fire suppression, and community preparedness, as opposed to fuel treatments that are unlikely to mitigate fire severity during extreme weather. While scientific uncertainties remain regarding the nature of infrequent, high‐severity fires in westside forests, particularly under climate change, adapting to their future occurrence will require different strategies than those in interior, dry forests.

This report assesses how carbon stocks at regional scales and in individual national forests are affected by factors such as timber harvesting, natural disturbances, climate variability, increasing atmospheric carbon dioxide concentrations, and nitrogen deposition.Previous baseline assessments of carbon stocks (https://www.fs.fed.us/managing-land/sc/carbon)evaluated observed trends based on forest inventory data but were limited in ability to reveal detailed causes of these trends.The expanded assessments reported here are based on an extensive disturbance and climate history for each national forest, and two forest carbon models, to estimate the relative impacts of disturbance (e.g., fires, harvests, insect outbreaks, disease) and nondisturbance factors (climate, carbon dioxide concentration, nitrogen deposition).Results are summarized for each region of the National Forest System in the main document.A set of regional appendices to this report provides more detailed information about individual national forests within each region.Results are highly variable across the United States.Generally, carbon stocks are increasing in forests of the eastern United States as these forests continue to recover and grow older after higher historical harvesting rates and periods of nonforest land use.In contrast, carbon stocks in forests of the western United States may be either increasing or decreasing, depending on recent effects of natural disturbances and climate change.The information supports national forest units in assessing carbon stocks, quantifying carbon outcomes of broad forest management strategies and planning, and meeting carbon assessment requirements of the 2012 Planning Rule and directives.Results of these expanded assessments will provide context for project-level decisions, separated from the effects of factors that are beyond land managers' control.

Management of forest carbon stocks on public lands is critical to maintaining or enhancing carbon dioxide removal from the atmosphere. Acknowledging this, an array of federal regulations and policies have emerged that requires US National Forests to report baseline carbon stocks and changes due to disturbance and management and assess how management activities and forest plans affect carbon stocks. To address these requirements with the best-available science, we compiled empirical and remotely sensed data covering the National Forests (one fifth of the area of US forest land) and analyzed this information using a carbon modeling framework. We demonstrate how integration of various data and models provides a comprehensive evaluation of key drivers of observed carbon trends, for individual National Forests. The models in this framework complement each other with different strengths: the Carbon Calculation Tool uses inventory data to report baseline carbon stocks; the Forest Carbon Management Framework integrates inventory data, disturbance histories, and growth and yield trajectories to report relative effects of disturbances on carbon stocks; and the Integrated Terrestrial Ecosystem Carbon Model incorporates disturbance, climate, and atmospheric data to determine their relative impacts on forest carbon accumulation and loss. We report results for several National Forests across the USA and compare their carbon dynamics. Results show that recent disturbances are causing some forests to transition from carbon sinks to sources, particularly in the West. Meanwhile, elevated atmospheric carbon dioxide and nitrogen deposition are consistently increasing carbon stocks, partially offsetting declines due to disturbances and aging. Climate variability introduces concomitant interannual variability in net carbon uptake or release. Targeting forest disturbance and post-disturbance regrowth is critical to management objectives that involve maintaining or enhancing future carbon sequestration.

Abstract Root diseases are known to suppress forest regeneration and reduce growth rates, and they may become more common as susceptible tree species become maladapted in parts of their historic ranges due to climate change. However, current ecosystem models do not track the effects of root disease on net productivity, and there has been little research on how the dynamics of root disease affect carbon (C) storage and productivity across infected landscapes. We compared the effects of root disease against the effects of other types of forest disturbance across six national forest landscapes, 1990–2011. This was enabled by a monitoring tool called the Forest Carbon Management Framework (ForCaMF), which makes use of ground inventory data, an empirical growth model, and time series of Landsat satellite imagery. Despite several large fires that burned across these landscapes during the study period, retrospective ForCaMF analysis showed that fire and root disease had approximately equal impacts on C storage. Relative to C accumulation that would have occurred in their absence, fires from 1990 to 2011 were estimated to reduce regionwide C storage by 215.3 ± 19.1 g/m2 C, while disease in the same period was estimated to reduce storage by 211.4 ± 59.9 g/m2 C. Harvest (75.5 ± 13.5 g/m2 C) and bark beetle activity (14.8 ± 12.5 g/m2 C) were less important. While long‐term disturbance processes such as root disease have generally been ignored by tools informing management of forest C storage, the recent history of several national forests suggests that such disturbances can be just as important to the C cycle as more conspicuous events like wildfires.

