Wetland area in agricultural landscapes has been heavily reduced to gain land for crop production, but in recent years there is increased societal recognition of the negative consequences from wetland loss on nutrient retention, biodiversity and a range of other benefits to humans. The current trend is therefore to re-establish wetlands, often with an aim to achieve the simultaneous delivery of multiple ecosystem services, i.e., multifunctionality. Here we review the literature on key objectives used to motivate wetland re-establishment in temperate agricultural landscapes (provision of flow regulation, nutrient retention, climate mitigation, biodiversity conservation and cultural ecosystem services), and their relationships to environmental properties, in order to identify potential for tradeoffs and synergies concerning the development of multifunctional wetlands. Through this process, we find that there is a need for a change in scale from a focus on single wetlands to wetlandscapes (multiple neighboring wetlands including their catchments and surrounding landscape features) if multiple societal and environmental goals are to be achieved. Finally, we discuss the key factors to be considered when planning for re-establishment of wetlands that can support achievement of a wide range of objectives at the landscape scale.
NbS Target: Climate Change Mitigation
Climate change mitigation
Justice and fairness have become key considerations in sustainability pathways and nature-based solutions (NBS), following activists and critical scholars who have long argued that the urban environment is an inherently political space that requires an analysis of benefits and burdens associated with its existence, use, and access. However, what justice means and how it is expressed, recognized, or achieved is often implicit in the literature on NBS, even though underlying notions of justice shape the analysis done and actions proposed. This paper starts from the premise that justice knows many different interpretations, therefore warranting scholars and practitioners working on NBS to carefully consider the differences and frictions between competing meanings of justice. Drawing from the history of social and environmental justice theory, we give an account of some key justice dilemmas and discuss their tenets as it relates to the end, means, and participants in the making of justice. From this, we draw out questions and commitments academics and practitioners in the NBS space should grapple with more explicitly. We argue that the emergent tension between pragmatic policy approaches and critical theoretical engagement is hindering a version of NBS that goes beyond a reflection of the justice implications of NBS to ensuring that NBS contributes to the furthering of justice. We advocate for the inclusion of critical social sciences and humanities perspectives and approaches beyond tokenism to instead encourage ontological, epistemological, and political reflection of the work academics and practitioners do in the NBS space.
Sustaining the organisms, ecosystems and processes that underpin human wellbeing is necessary to achieve sustainable development. Here we define critical natural assets as the natural and semi-natural ecosystems that provide 90% of the total current magnitude of 14 types of nature’s contributions to people (NCP), and we map the global locations of these critical natural assets at 2 km resolution. Critical natural assets for maintaining local-scale NCP (12 of the 14 NCP) account for 30% of total global land area and 24% of national territorial waters, while 44% of land area is required to also maintain two global-scale NCP (carbon storage and moisture recycling). These areas overlap substantially with cultural diversity (areas containing 96% of global languages) and biodiversity (covering area requirements for 73% of birds and 66% of mammals). At least 87% of the world’s population live in the areas benefitting from critical natural assets for local-scale NCP, while only 16% live on the lands containing these assets. Many of the NCP mapped here are left out of international agreements focused on conserving species or mitigating climate change, yet this analysis shows that explicitly prioritizing critical natural assets and the NCP they provide could simultaneously advance development, climate and conservation goals.
Cities increasingly have to find innovative ways to address challenges arising from climate change and urbanization. Nature-based solutions (NBS) have been gaining attention as multifunctional solutions that may help cities to address these challenges. However, the adoption and implementation of these solutions have been limited due to various barriers. This study aims to identify a taxonomy of dominant barriers to the uptake and implementation of NBS and their relationships. Fifteen barriers are identified from the literature and expert interviews and then ranked through a questionnaire. Interpretive Structural Modeling (ISM) serves to identify the mutual interdependencies among these barriers, which results in a structural model of six levels. Subsequently, Cross-impact matrix multiplication applied to classification (MICMAC analysis) is used to classify the barriers into four categories. The results suggest that political, institutional and knowledge-related barriers are the most dominant barriers to NBS. Cities that intend to apply NBS can draw on these findings, especially by more effectively prioritizing and managing their actions.
