Mapping forest-based natural climate solutions

Climate change mitigation necessitates significant reductions in greenhouse gas emissions. While reducing fossil fuel emissions is crucial, natural climate solutions (NCS), including improved land management, restoration, and protection actions, offer additional emission reduction and CO2 sequestration potential. Forests currently sequester up to 20% of annual anthropogenic emissions, making forest conservation and improved forest management key climate change mitigation strategies. However, current forest carbon management is often constrained by diverse land ownership and varying suitability for different strategies. This study addresses the need for high-resolution, spatially explicit approaches incorporating landowner management restrictions to identify suitable NCS actions at an ecoregional scale. A mapping approach can prioritize NCS projects by highlighting relationships between carbon mitigation strategies, land ownership, and opportunity scale, facilitating action planning and policy interventions. The recent availability of globally consistent data on forest carbon flux and aboveground carbon stocks with robust uncertainty estimates has enabled this ecoregional-scale assessment at a resolution suitable for decision-making. The study focuses on the coastal temperate rainforests of western North America, a region of high carbon density and sequestration rates, but facing threats from timber harvest and wildfire. This region presents a globally significant opportunity to invest in forest conservation and improved forest management for NCS.

Existing research highlights the importance of NCS in mitigating climate change (Griscom et al., 2017; Fargione et al., 2018; Drever et al., 2021). Studies emphasize the significant carbon sequestration potential of forests and the need for ambitious forest protection and management scenarios (Griscom et al., 2017; Fargione et al., 2018; Drever et al., 2021). However, challenges in quantifying NCS opportunities at an ecoregional scale stem from a lack of globally consistent data and the need to incorporate landowner management restrictions (Le Quéré et al., 2018). Previous research has demonstrated the value of spatial mapping approaches in identifying and prioritizing NCS projects (Tallis et al., 2021; Robertson et al., 2021), considering the scale of opportunity, ownership types, and potential emissions reductions. However, data limitations have hampered the quantification of NCS opportunities at the necessary resolution for effective decision-making (Le Quéré et al., 2018). Studies have also highlighted the importance of considering wildfire risks to long-term carbon storage in different forest types (Anderegg et al., 2020; Noon et al., 2022; Peeler et al., 2023) and the need for diverse forest-based strategies for climate change mitigation (Roe et al., 2021). The study area's relatively low risk of carbon loss reversal from wildfire makes it a particularly important target for NCS (Shanley et al., 2015).

The study employed a generalizable approach combining land management restrictions with high-resolution aboveground forest carbon stock and flux models. A spatial data hierarchy was developed using a jurisdictionally nested database structure, including country, state/province, landowner type, and land management designations. Data sources included publicly available subnational land ownership and land designation datasets, supplemented and verified with finer-scale data from local agencies. Where spatial data were unavailable, GIS analysis was used to map regulations (e.g., riparian buffers). A classification framework categorized areas for NCS action: NCS Action Category 1 (NCS1, available for action), NCS Action Category 2 (NCS2, not currently available due to administrative restrictions), and NCS Action Category 3 (NCS3, permanently protected). The best available high-resolution aboveground forest carbon stock and flux maps (emissions and sequestration) were analyzed at global, national, and regional scales. The global AGB model (Santoro et al., 2021), national and regional AGB models (Williams et al., 2020; Matasci et al., 2018; Hudak et al., 2020), and global carbon flux model (Harris et al., 2021) were used. A forest vegetation mask was created from NALCMS data, and data were processed to align resolutions and units for consistent analysis. Uncertainty estimates for carbon stocks were obtained from the global model and incorporated into calculations. Zonal statistics were computed to summarize carbon stocks and fluxes by jurisdiction and landowner type. To estimate the potential NCS opportunity from improved forest management and conservation, the historical range of variability in timber harvest was compared to 2030 land-based climate commitments of the US and Canada. A 10% reduction in average annual forest carbon loss was used as a conservative emissions reduction target.

The analysis revealed that 13 million hectares (40%) of the coastal temperate rainforests in western North America are potentially available for NCS action (NCS1), representing 45% of regional and 0.5% of global aboveground forest carbon stocks (4,900 ± 640 MtCO2e). The average carbon density of NCS1 forests (370 MgCO2e ha⁻¹) is higher than 90% of the world's forests. Between 2001 and 2021, these forests exhibited a net carbon sequestration of 36 MtCO2 yr⁻¹. A conservative estimate suggests a 10% reduction in average annual forest carbon losses through improved management and conservation could reduce emissions by 9.1 MtCO2e yr⁻¹, equivalent to 5.2% of the combined 2030 land-based climate commitments of the US and Canada. 61% of carbon stocks available for NCS action are publicly owned, with higher density carbon stocks than other ownership types. Private forest lands comprise 32%, and Indigenous ownerships 7% of the available carbon stocks. Oregon and Washington show the largest regional opportunities to transition towards additional carbon storage and sequestration. Private forest lands are significant contributors to carbon losses in these states. British Columbia is the largest forest manager in the region, managing 40% of the total aboveground forest carbon stocks available for NCS action. Indigenous-owned and managed lands in British Columbia and Alaska represent substantial opportunities for forest-based climate mitigation. Comparisons across global, national, and regional forest carbon maps showed similar carbon density estimates, with global maps generally indicating higher aboveground carbon stocks than national and regional models.

This study provides a generalizable spatial framework for identifying forest-based NCS opportunities by integrating land management restrictions with high-resolution carbon data. The approach improves upon previous NCS assessments by using fine-resolution land use information and offers a guide for assessments in globally significant forest landscapes. Identifying suitable and feasible intervention locations enhances planning, implementation, and ecoregional assessments of NCS potential. The spatial approach minimizes overestimation of NCS opportunities by accounting for administrative protections not captured in global and national databases. The results highlight the importance of utilizing high-resolution carbon stock and flux data combined with land management restrictions for accurate assessments of annual NCS opportunities. The analysis demonstrates that a 10% reduction in annual forest loss due to timber harvest within the historical range of variation could result in a meaningful reduction in carbon emissions. However, the success of improved forest management actions will depend on local contexts and the feasibility of large-scale project implementation. The study underscores the critical role of rural and Indigenous communities in implementing NCS projects aligned with community objectives. Integrating spatial data with traditional knowledge is vital for developing equitable and socially responsible NCS projects.

This paper demonstrates a generalizable spatial approach for identifying forest-based NCS opportunities by incorporating land management restrictions with high-resolution carbon stock and flux models. The study quantifies the significant potential of the coastal temperate rainforests of western North America for carbon sequestration and emission reduction through improved forest management and conservation. The findings highlight the need for collaborative planning with communities, industry, governments, and Indigenous peoples to maximize the effectiveness and equity of large-scale NCS implementation. Future research should focus on refining site-specific assessments, exploring synergies and trade-offs among different NCS pathways, and integrating traditional ecological knowledge into regional-scale assessments.

The study assumes that historical carbon loss patterns will continue into the future and that increased improved forest management and conservation will not be counteracted by forest carbon losses elsewhere (leakage). The analysis focuses on improved forest management and does not fully address other NCS pathways such as forest restoration or interventions to enhance the growth of regenerating forests. The study's reliance on publicly available data may not capture the full extent of Indigenous land ownership and management practices, particularly where treaties are still under negotiation. Finally, the study's carbon flux estimates do not account for carbon transferred to harvested wood products, which would lead to a different net carbon accounting.