Research scientists collaborated with federal land managers of two national parks and two national forests to conduct a climate change vulnerability assessment and to identify adaptation strategies for a transportation network covering 28,900 km of roads and trails in north-central Washington, U.S.A. The assessment employed observations of sensitivity and response to climatic variability, downscaled climate projections, literature reviews, current management policies and practices, expert knowledge, and stakeholder engagement. Primary pathways for climate impacts focused on projected increases in extreme high flows and flooding, elevated winter soil moisture and landslide hazards, and loss of snowpack. The biggest impacts to roads and trails are expected from temperature-induced changes in hydrologic regimes that enhance autumn flooding and reduce spring snowpack. Projected higher winter soil moisture caused by changes in seasonal precipitation and snow accumulation could reduce slope stability. Earlier snowmelt may lengthen the snow-free season for visitor use and agency operations. Infrastructure age, design, maintenance, location, use, and limited redundancy along with funding policies and management, influence the sensitivities of the transportation system. Vulnerabilities were identified based on when and where these sensitivities to changes in climate may emerge. Adaptation strategies and tactics identified to address these vulnerabilities included: upgrading stream crossing and drainage design, changing use and maintenance, relocating or closing roads and trails, modifying funding policies, and expanding pubic engagement. Many adaptation options are “no regrets” approaches to changes in climate projected for the 2040s and 2080s that can be applied to other resource sectors and mountainous regions.

Disturbance is a key driver of carbon (C) dynamics in forests. Insect epidemics, wildfires, and timber harvest have greatly affected North American C budgets in the last century. Research is needed to understand how forest C dynamics (source duration and recovery time) following disturbance vary as a function of disturbance type, severity, forest type, and initial C stocks. We used the Forest Vegetation Simulator (FVS) to simulate total C stocks (excluding soil) for 100 years following three types of disturbance (fire, harvest, and insects) with four levels of severity. We initiated the model using empirical data from a large representative sample of forest conditions on the national forest ownership in the Rocky Mountain region (Forest Inventory and Analysis data). Unlike analyses based on stand age, an ambiguous quantity with respect to disturbance history, our approach enables explicit consideration of disturbance type and severity, as well as pre-disturbance forest C. On average, stands became a C sink after fire in 5, 6, 14, and 23 years for low to high-severity fire. Pre-fire C stocks were reached 25-55 years later. Following bark beetle epidemics, on average stands continued to be a C source for 10 years longer than fire and up to 40 years longer in some cases, but pre-disturbance C stocks were reached in a similar amount of time. C stocks following harvest showed the largest initial decline, but on average stands became a sink sooner at 1, 5, 15, and 12 years post-harvest for low to high-severity harvests. Differences in C dynamics based on disturbance type and severity, initial conditions, and forest type demonstrate the importance of considering this variability when modeling forest C dynamics. The regionally averaged models of C response quantified in this study can be combined with remotely sensed data on disturbance type and severity and used with C accounting approaches that rely on growth and yield or state and transition models. Published by Elsevier B.V.

The effects of climate change on insect outbreaks, wildfire, invasive species, and pathogens in forest ecosystems will greatly exceed the effects of warmer temperature on gradual changes in forest processes. Increased frequency and extent of these disturbances will lead to rapid changes in vegetation age and structure, plant species composition, productivity, carbon storage, and water yield. Insect outbreaks are the most pervasive forest disturbance in the United States, and rapid spread of bark beetles in the western United States has been attributed to a recent increase in temperature. Wildfire area burned has increased in recent decades, although frequency and severity have not changed, and is expected to greatly increase by 2050 (at least twice as much area burned annually in the West). More frequent occurrence of fire and insects will create landscapes in which regeneration of vegetation will occur in a warmer environment, possibly with new species assemblages, younger age classes, and altered forest structure. Increased fire and insects may in turn cause more erosion and landslides. Invasive plant species are already a component of all forest ecosystems, and a warmer climate will likely facilitate the spread of current and new invasives, particularly annuals that compete effectively in an environment with higher temperature and frequent disturbance. The interaction of multiple disturbances and stressors, or stress complexes, has the potential to alter the structure and function of forest ecosystems, especially when considered in the context of human land-use change. Occurring across large landscapes over time, these stress complexes will have mostly negative effects on ecosystem services, requiring costly responses to mitigate them and active management of forest ecosystems to enhance resilience.

Accurate estimates of foliar biomass (FB) are important for quantifying carbon storage in forest ecosystems, but FB is not always reported in regional or national inventories. Foliar biomass also drives key ecological processes in ecosystem models. Published algorithms for estimating FB in conifer species of the Pacific Northwest can yield signifi cantly different results, but have not been rigorously compared for species other than Douglas-fi r (Pseudotsuga menziesii (Mirb.) Franco). We compared fi ve algorithms for estimating FB for seven common coniferous species in the Pacifi c Northwest. Algorithms based on diameter at breast height (DBH), or on DBH and height, consistently yield higher estimates of FB than algorithms based on sapwood area. At the tree level, differences between algorithms increased with increasing DBH for all species, but their order and magnitude differed by species. At the stand level, differences among algorithms were muted by the mix of species and diameter classes that contributed to total FB of stands of different seral stages and species composition. Signifi cant differences among estimates of FB from different algorithms show the need for consistent methods for estimating FB for carbon accounting, tests of the sensitivity of ecosystem models to these differences, and more field observations to compare algorithms.

