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Edited by Alan Organschi, Yale University, New Haven, CT, accepted by the editorial board on October 15, 2021 (received on September 10, 2020)

Natural carbon sinks help alleviate climate change, but climate risks (such as increased wildfires) threaten the ability of forests to store carbon. California recently set ambitious forest management goals to reduce these risks. However, management may result in carbon loss, as wood residues are often burned or rotted. This study uses a systematic approach to assess the climate change mitigation potential and wildfire results in forest management scenarios and several wood products. We have found that the innovative use of wood residues helps to reduce wildfire hazards and maximize carbon efficiency. Long-life products that replace carbon-intensive alternatives have the greatest benefits, including wood-structured products. Our research results suggest a low-cost way to reduce carbon emissions and support climate adaptation in temperate forests.

Responsible management of temperate forests can address the main challenges posed by climate change by sequestering carbon, producing low-carbon products, and mitigating climate risks. Thinning forests and reducing fuel can reduce climate-related risks, such as catastrophic wildfires. However, these treatments are often costly, partly because of the low demand for low-value wood "residues". Where it is processed, this low-value wood is often burned or left to rot, thereby releasing carbon. In this study, we proved that the innovative use of low-value wood can increase potential income and carbon benefits, and can support California's economic and carbon-friendly forest management results. As the demand for wood residues increases, forest health-oriented thinning can produce up to 7.3 million (M) tons of dried forest residues each year, eight times the current level. Consider the impact of management, wildfires, carbon storage in products, and substitution of fossil carbon-intensive substitutes over a 40-y period. We have found that products with long-lasting carbon storage bring the greatest benefits, as well as products that reduce emissions from sectors that are difficult to decarbonize, such as industrial heating. At the same time, treatment can reduce 4.9 M hectares (12.1 M ac) of wildfire hazards, of which a quarter may experience stand replacement effects without treatment. Our results show that innovative wood use can support widespread fire hazard mitigation and reduce California’s net carbon dioxide emissions.

Climate change poses great challenges to the management of temperate forests, especially in California (1, 2). Due to massive timber logging and fire elimination in the 20th century, California forests are younger, denser, and more uniform than historical conditions (3, 4). These changes make California's forests vulnerable to large-scale disturbances such as drought, insects, diseases, and wildfires. Like other temperate forests, California forests face the risk of increased fire severity and frequency caused by climate change (5⇓ –7). Extreme wildfires with large-scale tree replacement effects have become more common and pose an existential threat to forest ecosystems and their ability to sequester carbon, especially on federal land (1, 8⇓ ⇓ ⇓ ⇓ –13).

At the same time, recent work has highlighted the potential of forests to help achieve near- and long-term climate goals (14⇓ ⇓ –17). Nevertheless, there are still huge uncertainties in adjusting forest treatment and climate targets. Estimates of how forest treatment will affect net carbon emissions from temperate forests vary widely (10, 18, 19). It is generally believed that more efficient use of harvested wood can improve the carbon balance of management, but different wood products vary greatly due to production emissions, replacement benefits, and end-of-life emissions (20⇓ ⇓ –23).

In response to the increasing risk of wildfires, the California Forest Climate Action Group and the State of California have set a goal to reduce wildfire hazards in public and private forests by 1 million (M) acres (0.4 million hectares) each year (24). These plans involve fuel reduction processing, timber harvesting, and expanded use of harvested timber products. Active management—such as prescribed fires and mechanical thinning—can mitigate the effects of wildfires and provide many common benefits (25, 26). However, even where larger harvested trees (sawn timber) can be sold, these treatments are usually expensive. In addition, the effectiveness of fuel treatment mainly depends on the removal of small trees, which constitute most of the "ladder" fuel in the forest (27). The sale of small trees and residues (for example, as biomass chips or pulp logs) can offset some of the processing costs, but current market demand is limited. As a result, large amounts of low-value wood rot or burn after treatment, releasing stored carbon into the atmosphere. We suggest that another destiny of this wood may expand the scope of processing and the flexibility to manage multiple goals.