More than 90 systematic reviews have been conducted on the topic of nature-based solutions for climate change adaptation (NBS-CCA). These prior reviews, however, are scattered across more than 45 different peer-reviewed journals and gray literature sources, making it difficult to follow all of the knowledge generated and remaining research gaps. In this study, we conducted a systematic review of systematic reviews on the topic of NBS-CCA, with the objective of mapping and analyzing these prior reviews. We found that most of the prior systematic reviews had relatively narrow research focuses, typically focusing on a particular geographic context of NBS-CCA (mainly in urban and coastal areas) or on a particular aspect of NBS-CCA planning/implementation (mainly outcomes assessment and policy/governance issues). Fewer reviews focused on mountainous areas or on social and financial aspects of NBS-CCA planning/implementation. The majority reviews relied solely on peer-reviewed literature for analysis, with only 26% including gray literature, despite the large amount and variety of gray literature on NBS-CCA that exists. Notably, we found that no prior systematic reviews have yet attempted to comprehensively analyze all geographic contexts and all aspects of NBS-CCA, e.g. through a review and meta-analysis of all available peer-reviewed and gray literature on the topic. This would likely require a massive multidisciplinary effort, but could be a worthy endeavor considering the realized need to integrate NBS-CCA into national/subnational policies and various international environmental agreements pertaining to climate change (e.g., Paris Agreement) and biodiversity conservation (e.g., Post-2020 Global Biodiversity Framework).
Arising from B. B. N. Strassburg et al. Nature https://doi.org/10.1038/s41586-020-2784-9 (2020
Preventing dangerous climate change and halting the global loss of biodiversity are considered crucial goals to ensure a sustainable future on Earth1,2. Strassburg et al.3 present a high-resolution method to identify optimal locations for ecosystem restoration globally for conserving biodiversity and increasing carbon sequestration. Their most prominently presented conclusion is that 30% of the total CO2 increase in the atmosphere since the Industrial Revolution can be sequestered by restoring 15% of converted lands. Here we argue that this is an overly optimistic message that is partly based on inaccurate assumptions and that this creates unrealistic expectations for the contribution of restoration to the mitigation of climate change.
Mangrove forests, seagrass meadows and tidal saltmarshes are vegetated coastal ecosystems that accumulate and store large quantities of carbon in their sediments. Many recent studies and reviews have favorably identified the potential for such coastal “blue carbon” ecosystems to provide a natural climate solution in two ways: by conservation, reducing the greenhouse gas emissions arising from the loss and degradation of such habitats, and by restoration, to increase carbon dioxide drawdown and its long-term storage. The focus here is on the latter, assessing the feasibility of achieving quantified and secure carbon removal (negative emissions) through the restoration of coastal vegetation. Seven issues that affect the reliability of carbon accounting for this approach are considered: high variability in carbon burial rates; errors in determining carbon burial rates; lateral carbon transport; fluxes of methane and nitrous oxide; carbonate formation and dissolution; vulnerability to future climate change; and vulnerability to non-climatic factors. Information on restoration costs is also reviewed, with the conclusion that costs are highly uncertain, with lower-range estimates unrealistic for wider application. CO2 removal using coastal blue carbon restoration therefore has questionable cost-effectiveness when considered only as a climate mitigation action, either for carbon-offsetting or for inclusion in Nationally Determined Contributions. Many important issues relating to the measurement of carbon fluxes and storage have yet to be resolved, affecting certification and resulting in potential over-crediting. The restoration of coastal blue carbon ecosystems is nevertheless highly advantageous for climate adaptation, coastal protection, food provision and biodiversity conservation. Such action can therefore be societally justified in very many circumstances, based on the multiple benefits that such habitats provide at the local scale.