The U.S. Forest Service (USFS) and National Park Service (NPS) have highlighted climate change as an agency priority and issued direction to administrative units for responding to climate change. In response, the USFS and NPS initiated the North Cascadia Adaptation Partnership (NCAP) in 2010. The goals of the NCAP were to build an inclusive partnership, increase climate change awareness, assess vulnerability, and develop science-based adaptation strategies to reduce these vulnerabilities. The NCAP expanded previous science-management partnerships on federal lands to a larger, more ecologically and geographically complex region and extended the approach to a broader range of stakeholders. The NCAP focused on two national forests and two national parks in the North Cascades Range, Washington (USA), a total land area of 2.4 million ha, making it the largest science-management partnership of its kind. The NCAP assessed climate change vulnerability for four resource sectors (hydrology and access; vegetation and ecological disturbance; wildlife; and fish) and developed adaptation options for each sector. The NCAP process has proven to be a successful approach for implementing climate change adaptation across a region and can be emulated by other land management agencies in North America and beyond.

The U.S. Forest Service (USFS) and National Park Service (NPS) have highlighted climate change as an agency priority and issued direction to administrative units for responding to climate change. In response, the USFS and NPS initiated the North Cascadia Adaptation Partnership (NCAP) in 2010. The goals of the NCAP were to build an inclusive partnership, increase climate change awareness, assess vulnerability, and develop science-based adaptation strategies to reduce these vulnerabilities. The NCAP expanded previous science-management partnerships on federal lands to a larger, more ecologically and geographically complex region and extended the approach to a broader range of stakeholders. The NCAP focused on two national forests and two national parks in the North Cascades Range, Washington (USA), a total land area of 2.4 million ha, making it the largest science-management partnership of its kind. The NCAP assessed climate change vulnerability for four resource sectors (hydrology and access; vegetation and ecological disturbance; wildlife; and fish) and developed adaptation options for each sector. The NCAP process has proven to be a successful approach for implementing climate change adaptation across a region and can be emulated by other land management agencies in North America and beyond.

•We fit empirical models of forest carbon dynamics at two spatial scales.•Theoretical models of carbon fit the data better at the finer scale of forest types.•Potential carbon storage and uptake varied greatly among forest types.•Models for some forest types demonstrated an optimum age for carbon management.•Models for other forest types suggest a tradeoff between carbon storage and uptake.

During the 21st century, climate-driven changes in fire regimes will be a key agent of change in forests of the U.S. Pacific Northwest (PNW). Understanding the response of forest carbon (C) dynamics to increases in fire will help quantify limits on the contribution of forest C storage to climate change mitigation and prioritize forest types for monitoring C storage and fire management to minimize C loss. In this study, we used projections of 21st century area burned to explore the consequences of changes in fire regimes on C dynamics in forests of Washington State. We used a novel empirical approach that takes advantage of chronosequences of C pools and fluxes and statistical properties of fire regimes to explore the effects of shifting age class distributions on C dynamics. Forests of the western Cascades are projected to be more sensitive to climate-driven increases in fire, and thus projected changes in C dynamics, than forests of the eastern Cascades. In the western Cascades, mean live biomass C is projected to decrease by 24-37%, and coarse woody debris (CWD) biomass C by 15-25% for the 2040s. Loss of live biomass C is projected to be lower for forests of the eastern Cascades and Okanogan Highlands (17-26%), and CWD biomass is projected to increase. Landscape mean net primary productivity is projected to increase in wet low-elevation forests of the western Cascades, but decrease elsewhere. These forests, and moist forests of the Okanogan Highlands, are projected to have the greatest percentage increases in consumption of live biomass. Percentage increases in consumption of CWD biomass are greater than 50% for all regions and up to four times greater than increases in consumption of live biomass. Carbon sequestration in PNW forests will be highly sensitive to increases in fire, suggesting a cautious approach to managing these forests for C sequestration to mitigate anthropogenic CO2 emissions.

Alaskan forests cover one-third of the state’s 52 million ha of land (Parson et al. 2001), and are regionally and globally significant. Ninety percent of Alaskan forests are classified as boreal, representing 4 percent of the world’s boreal forests, and are located throughout interior and south-central Alaska (fig. A1-1). The remaining 10 percent of Alaskan forests are classified as coastal-temperate, representing 19 percent of the world’s coastal-temperate forests (National Synthesis Assessment Team 2003), and are located in southeast Alaska (fig. A1-1). Regional changes in the disturbance regimes of Alaskan forests (Wolken et al. 2011) directly affect the global climate system through greenhouse gas emissions (Tan et al. 2007) and altered surface energy budgets (Chapin et al. 2000, Randerson et al. 2006). Climaterelated changes in Alaskan forests also have regional societal consequences, because some forests are in proximity to communities (both urban and rural) and provide a diversity of ecosystem services (Reid et al. 2005, Wolken et al. 2011).