Accounting for albedo change to identify climate-positive tree cover restoration.

Restoring tree cover changes albedo, which is the fraction of sunlight reflected from the Earth's surface. In most locations, these changes in albedo offset or even negate the carbon removal benefits with the latter leading to global warming. Previous efforts to quantify the global climate mitigation benefit of restoring tree cover have not accounted robustly for albedo given a lack of spatially explicit data. Here we produce maps that show that carbon-only estimates may be up to 81% too high. While dryland and boreal settings have especially severe albedo offsets, it is possible to find places that provide net-positive climate mitigation benefits in all biomes. We further find that on-the-ground projects are concentrated in these more climate-positive locations, but that the majority still face at least a 20% albedo offset. Thus, strategically deploying restoration of tree cover for maximum climate benefit requires accounting for albedo change and we provide the tools to do so.

The principles of natural climate solutions.

Natural climate solutions can mitigate climate change in the near-term, during a climate-critical window. Yet, persistent misunderstandings about what constitutes a natural climate solution generate unnecessary confusion and controversy, thereby delaying critical mitigation action. Based on a review of scientific literature and best practices, we distill five foundational principles of natural climate solutions (nature-based, sustainable, climate-additional, measurable, and equitable) and fifteen operational principles for practical implementation. By adhering to these principles, practitioners can activate effective and durable natural climate solutions, enabling the rapid and wide-scale adoption necessary to meaningfully contribute to climate change mitigation.

Quantifying the Effect Size of Management Actions on Aboveground Carbon Stocks in Forest Plantations.

Purpose of the Review: Improved forest management is a promising avenue for climate change mitigation. However, we lack synthetic understanding of how different management actions impact aboveground carbon stocks, particularly at scales relevant for designing and implementing forest-based climate solutions. Here, we quantitatively assess and review the impacts of three common practices-application of inorganic NPK fertilizer, interplanting with N-fixing species, and thinning-on aboveground carbon stocks in plantation forests. Recent Findings: Site-level empirical studies show both positive and negative effects of inorganic fertilization, interplanting, and thinning on aboveground carbon stocks in plantation forests. Recent findings and the results of our analysis suggest that these effects are heavily moderated by factors such as species selection, precipitation, time since practice, soil moisture regime, and previous land use. Interplanting of N-fixing crops initially has no effect on carbon storage in main tree crops, but the effect becomes positive in older stands. Conversely, the application of NPK fertilizers increases aboveground carbon stocks, though the effect lessens with time. Moreover, increases in aboveground carbon stocks may be partially or completely offset by emissions from the application of inorganic fertilizer. Thinning results in a strong reduction of aboveground carbon stocks, though the effect lessens with time. Summary: Management practices tend to have strong directional effects on aboveground carbon stocks in plantation forests but are moderated by site-specific management, climatic, and edaphic factors. The effect sizes quantified in our meta-analysis can serve as benchmarks for the design and scoping of improved forest management projects as forest-based climate solutions. Overall, management actions can enhance the climate mitigation potential of plantation forests, if performed with sufficient attention to the nuances of local conditions. Supplementary Information: The online version contains supplementary material available at 10.1007/s40725-023-00182-5.

Rates and drivers of aboveground carbon accumulation in global monoculture plantation forests.

Restoring forest cover is a key action for mitigating climate change. Although monoculture plantations dominate existing commitments to restore forest cover, we lack a synthetic view of how carbon accumulates in these systems. Here, we assemble a global database of 4756 field-plot measurements from monoculture plantations across all forested continents. With these data, we model carbon accumulation in aboveground live tree biomass and examine the biological, environmental, and human drivers that influence this growth. Our results identify four-fold variation in carbon accumulation rates across tree genera, plant functional types, and biomes, as well as the key mediators (e.g., genus of tree, endemism of species, prior land use) of variation in these rates. Our nonlinear growth models advance our understanding of carbon accumulation in forests relative to mean annual rates, particularly during the next few decades that are critical for mitigating climate change.

The global potential for increased storage of carbon on land.