The European Union (EU) has firmly positioned itself as a global leader in promoting and implementing nature-based solutions (NBS). The recently released EU Biodiversity Strategy for 2030, Strategy on Adaptation to Climate Change, and Forest Strategy – all representing key pillars of the ambitious European Green Deal (EGD) – rely on NBS to both preserve and restore ecosystem integrity and increase climate resilience. Although research and policy in Europe have advanced the conceptualization and operationalization of NBS, a much wider adoption is needed to reach the ambitious goals of the EGD and fulfil its vision of transforming into a sustainable, climate-neutral, climate resilient, fair, and prosperous EU by 2050. In this paper, we review recent EU-supported research, policy, and practices to identify critical dimensions that still need to be addressed for greater uptake of NBS. While recognising the multiple societal challenges NBS can target, we build on the key messages from the ‘5th European Climate Change Adaptation conference ECCA 2021′ and focus our analysis on NBS for climate change adaptation and disaster risk reduction. We screen a wide range of NBS cases across the EU and identify-three core challenges to implementation: the lack of a comprehensive evidence base on the effectiveness of NBS to address targeted challenges; the need for a greater involvement of the private sector in financing NBS; and opportunities for enhancing stakeholder engagement in the successful design and implementation of NBS. We take these challenges as the starting point for a broader reflection and critical discussion on the role of i) knowledge, i) finance, including investments in NBS and divestments from nature-negative projects, and iii) governance and policy frameworks to enable the uptake of NBS. We conclude by identifying options for the EU to foster the uptake of NBS in research, policy and practice.
Seagrasses are remarkable plants that have adapted to live in a marine environment. They form extensive meadows found globally that bioengineer their local environments and preserve the coastal seascape. With the increasing realization of the planetary emergency that we face, there is growing interest in using seagrasses as a nature-based solution for greenhouse gas mitigation. However, seagrass sensitivity to stressors is acute, and in many places, the risk of loss and degradation persists. If the ecological state of seagrasses remains compromised, then their ability to contribute to nature-based solutions for the climate emergency and biodiversity crisis remains in doubt. We examine the major ecological role that seagrasses play and how rethinking their conservation is critical to understanding their part in fighting our planetary emergency.
Transformative governance is key to addressing the global environmental crisis. We explore how transformative governance of complex biodiversity–climate–society interactions can be achieved, drawing on the first joint report between the Intergovernmental Panel on Climate Change and the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services to reflect on the current opportunities, barriers, and challenges for transformative governance. We identify principles for transformative governance under a biodiversity–climate–society nexus frame using four case studies: forest ecosystems, marine ecosystems, urban environments, and the Arctic. The principles are focused on creating conditions to build multifunctional interventions, integration, and innovation across scales; coalitions of support; equitable approaches; and positive social tipping dynamics. We posit that building on such transformative governance principles is not only possible but essential to effectively keep climate change within the desired 1.5 degrees Celsius global mean temperature increase, halt the ongoing accelerated decline of global biodiversity, and promote human well-being.
Globally, rising seas threaten massive numbers of people and significant infrastructure. Adaptation strategies increasingly incorporate nature-based solutions. New science can illuminate where these solutions are appropriate in urban environments and what benefits they provide to people. Together with stakeholders in San Mateo County, California, USA, we co-developed nature-based solutions to support adaptation planning. We created six guiding principles to shape planning, summarized vulnerability to sea-level rise and opportunities for nature-based solutions, created three adaptation scenarios, and compared multiple benefits provided by each scenario. Adaptation scenarios that included investments in nature-based solutions deliver up to eight times the benefits of a traditionally engineered baseline as well as additional habitat for key species. The magnitude and distribution of benefits varied at subregional scales along the coastline. Our results demonstrate practical tools and engagement approaches to assessing the multiple benefits of nature-based solutions in an urban estuary that can be replicated in other regions.
Natural climate solutions (NCS)—actions to conserve, restore, and modify natural and modified ecosystems to increase carbon storage or avoid greenhouse gas (GHG) emissions—are increasingly regarded as important pathways for climate change mitigation, while contributing to our global conservation efforts, overall planetary resilience, and sustainable development goals. Recently, projections posit that terrestrial-based NCS can potentially capture or avoid the emission of at least 11 Gt (gigatons) of carbon dioxide equivalent a year, or roughly encompassing one third of the emissions reductions needed to meet the Paris Climate Agreement goals by 2030. NCS interventions also purport to provide co-benefits such as improved productivity and livelihoods from sustainable natural resource management, protection of locally and culturally important natural areas, and downstream climate adaptation benefits. Attention on implementing NCS to address climate change across global and national agendas has grown—however, clear understanding of which types of NCS interventions have undergone substantial study versus those that require additional evidence is still lacking. This study aims to conduct a systematic map to collate and describe the current state, distribution, and methods used for evidence on the links between NCS interventions and climate change mitigation outcomes within tropical and sub-tropical terrestrial ecosystems. Results of this study can be used to inform program and policy design and highlight critical knowledge gaps where future evaluation, research, and syntheses are needed.