In this study, we investigated how a strong forest residue market affects the scale and impact of California forest processing. First, we used Forest Inventory and Analysis (FIA) data to model forest health-oriented thinning treatments (materials and methods) and potential wildfire outcomes on public and private forest lands in California (Figure 1). We consider three management options: 1) Business as usual, limited management (low BAU), 2) Business as usual, expanded management (high BAU), and 3) Innovative wood products (IWP). In the IWP scenario, the potential revenue generated by IWP supports increased management of any BAU. Second, we use attributable life cycle assessment to examine the carbon benefits of several ways of logging wood, including production emissions, carbon storage, replacement of carbon-intensive alternatives, and end-of-life emissions. In Figure 2A, we show the net carbon balance of California's expanded forest management and wood products market.

The low BAU scenario represents a low-management future, similar to but different from the current practice in California (see discussion). In low BAU, we assume that there is no thinning of public and private (ie family owned) forests. On the land owned by the company, we modeled the thinning of 0.8 M hectares (2 M ac), where the net income is> 2,500 USD/ha, and there is no income from forest residues. Under this management plan, an average of 1.6 million ODT (4.1 million cubic meters) of sawn timber can be harvested annually for the next 40 years. In contrast, California has produced an average of 3.8 million cubic meters of sawn timber annually over the past ten years (13). The low BAU scenario is characterized by high fire risks and high carbon storage rates in untreated forests. Taking into account the occurrence and impact of wildfires through random simulations, in the next 40 years, this woodland will be isolated by 0.89 ± 0.02 tC·ha−1 · y−1 (± means 95% CI from Monte Carlo simulation). This value is close to previous estimates for temperate coniferous forests in the western United States (for example, Reference 28). The direct emissions from the fire were 0.40 ± 0.01 tC · ha-1 · y-1, but the attenuation after the fire increased the total emissions by 0.17 ± 0.007 tC · ha-1 · y-1 over a 40-year period (SI appendix, figure S4). We proposed an alternative formula for BAU in SI, with similar results.

In the high BAU scenario, we consider the impact of maximizing management scale without subsidies (ie, net income is positive) and without income from forest residues. In this case, 3.3 M unique hectares can be managed on public and private land for more than 40 years (8.1 M ac). The harvested lumber flow is almost three times the low BAU, 5.12 M ODT (13 M cubic meters) per year, which is comparable to historical production (13). Most of this wood comes from trees with a breast height (DBH) of less than 53 cm in diameter (Figure 3). In addition, in this scenario, a forest residue of 4.4 M ODT can be technically obtained. However, if the price of forest residues is not enough to cover the cost of removal and transportation, the wood is likely to rot or burn in the forest. In the presence of subsidies, forest residues may be sent to biological power generation facilities. Compared with low BAU, increased management led to a reduction in emissions related to wildfires: direct emissions from wildfires were 0.32 ± 0.01 tC · ha−1 · y−1, and attenuation increased by 0.12 ± 0.005 tC · ha−1 · y−1 Over 40 years old. Although the high BAU scenario reduces more hectares of wildfire hazards than the low BAU scenario, it poses two key challenges: 1) Stand management with small trees is costly without subsidies, and 2) low value The burning or decay of wood conflicts with climate goals.

In the “IWP” scenario, we studied how the innovative use of forest residues can be managed to achieve better economic and carbon results. We evaluated several commercially and technologically mature products and estimated that the market size is equivalent to> 1 M ODT wood per year in California (29). We estimate that low-carbon fuel and oriented strand board (OSB) production can prove that the price of forest residues delivered exceeds $100 per ODT delivery (SI appendix, Figure S6), similar to other technical and economic analyses (30⇓ –32). In IWP , We assume that the delivery price is as high as $100/ODT, which supports management beyond the economically possible high BAU. However, the prices of most forest residues are lower (Figure 3).

