Warming, drought, and disturbances lead to shifts in functional composition: A millennial-scale analysis for Amazonian and Andean sites.

Tropical forests are changing in composition and productivity, probably in response to changes in climate and disturbances. The responses to these multiple environmental drivers, and the mechanisms underlying the changes, remain largely unknown. Here, we use a functional trait approach on timescales of 10,000 years to assess how climate and disturbances influence the community-mean adult height, leaf area, seed mass, and wood density for eight lowland and highland forest landscapes. To do so, we combine data of eight fossil pollen records with functional traits and proxies for climate (temperature, precipitation, and El Niño frequency) and disturbances (fire and general disturbances). We found that temperature and disturbances were the most important drivers of changes in functional composition. Increased water availability (high precipitation and low El Niño frequency) generally led to more acquisitive trait composition (large leaves and soft wood). In lowland forests, warmer climates decreased community-mean height probably because of increased water stress, whereas in highland forests warmer climates increased height probably because of upslope migration of taller species. Disturbance increased the abundance of acquisitive, disturbance-adapted taxa with small seeds for quick colonization of disturbed sites, large leaves for light capture, and soft wood to attain fast height growth. Fire had weak effects on lowland forests but led to more stress-adapted taxa that are tall with fast life cycles and small seeds that can quickly colonize burned sites. Site-specific analyses were largely in line with cross-site analyses, except for varying site-level effects of El Niño frequency and fire activity, possibly because regional patterns in El Niño are not a good predictor of local changes, and charcoal abundances do not reflect fire intensity or severity. With future global changes, tropical Amazonian and Andean forests may transition toward shorter, drought- and disturbance-adapted forests in the lowlands but taller forests in the highlands.

Biomass resilience of Neotropical secondary forests.

Land-use change occurs nowhere more rapidly than in the tropics, where the imbalance between deforestation and forest regrowth has large consequences for the global carbon cycle. However, considerable uncertainty remains about the rate of biomass recovery in secondary forests, and how these rates are influenced by climate, landscape, and prior land use. Here we analyse aboveground biomass recovery during secondary succession in 45 forest sites and about 1,500 forest plots covering the major environmental gradients in the Neotropics. The studied secondary forests are highly productive and resilient. Aboveground biomass recovery after 20 years was on average 122 megagrams per hectare (Mg ha(-1)), corresponding to a net carbon uptake of 3.05 Mg C ha(-1) yr(-1), 11 times the uptake rate of old-growth forests. Aboveground biomass stocks took a median time of 66 years to recover to 90% of old-growth values. Aboveground biomass recovery after 20 years varied 11.3-fold (from 20 to 225 Mg ha(-1)) across sites, and this recovery increased with water availability (higher local rainfall and lower climatic water deficit). We present a biomass recovery map of Latin America, which illustrates geographical and climatic variation in carbon sequestration potential during forest regrowth. The map will support policies to minimize forest loss in areas where biomass resilience is naturally low (such as seasonally dry forest regions) and promote forest regeneration and restoration in humid tropical lowland areas with high biomass resilience.

Land-use intensification effects on functional properties in tropical plant communities.