Background
Human impacts on earth-system processes are overshooting several planetary boundaries, driving a crisis of ecological breakdown. This crisis is being caused in large part by global resource extraction, which has increased dramatically over the past half century. We propose a novel method for quantifying national responsibility for ecological breakdown by assessing nations’ cumulative material use in excess of equitable and sustainable boundaries.
Methods
For this analysis, we derived national fair shares of a sustainable resource corridor. These fair shares were then subtracted from countries’ actual resource use to determine the extent to which each country has overshot its fair share over the period 1970–2017. Through this approach, each country’s share of responsibility for global excess resource use was calculated.
Findings
High-income nations are responsible for 74% of global excess material use, driven primarily by the USA (27%) and the EU-28 high-income countries (25%). China is responsible for 15% of global excess material use, and the rest of the Global South (ie, the low-income and middle-income countries of Latin America and the Caribbean, Africa, the Middle East, and Asia) is responsible for only 8%. Overshoot in higher-income nations is driven disproportionately by the use of abiotic materials, whereas in lower-income nations it is driven disproportionately by the use of biomass.
Interpretation
These results show that high-income nations are the primary drivers of global ecological breakdown and they need to urgently reduce their resource use to fair and sustainable levels. Achieving sufficient reductions will likely require high-income nations to adopt transformative post-growth and degrowth approaches.
To counteract undesirable impacts of climate change, several different mitigation instruments have been proposed to sequester carbon through reforestation or avert greenhouse gas emissions due to land use change through forest carbon offset projects. Such projects will require an explicit focus on equitable benefit sharing to generate sustainable and alternative livelihoods. However, research on the impacts of forest carbon offset projects for individuals and communities has often been conducted without baseline data or counterfactuals built into the research methods. We conducted a study in a small Indigenous community in eastern Panama with participants and non-participants in a forest carbon offset project, across wealth groups. In this mixed methods study, participants and non-participants completed surveys before, during, and after implementation over 14 years to assess changes to natural and financial assets. We also assessed major concerns and perceived benefits of the carbon offset project via open-ended questions. Quantitative data show that participants continued to engage in reforestation practices even after payment cessation. Quantitative data also suggest carbon offset payments provided financial stability for poorer participants to diversify into other sources of income over time, while income inequality remained stable across wealth groups. Qualitative data indicate that the greatest benefit of the carbon offset project for participants was economic security for future generations, while concerns about basic needs like food and money declined over time for both participants and non-participants. This research suggests that forest carbon offset projects can be effective for encouraging long-term adoption of forestry practices, specifically reforestation and agroforestry, while providing social co-benefits for rural livelihoods, across wealth dimensions.
Climate policy has thus far focused solely on carbon stocks and sequestration to evaluate the potential of forests to mitigate global warming. These factors are used to assess the impacts of different drivers of deforestation and forest degradation as well as alternative forest management. However, when forest cover, structure and composition change, shifts in biophysical processes (the water and energy balances) may enhance or diminish the climate effects of carbon released from forest aboveground biomass. The net climate impact of carbon effects and biophysical effects determines outcomes for forest and agricultural species as well as the humans who depend on them. Evaluating the net impact is complicated by the disparate spatio-temporal scales at which they operate. Here we review the biophysical mechanisms by which forests influence climate and synthesize recent work on the biophysical climate forcing of forests across latitudes. We then combine published data on the biophysical effects of deforestation on climate by latitude with a new analysis of the climate impact of the CO2 in forest aboveground biomass by latitude to quantitatively assess how these processes combine to shape local and global climate. We find that tropical deforestation leads to strong net global warming as a result of both CO2 and biophysical effects. From the tropics to a point between 30°N and 40°N, biophysical cooling by standing forests is both local and global, adding to the global cooling effect of CO2 sequestered by forests. In the mid-latitudes up to 50°N, deforestation leads to modest net global warming as warming from released forest carbon outweighs a small opposing biophysical cooling. Beyond 50°N large scale deforestation leads to a net global cooling due to the dominance of biophysical processes (particularly increased albedo) over warming from CO2 released. Locally at all latitudes, forest biophysical impacts far outweigh CO2 effects, promoting local climate stability by reducing extreme temperatures in all seasons and times of day. The importance of forests for both global climate change mitigation and local adaptation by human and non-human species is not adequately captured by current carbon-centric metrics, particularly in the context of future climate warming.