Constraining the climate crisis requires urgent action to reduce anthropogenic emissions while simultaneously removing carbon dioxide from the atmosphere. Improved information about the maximum magnitude and spatial distribution of opportunities for additional land-based removals of CO2 is needed to guide on-the-ground decision-making about where to implement climate change mitigation strategies. Here, we present a globally consistent spatial dataset (approximately 500-m resolution) of current, potential, and unrealized potential carbon storage in woody plant biomass and soil organic matter. We also provide a framework for prioritizing actions related to the restoration, management, and maintenance of woody carbon stocks and associated soils. By comparing current to potential carbon storage, while excluding areas critical to food production and human habitation, we find 287 petagrams (PgC) of unrealized potential storage opportunity, of which 78% (224 PgC) is in biomass and 22% (63 PgC) is in soil. Improved management of existing forests may offer nearly three-fourths (206 PgC) of the total unrealized potential, with the majority (71%) concentrated in tropical ecosystems. However, climate change is a source of considerable uncertainty. While additional research is needed to understand the impact of natural disturbances and biophysical feedbacks, we project that the potential for additional carbon storage in woody biomass will increase (+17%) by 2050 despite projected decreases (−12%) in the tropics. Our results establish an absolute reference point and conceptual framework for national and jurisdictional prioritization of locations and actions to increase land-based carbon storage.

Consistent cooling benefits of silvopasture in the tropics.

Agroforestry systems have the potential to sequester carbon and offer numerous benefits to rural communities, but their capacity to offer valuable cooling services has not been quantified on continental scales. Here, we find that trees in pasturelands ("silvopasture") across Latin America and Africa can offer substantial cooling benefits. These cooling benefits increase linearly by -0.32 °C to -2.4 °C per 10 metric tons of woody carbon per hectare, and importantly do not depend on the spatial extent of the silvopasture systems. Thus, even smallholders can reap important cooling services from intensifying their silvopasture practices. We then map where realistic (but ambitious) silvopasture expansion could counteract a substantial fraction of the local projected warming in 2050 due to climate change. Our findings indicate where and to what extent silvopasture systems can counteract local temperature increases from global climate change and help vulnerable communities adapt to a warming world.

Natural climate solutions for Canada.

Alongside the steep reductions needed in fossil fuel emissions, natural climate solutions (NCS) represent readily deployable options that can contribute to Canada's goals for emission reductions. We estimate the mitigation potential of 24 NCS related to the protection, management, and restoration of natural systems that can also deliver numerous co-benefits, such as enhanced soil productivity, clean air and water, and biodiversity conservation. NCS can provide up to 78.2 (41.0 to 115.1) Tg CO2e/year (95% CI) of mitigation annually in 2030 and 394.4 (173.2 to 612.4) Tg CO2e cumulatively between 2021 and 2030, with 34% available at ≤CAD 50/Mg CO2e. Avoided conversion of grassland, avoided peatland disturbance, cover crops, and improved forest management offer the largest mitigation opportunities. The mitigation identified here represents an important potential contribution to the Paris Agreement, such that NCS combined with existing mitigation plans could help Canada to meet or exceed its climate goals.

Mapping carbon accumulation potential from global natural forest regrowth.

To constrain global warming, we must strongly curtail greenhouse gas emissions and capture excess atmospheric carbon dioxide1,2. Regrowing natural forests is a prominent strategy for capturing additional carbon3, but accurate assessments of its potential are limited by uncertainty and variability in carbon accumulation rates2,3. To assess why and where rates differ, here we compile 13,112 georeferenced measurements of carbon accumulation. Climatic factors explain variation in rates better than land-use history, so we combine the field measurements with 66 environmental covariate layers to create a global, one-kilometre-resolution map of potential aboveground carbon accumulation rates for the first 30 years of natural forest regrowth. This map shows over 100-fold variation in rates across the globe, and indicates that default rates from the Intergovernmental Panel on Climate Change (IPCC)4,5 may underestimate aboveground carbon accumulation rates by 32 per cent on average and do not capture eight-fold variation within ecozones. Conversely, we conclude that maximum climate mitigation potential from natural forest regrowth is 11 per cent lower than previously reported3 owing to the use of overly high rates for the location of potential new forest. Although our data compilation includes more studies and sites than previous efforts, our results depend on data availability, which is concentrated in ten countries, and data quality, which varies across studies. However, the plots cover most of the environmental conditions across the areas for which we predicted carbon accumulation rates (except for northern Africa and northeast Asia). We therefore provide a robust and globally consistent tool for assessing natural forest regrowth as a climate mitigation strategy.