With this additional income, 4.9 million hectares (12.1 million acres) of forest can be managed in the next 40 years without subsidies. Some of these areas have been treated more than once, so on average, about 0.2M hectares (about 0.5M ac) of forest can be treated each year. Most of this treatment was technically feasible in the first 20 years (SI appendix, Figure S1). We estimate that at this price, thinning can produce 7.3 M ODT of forest residues and 14.8 M m3 (5.7 M ODT) of sawn timber per year within 40 years. This will represent a nearly eight-fold increase in the current supply of forest residues and a four-fold increase in sawn timber production (13, 33). The increase in residue prices does not significantly increase the harvest of sawn timber: although the supply of residues has increased sharply by 62% to a price of $100/ODT, the supply of sawn timber has only increased in the smallest saleable diameter class (Figure 3). Even if the residue price is US$200/ODT, the supply of sawn timber will only increase by 18% compared to the residue-free price. Calculated at $100/ODT, a small portion of forest residues (40 million ODT, 19%) come from small trees (10 to 20 cm DBH). Most residues are by-products of whole tree harvests from large trees and non-commercial tree species.

In IWP, 1.3 M hectares (3.1 M ac) of forest land can be treated. These forest lands may experience stand replacement wildfire effects (> 95% mortality) without treatment, and the potential base area mortality of these stands is reduced by an average of 28 ± 1% (SI appendix), Figure S3). Of all treated areas, 47% occurred in landscapes designated by CalFire as high priority (area 4 and 5) to reduce the risk of wildfires to ecosystem services (Figure 4). The annual average combustion emissions of wildfires are 0.27 ± 0.01 tC · ha-1 · y-1, and the attenuation after the fire has increased by 0.07 ± 0.005 tC · ha-1 · y-1. This means that fire-driven emissions are reduced by 39% compared to low BAU and 19% less than high BAU.

For the current mix of end-use California sawmills (13), we estimate the net replacement factor per tC harvest to be 0.75 tC gains (tC/tC), where “net” is the sum of production emissions and carbon-intensive substitute substitution. The value is slightly higher than the Canadian estimate (34) because most of California's wood products are used in construction. However, it is lower than similar studies, partly because it excludes emissions from building operations (34). Wood, which replaces steel and concrete, has the greatest carbon benefit of any use studied here. For this reason, we considered the impact of transferring all the extra (relative to low BAU) sawnwood produced in IWP to multi-family and multi-purpose buildings. This "IWP Housing" program represents a future where affordable, medium-density housing is prioritized. In IWP Housing, due to the increase in steel and concrete substitutes, the net substitution factor is 1.75 tC/tC. Although we are optimistic about the timber use assumption, this value is similar to the net substitution factor (34, 35) in other regions. When we include the end of the life cycle (simulated as 40 years), we find that the weighted net carbon benefit of all sawn timber products is 1.35 tC/tC. In the IWP Housing scheme, it is 2.35 tC/tC.

For forest residue products, carbon benefits vary widely (Figure 2B). Bioenergy is currently the most common use of forest residues in California. Compared with more innovative technologies, it has low carbon benefits (0.11 tC/tC), mainly because there is no carbon dioxide storage and replacement of relatively clean California Grid electricity. On the contrary, technologies with most carbon storage have the greatest benefits. Bioenergy with carbon capture and storage (CCS) function has relatively high carbon efficiency (0.81 tC/tC) because most of the CO2 emitted is captured and stored. Hydrogen and CCS, GluLam and OSB have the highest carbon efficiency (1.18 to 1.65 tC/tC) because of the high substitution efficiency and carbon storage in wood products or through CCS. In addition, these three products will reduce emissions from industries that are "difficult to reduce emissions" such as cement and industrial heating. Although all these residue-based products are technically feasible, they rely on different forest residue components. For example, OSB and GluLam require small diameter (pulp) logs, while hydrogen production can use mixed biomass including leaves and bark. In IWP, we only show an equivalent combination of products that exceed the 0.5 tC/tC carbon revenue threshold and can use mixed biomass (fuel) or pulp logs (OSB and GluLam) on a commercial scale (Figure 2B).