Meeting the Paris Agreement’s climate objectives will require the world to achieve net-zero CO2 emissions around or before mid-century. Nature-based climate solutions, which aim to preserve and enhance carbon storage in terrestrial or aquatic ecosystems, could be a potential contributor to net-zero emissions targets. However, there is a risk that successfully stored land carbon could be subsequently lost back to the atmosphere as a result of disturbances such as wildfire or deforestation. Here we quantify the climate effect of nature-based climate solutions in a scenario where land-based carbon storage is enhanced over the next several decades, and then returned to the atmosphere during the second half of this century. We show that temporary carbon sequestration has the potential to decrease the peak temperature increase, but only if implemented alongside an ambitious mitigation scenario where fossil fuel CO2 emissions were also decreased to net-zero. We also show that non-CO2 effects such as surface albedo decreases associated with reforestation could counter almost half of the climate effect of carbon sequestration. Our results suggest that there is climate benefit associated with temporary nature-based carbon storage, but only if implemented as a complement (and not an alternative) to ambitious fossil fuel CO2 emissions reductions.
Sustainable management of intact tropical peatlands is crucial for climate change mitigation, for biodiversity conservation and to support the livelihoods of local communities. Here, we explore whether sustainable fruit harvesting from Mauritia flexuosa palms could support these linked goals by increasing fruit production and incomes across the 2.8 million hectares of the most carbon-dense ecosystem in Amazonia: the lowland peatlands of northeastern Peru. M. flexuosa is dioecious, and fruits are typically harvested by felling female palms; the proportion of female palms therefore provides a good indicator of the health of a stand. Across 93 widely distributed sites, we found that the proportion of female palms increases with travel time to the urban market, and overall, fruit harvesting has halved the current potential production and income from this resource. However, significantly more female palms are found where fruit are harvested by climbing. We estimate that region-wide uptake of climbing could eventually increase potential fruit production by 51% and increase its gross value to US$62 ± 28.2 million yr–1. These findings demonstrate the high cost of unsustainable resource extraction in Neotropical forests and outline a practical path to conserve and sustainably exploit one of the most carbon-rich landscapes on the planet.
To address climate change and meet global commitments, nature-based climate (NbCS) solutions i.e. actions that aim to address climate change in sustainable way, are becoming increasingly popular. This is often expressed in so called “green pledges” that promote large scale programs of tree planting, often in plantations. Establishment of such plantations could deliver benefits to biodiversity, but this is not guaranteed, and recommendations on how to manage nature-based solutions (NbS) for biodiversity are limited and not embedded in scientific knowledge. Evidence from landscape ecology can inform general principles of NbCS design. Here, we synthesise evidence and make recommendations that can help “green pledges” to benefit biodiversity conservation. We call for an integrated approach, where NbCS tree planting projects move beyond carbon storage to create functional and diverse habitats providing multiple long-term services while safeguarding biodiversity.