There is consensus that plant diversity and ecosystem processes are negatively affected by land-use intensification (LUI), but, at the same time, there is empirical evidence that a large heterogeneity can be found in the responses. This heterogeneity is especially poorly understood in tropical ecosystems. We evaluated changes in community functional properties across five common land-use types in the wet tropics with different land-use intensity: mature forest, logged forest, secondary forest, agricultural land, and pastureland, located in the lowlands of Bolivia. For the dominant plant species, we measured 12 functional response traits related to their life history, acquisition and conservation of resources, plant domestication, and breeding. We used three single-trait metrics to describe community functional properties: community abundance-weighted mean (CWM) traits values, coefficient of variation, and kurtosis of distribution. The CWM of all 12 traits clearly responded to LUI. Overall, we found that an increase in LUI resulted in communities dominated by plants with acquisitive leaf trait values. However, contrary to our expectations, secondary forests had more conservative trait values (i.e., lower specific leaf area) than mature and logged forest, probably because they were dominated by palm species. Functional variation peaked at intermediate land-use intensity (high coefficient of variation and low kurtosis), which included secondary forest but, unexpectedly, also agricultural land, which is an intensely managed system. The high functional variation of these systems is due to a combination of how response traits (and species) are filtered out by biophysical filters and how management practices introduced a range of exotic species and their trait values into the local species pool. Our results showed that, at local scales and depending on prevailing environmental and management practices, LUI does not necessarily result in communities with more acquisitive trait values or with less functional variation. Instead of the widely expected negative impacts of LUI on plant diversity, we found varying responses of functional variation, with possible repercussions on many ecosystem services. These findings provide a background for actively mitigating negative effects of LUI while meeting the needs of local communities that rely mainly on provisioning ecosystem services for their livelihoods.

Markedly divergent estimates of Amazon forest carbon density from ground plots and satellites.

AIM: The accurate mapping of forest carbon stocks is essential for understanding the global carbon cycle, for assessing emissions from deforestation, and for rational land-use planning. Remote sensing (RS) is currently the key tool for this purpose, but RS does not estimate vegetation biomass directly, and thus may miss significant spatial variations in forest structure. We test the stated accuracy of pantropical carbon maps using a large independent field dataset. LOCATION: Tropical forests of the Amazon basin. The permanent archive of the field plot data can be accessed at: http://dx.doi.org/10.5521/FORESTPLOTS.NET/2014_1. METHODS: Two recent pantropical RS maps of vegetation carbon are compared to a unique ground-plot dataset, involving tree measurements in 413 large inventory plots located in nine countries. The RS maps were compared directly to field plots, and kriging of the field data was used to allow area-based comparisons. RESULTS: The two RS carbon maps fail to capture the main gradient in Amazon forest carbon detected using 413 ground plots, from the densely wooded tall forests of the north-east, to the light-wooded, shorter forests of the south-west. The differences between plots and RS maps far exceed the uncertainties given in these studies, with whole regions over- or under-estimated by > 25%, whereas regional uncertainties for the maps were reported to be < 5%. MAIN CONCLUSIONS: Pantropical biomass maps are widely used by governments and by projects aiming to reduce deforestation using carbon offsets, but may have significant regional biases. Carbon-mapping techniques must be revised to account for the known ecological variation in tree wood density and allometry to create maps suitable for carbon accounting. The use of single relationships between tree canopy height and above-ground biomass inevitably yields large, spatially correlated errors. This presents a significant challenge to both the forest conservation and remote sensing communities, because neither wood density nor species assemblages can be reliably mapped from space.

Linking functional diversity and social actor strategies in a framework for interdisciplinary analysis of nature's benefits to society.

The crucial role of biodiversity in the links between ecosystems and societies has been repeatedly highlighted both as source of wellbeing and as a target of human actions, but not all aspects of biodiversity are equally important to different ecosystem services. Similarly, different social actors have different perceptions of and access to ecosystem services, and therefore, they have different wants and capacities to select directly or indirectly for particular biodiversity and ecosystem characteristics. Their choices feed back onto the ecosystem services provided to all parties involved and in turn, affect future decisions. Despite this recognition, the research communities addressing biodiversity, ecosystem services, and human outcomes have yet to develop frameworks that adequately treat the multiple dimensions and interactions in the relationship. Here, we present an interdisciplinary framework for the analysis of relationships between functional diversity, ecosystem services, and human actions that is applicable to specific social environmental systems at local scales. We connect the mechanistic understanding of the ecological role of diversity with its social relevance: ecosystem services. The framework permits connections between functional diversity components and priorities of social actors using land use decisions and ecosystem services as the main links between these ecological and social components. We propose a matrix-based method that provides a transparent and flexible platform for quantifying and integrating social and ecological information and negotiating potentially conflicting land uses among multiple social actors. We illustrate the applicability of our framework by way of land use examples from temperate to subtropical South America, an area of rapid social and ecological change.