We estimate the net climate impact of management on the entire economy by combining carbon changes in the forest with the carbon benefits harvested (Figure 2A). Therefore, the net carbon balance is a combination of sequestration, storage, emission, and emission avoidance. In all three scenarios, the forestry sector is a net carbon sink. In low BAU and high BAU, we find similar net carbon benefits of 10.2 M and 9.5 M tCO2e per year, respectively, over 40 years. The IWP scenario has greater carbon benefits, 16.6 M tCO2e per year. In all three cases, traditional sawn timber products play an important role in supporting positive net carbon balance management. However, the IWP scenario shows that the innovative use of forest residues and sawn timber brings clear benefits. In terms of climate goals, switching from a low BAU to an IWP can bring net climate benefits of 6.4 million tons of carbon dioxide equivalent each year, mainly due to innovative use of forest residues. IWP Housing generates higher net income, 27.1 M tCO2e per year, or 16.9 M tCO2e per year than Low BAU, which is mainly due to the use of sawn timber instead of steel and concrete. The IWP Housing Scenario has the most obvious direct benefit on the time scale related to California's recent climate target (2045) (SI Appendix, Figure S5). In short, innovative wood use may be critical to achieving California's dual goal of reducing wildfire hazards and carbon dioxide emissions.

Our research results show that effective use of wood can play an important role in building California's forests into resilient long-term carbon sinks. We found that IWP would increase the management scale and carbon benefits of forest residues, which would otherwise rot or burn. These products can simultaneously advance California's existing forest management and climate goals. Below, we will review our results in the context of forest management, innovative wood utilization technologies and climate policies. We also emphasize that although this research integrates several key elements of a complex system, it has important limitations. This analytical framework can be used as a template and starting point to further study the complex relationship between wood use and management in high-disturbance forests. In addition, large-scale forest treatments such as those discussed here may have unforeseen consequences. Investigating the ecological results not analyzed in this study, such as the comparative impact of wildfires and expanded forest management on ecosystem services such as biodiversity, will be a fruitful research area to expand the framework.

In this study, we emphasized thinning and surface fuel treatment consistent with California guidelines (materials and methods). These treatments promote multiple ecosystem benefits and restore historical forest structure by reducing forest density and retaining the largest and most fire-resistant trees (3, 4, 36). Forest management plans are bound to depend on the environment and will deviate to varying degrees from the plans we assume here. Future management plans may also require new methods to effectively deal with climatic conditions, which is unprecedented in history (37, 38). In order to achieve multiple social and ecological goals under constantly changing environmental conditions, management planning is best viewed as an active and adaptable process (37, 38). However, we have found that in most woodlands in California, carbon-beneficial treatment is not feasible if wood products are not included. Innovative use of wood may be required to ensure that treatments motivated by wildfires produce climate benefits. This strategy is complementary to other strategies that emphasize reforestation or long-term retention of larger trees to help achieve climate goals (14, 15, 17).

Innovative wood use has two main value propositions in California: increasing income from logging wood and improving the carbon balance of forest management. Two types of promising products have emerged in recent reviews: low-carbon and negative-carbon fuels and engineered wood products (eg, bulk wood) (29, 32, 39). Thanks to the support of state and federal fuel policies, including California's low-carbon fuel standards, low-carbon fuels derived from woody biomass have shown economic prospects. Several large-scale transportation fuel projects that use California wood and biomass but are located in neighboring states are scheduled to start production in 2022. If more facilities are set up in California, low-carbon and negative-carbon fuels can promote additional regional economic development benefits.