Large climate mitigation potential from adding trees to agricultural lands.

While improved management of agricultural landscapes is promoted as a promising natural climate solution, available estimates of the mitigation potential are based on coarse assessments of both agricultural extent and aboveground carbon density. Here we combine 30 meter resolution global maps of aboveground woody carbon, tree cover, and cropland extent, as well as a 1 km resolution map of global pasture land, to estimate the current and potential carbon storage of trees in nonforested portions of agricultural lands. We find that global croplands currently store 3.07 Pg of carbon (C) in aboveground woody biomass (i.e., trees) and pasture lands account for an additional 3.86 Pg C across a combined 3.76 billion ha. We then estimate the climate mitigation potential of multiple scenarios of integration and avoided loss of trees in crop and pasture lands based on region-specific biomass distributions. We evaluate our findings in the context of nationally determined contributions and find that the majority of potential carbon storage from integration and avoided loss of trees in crop and pasture lands is in countries that do not identify agroforestry as a climate mitigation technique.

Tree diversity promotes growth of late successional species despite increasing deer damage in a restored forest.

The role of tree diversity in restored forests and its impact on key ecological processes like growth and resistance to herbivory has become increasingly important. We analyzed height growth and white-tailed deer Odocoileus virginianus browsing damage to saplings of 16 broadleaved tree species in a large-scale (13 ha) reforestation experiment in Maryland, USA, where we manipulated tree diversity in 70 1,225-m2 plots. After four growing seasons, higher plot-level tree richness led to increased deer browsing damage (i.e., associational susceptibility). Despite increased deer damage to saplings in mixed plots, tree richness had no overall effect on sapling height growth. However, diversity-height relationships were related to species functional traits. Light demanding species with large leaves and faster growth rates had reduced heights in mixtures, whereas shade-tolerant, slower-growing species generally had either increased or unchanged height growth in diverse tree communities, likely related to increased canopy closure in mixtures relative to monocultures. We show that tree diversity can improve growth of late successional species despite exacerbated mammalian herbivore damage. By facilitating the establishment of species with a range of life-history strategies, increased tree diversity may enhance ecosystem multi-functionality in the early stages of forest restoration.

National mitigation potential from natural climate solutions in the tropics.

Better land stewardship is needed to achieve the Paris Agreement's temperature goal, particularly in the tropics, where greenhouse gas emissions from the destruction of ecosystems are largest, and where the potential for additional land carbon storage is greatest. As countries enhance their nationally determined contributions (NDCs) to the Paris Agreement, confusion persists about the potential contribution of better land stewardship to meeting the Agreement's goal to hold global warming below 2°C. We assess cost-effective tropical country-level potential of natural climate solutions (NCS)-protection, improved management and restoration of ecosystems-to deliver climate mitigation linked with sustainable development goals (SDGs). We identify groups of countries with distinctive NCS portfolios, and we explore factors (governance, financial capacity) influencing the feasibility of unlocking national NCS potential. Cost-effective tropical NCS offers globally significant climate mitigation in the coming decades (6.56 Pg CO2e yr-1 at less than 100 US$ per Mg CO2e). In half of the tropical countries, cost-effective NCS could mitigate over half of national emissions. In more than a quarter of tropical countries, cost-effective NCS potential is greater than national emissions. We identify countries where, with international financing and political will, NCS can cost-effectively deliver the majority of enhanced NDCs while transforming national economies and contributing to SDGs. This article is part of the theme issue 'Climate change and ecosystems: threats, opportunities and solutions'.

Estimating aboveground net biomass change for tropical and subtropical forests: Refinement of IPCC default rates using forest plot data.