Although nature-based solutions (NBS) have been promoted as a key tool for solving diverse environmental and societal problems, the concept and its practical applications remain unclear. This ambiguity is linked to the fact that the NBS concept has emerged from the integration of multiple scientific fields. In addition, there has been a delay in establishing clear standards for NBS, hence a number of actions that today would be seen as complementary or related measures, are frequently branded as NBS. Thus, this paper paves the way to clarify NBS by identifying their core features and formulating criteria to exclude certain actions from the set of NBS. After reviewing 20 definitions of NBS, these actions are identified as interventions that: (1) are inspired and powered by nature; (2) address (societal) challenges or resolve problems; (3) provide multiple services/benefits, including biodiversity gain; and (4) are of high effectiveness and economic efficiency. The non-systematic review includes both peer-review research papers and relevant official reports, enabling the formulation of a set of criteria that exclude green/blue interventions from the set of NBS. These are: (1) lack of functioning ecosystems; (2) random actions; (3) post-implementation goal(s); (4) negative/no impact on biodiversity; (5) same benefits as grey infrastructure alone; (6) unfair distribution of benefits; (7) ‘copy-paste’ implementation approach; (8) top-down model of governance; (9) static management approach; (10) financial expenses disproportionate to benefits; and (11) ‘point scale’ approach. Ongoing and future practice will contribute to our understanding of the long-term operation of NBS as well as to the detection of synergies and trade-offs, thereby enabling us to better define this concept’s boundaries.
Forest restoration is being scaled-up globally to deliver critical ecosystem services and biodiversity benefits, yet we lack rigorous comparison of co-benefit delivery across different restoration approaches. In a global synthesis, we use 25,950 matched data pairs from 264 studies in 53 countries to assess how delivery of climate, soil, water, and wood production services as well as biodiversity compares across a range of tree plantations and native forests. Carbon storage, water provisioning, and especially soil erosion control and biodiversity benefits are all delivered better by native forests, with compositionally simpler, younger plantations in drier regions performing particularly poorly. However, plantations exhibit an advantage in wood production. These results underscore important trade-offs among environmental and production goals that policymakers must navigate in meeting forest restoration commitments.
Forests can play a significant role both in halting biodiversity loss and in mitigating climate change. A variety of payments for ecosystem services (PES) schemes exists to promote biodiversity conservation in forests. These schemes could be used to strengthen the role of forests as carbon sinks as well. This paper analyzes the implications of supplementing a PES scheme that targets boreal forest biodiversity with a carbon index. We use a site selection framework to examine how the proposed scheme impacts the promotion of both targets. We compare a case where the selection is done solely based on biodiversity values to a case where the selection is done based on both biodiversity and carbon benefits. The carbon index is formulated as current carbon storage or as future carbon sink. Correspondingly, biodiversity is maximized based on either current ecological values or potential ones. We compare equal or differing weights for biodiversity and carbon indexes, and examine trade-offs between biodiversity and CO2 in current and future values. Combined index values increase with the carbon index, but there is a trade-off between biodiversity and CO2 values if the conservation budget is not increased when the carbon index is introduced. There is a temporal trade-off in biodiversity and carbon values between selecting sites based on current or future values. Younger stands are preferred at the expense of old-growth stands with the carbon index. Weights can be used to balance the trade-off between biodiversity and carbon benefits. Overall, risks to losing significant ecological value from the conservation network are negligible, but the limited number of sites decreases the generalizability of the results.
Compared to technical infrastructure, nature-based solutions, NBS, strive to work with nature and to move beyond business-as-usual practices in order to address societal challenges such as flood risks. This research aims to spatially identify possible NBS areas and evaluate the areas capacity to provide selected ecosystem services, ES, for the Lahn river landscape in Germany. The research follows the functional landscape approach using hydromorphological landscape units, HLU, based on specific biophysical spatial criteria, such as slope, to then identify locations which may be considered suitable for NBS. The current ES delivery of these possible NBS areas is then evaluated. The three ES assessed are carbon storage, nutrient retention and recreation. We then undertake a geospatial comparison analysis to show the spatial relationships and patterns that emerge in regards to the ES configuration of the distinct NBS apt areas. Results show the HLU method serves to delineate and identify areas where NBS may exist or be implemented. The data depicts a distinct spatial pattern for each possible NBS space and complementary ES delivery. This explorative method is a useful spatial approach that can support NBS implementation and serve to investigate the multiple benefits NBS provide. The use of ecosystem services to compare and understand NBS is a viable prospect that must, however, be cautiously, locally and scientifically approached. Noticeable limitations regarding ES assessment remain, as available methods are often insufficiently inclusive of natural ecosystem processes and functions. Further research should assess a broader spectrum of NBS and their delivery of ES.