Bulk timber products such as cross-laminated timber (CLT) and GluLam are not common in the United States, but have been widely adopted in the European market. Other engineered wood products, such as OSB, can be made from pulp logs and are widely used, but are not produced in the western United States (40). Certain engineered wood products may have relatively high substitution benefits (for example, I-beams produced with oriented strand board) or higher carbon storage densities (for example, CLT). These products will usually replace steel and concrete and will support our IWP Housing program, which has the largest net carbon benefit of any program. The recent inclusion of CLT in California building codes may encourage widespread adoption and production. However, further research should verify the applicability of small-diameter wood, low-quality wood, and California tree species as raw materials for these products.

In these situations, climate policy can play a key role. For example, California's low-carbon fuel standards provided financial incentives ranging from US$160 to US$192 per ton of carbon dioxide between 2018 and 2019, and were recently extended to 2030. Revenues from such carbon payment schemes make the financial feasibility of innovative wood use possible. Similarly, the state has recently adopted other performance-based climate policies, such as buying clean California, which can promote the use of wood construction products. Investment mechanisms, such as new climate catalyst funds, can also play an important role in paying upfront costs, although these funds need to grow to support higher capital cost facilities (such as OSB). These facilities may also require long-term supply contracts to ensure that capital costs are recovered. Finally, workforce development initiatives can support the rapid expansion of forest processing. Such policies may help achieve the central goal of the state's forest carbon plan: to firmly build California's forests into more resilient and reliable long-term carbon sinks (24).

In our scenario analysis, we recommend low BAU and high BAU as baseline scenarios. Although neither of these situations are perfect representations of reality, we expect them to encompass a range of possible futures, independent of the use of innovative wood. We use low BAU as the basis for comparison because it is the closest to the state of California's forest management, where the active management rate under corporate ownership is high, while public and household-owned forests are much less. We also considered an alternative BAU, which includes a more representative combination of public and private management, but does not materially change the results presented here (SI appendix, SI methodology and results). Or, increased interest in reducing wildfire hazards and related policy changes may produce a future more similar to high BAU.

In this study, we used an attributable life cycle analysis (LCA) method, which includes the physical flow into and out of a given system. However, wood harvested in California is unlikely to be used exclusively in California, and the consequences of the influx of wood products into the global market may have unforeseen consequences. Localized policies, such as California’s green procurement strategy, or policies that support the use of wood products to replace carbon-intensive alternatives, may promote greater carbon gains from wood harvested in the state, rather than replacing wood found elsewhere. Products. Although the LCA values ​​used here represent current technology, the carbon benefits of these products may increase or decrease during the modeling period. As the mix of alternative products and carbon intensity changes, the benefits of alternatives may change significantly.

The prediction of the occurrence and outcome of future wildfires is inherently uncertain (41). In our simulation, the growth and wildfire emissions are very different, depending on the burning forest plots and the burning time. This effect is most pronounced in low BAU, where a large amount of carbon is stored in untreated forests, but the stability of this carbon is highly uncertain. The parameterized attenuation and combustion values ​​also have a great influence on wildfire emissions. The parameters we use exclude non-CO2 climate forcing factors and may underestimate actual wildfire emissions, thereby limiting the carbon benefits of treatment. In addition, we simulate forest growth in the Forest Vegetation Simulator (FVS), which is known to underestimate mortality and thus overestimate growth. We do not simulate the effects of non-fire climate effects, such as increased incidence of drought, insects, disease, or carbon dioxide fertilization. We also do not consider the continuous change of vegetation (for example, from woodland to shrubland). In general, we may have overestimated the stability of forest carbon and underestimated the carbon benefits of forest treatment (1).

The complete documentation of the method used to produce this work can be found in the SI appendix, SI Method and Results. Here, we briefly introduce these methods.