As countries advance in greenhouse gas (GHG) accounting for climate change mitigation, consistent estimates of aboveground net biomass change (∆AGB) are needed. Countries with limited forest monitoring capabilities in the tropics and subtropics rely on IPCC 2006 default ∆AGB rates, which are values per ecological zone, per continent. Similarly, research into forest biomass change at a large scale also makes use of these rates. IPCC 2006 default rates come from a handful of studies, provide no uncertainty indications and do not distinguish between older secondary forests and old-growth forests. As part of the 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories, we incorporate ∆AGB data available from 2006 onwards, comprising 176 chronosequences in secondary forests and 536 permanent plots in old-growth and managed/logged forests located in 42 countries in Africa, North and South America and Asia. We generated ∆AGB rate estimates for younger secondary forests (≤20 years), older secondary forests (>20 years and up to 100 years) and old-growth forests, and accounted for uncertainties in our estimates. In tropical rainforests, for which data availability was the highest, our ∆AGB rate estimates ranged from 3.4 (Asia) to 7.6 (Africa) Mg ha-1  year-1 in younger secondary forests, from 2.3 (North and South America) to 3.5 (Africa) Mg ha-1  year-1 in older secondary forests, and 0.7 (Asia) to 1.3 (Africa) Mg ha-1  year-1 in old-growth forests. We provide a rigorous and traceable refinement of the IPCC 2006 default rates in tropical and subtropical ecological zones, and identify which areas require more research on ∆AGB. In this respect, this study should be considered as an important step towards quantifying the role of tropical and subtropical forests as carbon sinks with higher accuracy; our new rates can be used for large-scale GHG accounting by governmental bodies, nongovernmental organizations and in scientific research.

We need both natural and energy solutions to stabilize our climate.

We respond to concerns raised by Baldocchi and Penuelas who question the potential for ecosystems to provide carbon sinks and storage, and conclude that we should focus on decarbonizing our energy systems. While we agree with many of their concerns, we arrive at a different conclusion: we need strong action to advance both clean energy solutions and natural climate solutions (NCS) if we are to stabilize warming well below 2°C. Cost-effective NCS can deliver 11.3 PgCO2 e yr-1 or ~30% of near-term climate mitigation needs through protection, improved management, and restoration of ecosystems, as we increase overall ambition.

Natural climate solutions for the United States.

Limiting climate warming to <2°C requires increased mitigation efforts, including land stewardship, whose potential in the United States is poorly understood. We quantified the potential of natural climate solutions (NCS)-21 conservation, restoration, and improved land management interventions on natural and agricultural lands-to increase carbon storage and avoid greenhouse gas emissions in the United States. We found a maximum potential of 1.2 (0.9 to 1.6) Pg CO2e year-1, the equivalent of 21% of current net annual emissions of the United States. At current carbon market prices (USD 10 per Mg CO2e), 299 Tg CO2e year-1 could be achieved. NCS would also provide air and water filtration, flood control, soil health, wildlife habitat, and climate resilience benefits.

Spillover of a biological control agent (Chrysolina quadrigemina) onto native St. Johnswort (Hypericum punctatum).

Biological control agents may have unintended effects on native biota, particularly species that are closely related to the target invader. Here, we explored how Chrysolina quadrigemina, a beetle introduced to control the invasive weed Hypericum perforatum, impacts native H. punctatum in Tompkins County, New York, USA. Using a suite of complementary field surveys and experimental manipulations, we examined beetle preference for native and exotic Hypericum species and whether beetle herbivory influences the spatial distribution of H. punctatum. We found that the introduced beetle readily consumes native H. punctatum in addition to its intended target, and that H. punctatum at our field sites generally occurs along forest edges despite higher performance of experimental plants in more open habitats. However, we found no evidence that the beetle limits H. punctatum to forest edge habitats.

The face of conservation responding to a dynamically changing world.

In its 40-year history, the science of conservation has faced unprecedented challenges in terms of environmental damage and rapid global change, and environmental problems are only increasing as greater demands are placed on limited natural resources. Conservation science has been adapting to keep pace with these changes. Here, we highlight contemporary and emerging trends and innovations in conservation science that we believe represent the most effective responses to biodiversity threats. We focus on specific areas where conservation science has had to adjust its approach to address emerging threats to biodiversity, including habitat destruction and degradation, climate change, declining populations and invasive species. We also document changes in attitudes, norms and practices among conservation scientists. A key component to success is engaging and maintaining public support for conservation, which can be facilitated through the use of technology. These recent trends in conservation and management are innovative and will assist in optimizing conservation strategies, increasing our leverage with the general public and tackling our current environmental challenges.