Forest carbon projects can deliver multiple benefits to society. Within Southeast Asia, 58% of forests threatened by loss could be protected as financially viable carbon projects, which would avoid 835 MtCO2e of emissions per year from deforestation, support dietary needs for an equivalent of 323,739 people annually from pollinator-dependent agriculture, retain 78% of the volume of nitrogen pollutants in watersheds yearly and safeguard 25 Mha of Key Biodiversity Areas.
Fire-prone dry forests often face increasing fires from climate change with low resistance and resilience due to logging of large, old fire-resistant trees. Their restoration across large landscapes is constrained by limited mature trees, physical settings, and protection. Active restoration has been costly and shown limited effectiveness, but lower cost passive restoration is less studied. I used GIS and machine learning to see whether passive restoration of old trees could overcome constraints in time, by 2060, across 667,000 ha of montane forests in the San Juan Mountains, Colorado, where temperatures are increasing faster than the global average. Random Forest models of physical locations of reconstructed historical old growth (OG) and relatively frequent fire (RFF) show historical OG with RFF was favored between 6.1 and 7.9℃ annual mean temperatures. Random Forest models projected that similar temperature-suitable locations were moved into the current middle montane ca 2015, and would be extended to just below the upper limit of the montane if the Paris 1.5℃ goal is reached, but beyond if not. US Forest Service common stand exam data, which covered ~15% of the study area and included 26,149 tree ages, show the highest potential for restoring resistance and resilience from old trees is a ≥120-year age class. This class could become a ≥160-year age class, which meets old-growth age criteria, over 81% of the area by ca 2060, nearly fully restoring historical old-growth levels. Half this age class is already protected, and much of the remainder could be retained using evidence-based diameter caps. Datasets thus are sufficient to show that passive restoration of old-tree resistance and resilience to fire is feasible by ca 2060 across a large montane landscape, although contingent on global success in achieving the Paris 1.5℃ goal. Passive restoration may be viable elsewhere.
- 1. The environmental benefits and lower implementation costs of (assisted) natural forest regrowth (NFR) compared to tree planting qualify it as a viable strategy to scale up forest restoration. However, NFR is not suitable in all places, because the potential for forest regeneration depends on the socio-environmental context and differs greatly over space and time. Therefore, it is critical to quantify the potential contribution of NFR for reaching forest restoration targets and complying with environmental policies.
- 2. Here, we quantify the socio-environmental consequences of NFR by considering four targets differing in restored area in the Atlantic Forest (6, 8, 15 and 22 Mha). We quantified the compliance with environmental policies, expected distribution of natural and restored vegetation within the biome and social fairness (distribution of restoration efforts and costs within small, medium and large-sized properties) of two hypothetical forest restoration scenarios.
- 3. We show that large-scale forest restoration prioritizing the areas with the highest potential for NFR (Scenario I) allows us to comply with one-third of the current environmental debt in the Atlantic Forest. Furthermore, this scenario disproportionately burdens specific types of land use, increases socioeconomic inequalities and concentrates restoration activities in regions in which the natural vegetation cover is already high.
- 4. By contrast, Scenario II—eradicating the environmental debt that results from environmental policies, then prioritizing areas with the lowest overall restoration costs until reaching the restoration targets—is socially fairer and maximizes compliance with environmental policies. Its outcomes are more homogeneously distributed among counties and small, medium and large-sized properties from the Brazilian Atlantic Forest. Despite doubling the implementation costs, the lower overall restoration costs in Scenario II result from significantly lower opportunity costs than in Scenario I.
- 5. Synthesis and application. The environmental, social and economic outputs of large-scale forest restoration in the Atlantic Forest can be maximized when NFR and tree planting are balanced (Scenario II). To achieve compliance with forest restoration commitments, we thus advocate for the site-specific selection of the best forest restoration strategy to guarantee social fairness and compliance with environmental policies at minimum overall restoration costs.