This analysis applies the FIA ​​BioSum modeling framework (42) to understand the management results of California forest land. We rely on data collected from 5,404 field-sampled FIA plots between 2005 and 2016, which represent approximately 13.4 M hectares (33 M ac) of California forest land. We refined this woodland sample to limit our analysis to forests classified as woodland and one of the four common California forest types: mixed conifers, Douglas fir, true firs, and ponderosa pine. We considered three types of ownership that account for almost all forest land in California: corporate, non-corporate private ("household"), and national forest system ("public"). We do not include federally reserved land management and land managed by state and local governments.

We use FVS and related fire and fuel expansion (FFE) to model forest growth, management, and potential fire outcomes over 40 years. For each FIA plot, we simulated five forest treatments (SI appendix, Table S1), designed to represent forest restoration incentive management compatible with the provisions of the Sierra Nevada Forest Program. These treatments differ in the method of thinning (from below or across the diameter class), the maximum size of trees that can be harvested, and the treatment of surface fuel. Each treatment can reduce the substrate area by up to 33%. Use a mechanical harvester (DBH <53 cm) or a hand-cut whole tree harvesting system for thinning. After dilution, the surface fuel is treated with prescribed fire or lop and scatter. We also simulated an alternative "growth only" to represent the untreated forest. Subsequently, we used BioSum to evaluate the cost and revenue of each treatment. The value of sawn timber is based on the California Equilibrium Commission's ratio. In the IWP scenario, the maximum value of residue deliverable is $100/ODT, although most can be delivered at a lower price (SI appendix, SI method and results). Residues include small trees (DBH <20 cm), tops of large trees, branches, and non-commercial tree species of all sizes. We have performed multi-criteria optimization in BioSum to select a treatment method that has a positive net income, reduces the risk of fire, and maximizes the carbon of living trees by the end of 40 years. Based on this optimization, BioSum calculates the amount of sawn timber and forest residues that can be transported to the existing network of processing facilities.

In order to understand the carbon balance in the managed forest, we randomly simulated wildfires based on the static potential fire results predicted by FVS-FFE. These potential fire results are modeled independently each year and represent "hypothetical" fire hazard indicators. We developed a stochastic model to understand how these potential results will behave under fire conditions consistent with current and possible fire activities in the future. We ran 5,000 Monte Carlo simulations for each management scenario to reflect the inherent spatial and temporal variability of wildfires. In each simulation, we randomized 1) how many plots burned, 2) which plots burned, and 3) when they burned. We assume that the average annual fire probability is 0.092%, which is slightly higher than historical conditions due to climate change (7, 12). We simulated fire weather conditions in the 90th and 97.5th percentile, and expected their frequency to increase during our modeling period (6, 43). These percentiles are related to large-scale wildfires in California forests, which have accounted for the vast majority of the total burned area in recent decades (44). In this study, we provided the average of these two fire weather conditions. For each forest stand and simulated wildfire, we estimate the storage and emissions associated with growth, fire, and decay. We use a scalar function to predict the growth after the fire. This scalar function is the predicted wildfire mortality rate based on the base area and FVS growth.​​ We use the published values ​​(45, 46) to parameterize direct combustion and post-fire attenuation. These parameters may underestimate the emissions caused by wildfires, which in turn underestimate the carbon benefits of management (SI appendix, SI methodology and results).

We rely on published values ​​to simulate the cradle-to-grave carbon benefits of harvested wood in four categories: logging and transportation emissions, production emissions, replacement of carbon-intensive products, and product obsolescence (Figure 1). We consider 1 ton of harvested carbon as the main unit of analysis. For products based on forest residues, we use data from LCA research with raw materials and system boundaries similar to what we modeled here. We only consider products that can use some or all of the raw materials modeled here (for example, top wood, but do not consider mixed biomass for OSB; SI appendix, SI methods and results). Where possible, we rely on research data using the greenhouse gas, regulated emissions, and transportation energy use (GREET) model (47). We standardize the harvest and transportation emissions of all products to align with the values ​​used in GREET. In all product paths, we assume that the average carbon intensity of the California grid is 225 g CO2e/kWh (48). The methods and assumptions for each product approach are described in the SI appendix, SI Methods and Results.