Integrating physiological threshold experiments with climate modeling to project mangrove species' range expansion.

Predictions of climate-related shifts in species ranges have largely been based on correlative models. Due to limitations of these models, there is a need for more integration of experimental approaches when studying impacts of climate change on species distributions. Here, we used controlled experiments to identify physiological thresholds that control poleward range limits of three species of mangroves found in North America. We found that all three species exhibited a threshold response to extreme cold, but freeze tolerance thresholds varied among species. From these experiments, we developed a climate metric, freeze degree days (FDD), which incorporates both the intensity and the frequency of freezes. When included in distribution models, FDD accurately predicted mangrove presence/absence. Using 28 years of satellite imagery, we linked FDD to observed changes in mangrove abundance in Florida, further exemplifying the importance of extreme cold. We then used downscaled climate projections of FDD to project that these range limits will move northward by 2.2-3.2 km yr(-1) over the next 50 years.

Cascading effects of a highly specialized beech-aphid-fungus interaction on forest regeneration.

Specialist herbivores are thought to often enhance or maintain plant diversity within ecosystems, because they prevent their host species from becoming competitively dominant. In contrast, specialist herbivores are not generally expected to have negative impacts on non-hosts. However, we describe a cascade of indirect interactions whereby a specialist sooty mold (Scorias spongiosa) colonizes the honeydew from a specialist beech aphid (Grylloprociphilus imbricator), ultimately decreasing the survival of seedlings beneath American beech trees (Fagus grandifolia). A common garden experiment indicated that this mortality resulted from moldy honeydew impairing leaf function rather than from chemical or microbial changes to the soil. In addition, aphids consistently and repeatedly colonized the same large beech trees, suggesting that seedling-depauperate islands may form beneath these trees. Thus this highly specialized three-way beech-aphid-fungus interaction has the potential to negatively impact local forest regeneration via a cascade of indirect effects.

Exotic plants contribute positively to biodiversity functions but reduce native seed production and arthropod richness.

Although exotic plants comprise a substantial portion of floristic biodiversity, their contributions to community and ecosystem processes are not well understood. We manipulated plant species richness in old-field communities to compare the impacts of native vs. exotic species on plant biomass, seed production, and arthropod community structure. Plants within diverse communities, regardless of whether they were native or exotic, had higher biomass and seed production than in monocultures and displayed positive complementarity. Increasing native or exotic plant richness also enhanced the richness of arthropods on plants, but exotics attracted fewer arthropod species for a given arthropod abundance than did natives. Additionally, when exotic and native plants grew together, exotics suppressed seed production of native species. Thus, exotic plants appear to contribute positively to some biodiversity functions, but may impact native communities over longer time frames by reducing native seed production and recruiting fewer arthropod species.

Positive interactions between herbivores and plant diversity shape forest regeneration.

The effects of herbivores and diversity on plant communities have been studied separately but rarely in combination. We conducted two concurrent experiments over 3 years to examine how tree seedling diversity, density and herbivory affected forest regeneration. One experiment factorially manipulated plant diversity (one versus 15 species) and the presence/absence of deer (Odocoileus virginianus). We found that mixtures outperformed monocultures only in the presence of deer. Selective browsing on competitive dominants and associational protection from less palatable species appear responsible for this herbivore-driven diversity effect. The other experiment manipulated monospecific plant density and found little evidence for negative density dependence. Combined, these experiments suggest that the higher performance in mixture was owing to the acquisition of positive interspecific interactions rather than the loss of negative intraspecific interactions. Overall, we emphasize that realistic predictions about the consequences of changing biodiversity will require a deeper understanding of the interaction between plant diversity and higher trophic levels. If we had manipulated only plant diversity, we would have missed an important positive interaction across trophic levels: diverse seedling communities better resist herbivores, and herbivores help to maintain seedling diversity.