Modeling framework, system boundaries and example results for a product, Oriented Strand Board (OSB). Product carbon benefits (right) are specific to OSB, while in-forest carbon flux (left) is common to all products in the IWP scenario. The carbon benefit value provided is accumulated over 40 years. For a complete description of all the above steps, please refer to the SI Appendix, SI Method.

Life cycle, forest carbon balance (A) three scenarios and (B) several technical approaches. The net carbon value is represented by the point in B and the black line in A. In B, the dashed line represents the threshold used to select the technology suite in the IWP. The significant drop in net living tree carbon value relative to zero-year carbon storage is related to harvest events. In Low BAU, we only conduct management modeling for potentially profitable company land (net income> 2,500 USD/ha). In High BAU, we model management when the net income is positive and the delivered residue price is $0. In Innovation (IWP), we model the management of positive net income and delivered residue prices of up to $100/ODT. The treatment area under IWP defines high BAU and low BAU study areas, including untreated forests.

For sawn timber, we adopted the method used in references. 34 California market background. This approach creates an economically wide displacement factor for sawn timber products, including emissions from the extraction, transportation, and production of representative building material kits. In this study, we kept all the values ​​in ref. 34 Except for the end use of the product, this is economically specific. We use California-specific historical end-use data instead of (13). In the IWP Housing scenario, it is assumed that 100% of the increased sawn timber supply (over low BAU) is used for multi-unit buildings, resulting in a larger net replacement factor (SI Appendix, Table S4). We conservatively assume that 24% of sawn timber is used for bioenergy and 75% is used for durable wood products, although sawmill residues have more carbon beneficial uses (49). We calculated the weighted average half-life of all primary wood products as 38 years (SI Appendix, Table S4) (50). After the initial use, we assume that 65% of decommissioned wood products are sent to landfills, 25% are sent to bio-power generation facilities, and 10% are not collected (49). In landfills, it is assumed that 90% of charcoal is permanently inert (51, 52), although this assumption has limited impact during our 40-year modeling period (Figure 2A).

Timber availability when the price of delivered forest residues is increased according to the DBH level. Residues include small trees, tops and branches of large trees, used for sawing wood, and whole trees of non-commercial species.

(A) CalFire fire priority zone and (B) reduction of fire hazard in the IWP scenario throughout the study area. The reduction in fire risk is defined as the difference in mortality between treated and untreated base areas in the event of a wildfire in severe fire weather. In B, each hexagon represents an FIA map, which statistically represents a larger forest area (usually 2,000 to 2,500 hectares). Empty hexagons represent untreated plots, with county boundaries shown in the background. In B, the values ​​are grouped by FVS variants, where CA is Central California, NC is North Coast, SO is Northeast California, and WS is Western Serra. The colors represent ownership groups, where "family" is non-corporate private land.

Forest inventory data and other data used in this analysis can be obtained from the BioSum portal at http://biosum.info/CEC/. All other research data is included in the article and/or SI appendix.

Carlin Starrs, Sara Loreno, and Benktesh Sharma made important contributions to the initial modeling of this work. We thank several colleagues who reviewed this work at different stages, including George Peridas, Sam Uden, the Conservation Strategy Team, etc. We thank the editor and four peer reviewers for their excellent suggestions to make the paper stronger. Finally, we thank the nearly 100 FIA field staff, information managers, and analysts for their dedication, who are responsible for the collection and quality assurance of inventory data that supports this modeling. We are grateful for the support of the Protection 2.0 project. BC completed this work during his tenure in the NSF Graduate Research Fellowship Program.

Author contributions: BC, BMC and DLS design research; BC and JW conducted research; JF and WS contributed basic data and analysis; DS takes learning as the concept; BC analyzes data; BC, JSF and DLS are under the contributions of all authors Wrote this paper.

The author declares no competing interests.

This article is directly contributed by PNAS. AO is a guest editor invited by the editorial board.

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