A regional approach to save the Amazon.

Early in August this year, a high-profile summit was held in Belém, Brazil, where the eight Amazonian countries discussed the future of the Amazon. The nations recognized that the Amazon is very close to reaching a tipping point for turning into a degraded ecosystem. The result of their discussions was the Belém Declaration, an ambitious plan to protect and conserve the Amazon forests and to support Indigenous Peoples and local communities. Concern arose, however, because they failed to agree on attaining zero deforestation by 2030 and on avoiding new explorations in the Amazon for fossil fuel. The Declaration also lacks specific and measurable indicators. The ministers of Foreign Affairs therefore have a very important role in further refining the agenda and deadlines so that the Belém Declaration can be implemented.

Successional theories.

Succession is a fundamental concept in ecology because it indicates how species populations, communities, and ecosystems change over time on new substrate or after a disturbance. A mechanistic understanding of succession is needed to predict how ecosystems will respond to land-use change and to design effective ecosystem restoration strategies. Yet, despite a century of conceptual advances a comprehensive successional theory is lacking. Here we provide an overview of 19 successional theories ('models') and their key points, group them based on conceptual similarity, explain conceptual development in successional ideas and provide suggestions how to move forward. Four groups of models can be recognised. The first group (patch & plants) focuses on plants at the patch level and consists of three subgroups that originated in the early 20th century. One subgroup focuses on the processes (dispersal, establishment, and performance) that operate sequentially during succession. Another subgroup emphasises individualistic species responses during succession, and how this is driven by species traits. A last subgroup focuses on how vegetation structure and underlying demographic processes change during succession. A second group of models (ecosystems) provides a more holistic view of succession by considering the ecosystem, its biota, interactions, diversity, and ecosystem structure and processes. The third group (landscape) considers a larger spatial scale and includes the effect of the surrounding landscape matrix on succession as the distance to neighbouring vegetation patches determines the potential for seed dispersal, and the quality of the neighbouring patches determines the abundance and composition of seed sources and biotic dispersal vectors. A fourth group (socio-ecological systems) includes the human component by focusing on socio-ecological systems where management practices have long-lasting legacies on successional pathways and where regrowing vegetations deliver a range of ecosystem services to local and global stakeholders. The four groups of models differ in spatial scale (patch, landscape) or organisational level (plant species, ecosystem, socio-ecological system), increase in scale and scope, and reflect the increasingly broader perspective on succession over time. They coincide approximately with four periods that reflect the prevailing view of succession of that time, although all views still coexist. The four successional views are: succession of plants (from 1910 onwards) where succession was seen through the lens of species replacement; succession of communities and ecosystems (from 1965 onwards) when there was a more holistic view of succession; succession in landscapes (from 2000 onwards) when it was realised that the structure and composition of landscapes strongly impact successional pathways, and increased remote-sensing technology allowed for a better quantification of the landscape context; and succession with people (from 2015 onwards) when it was realised that people and societal drivers have strong effects on successional pathways, that ecosystem processes and services are important for human well-being, and that restoration is most successful when it is done by and for local people. Our review suggests that the hierarchical successional framework of Pickett is the best starting point to move forward as this framework already includes several factors, and because it is flexible, enabling application to different systems. The framework focuses mainly on species replacement and could be improved by focusing on succession occurring at different hierarchical scales (population, community, ecosystem, socio-ecological system), and by integrating it with more recent developments and other successional models: by considering different spatial scales (landscape, region), temporal scales (ecosystem processes occurring over centuries, and evolution), and by taking the effects of the surrounding landscape (landscape integrity and composition, the disperser community) and societal factors (previous and current land-use intensity) into account. Such a new, comprehensive framework could be tested using a combination of empirical research, experiments, process-based modelling and novel tools. Applying the framework to seres across broadscale environmental and disturbance gradients allows a better insight into what successional processes matter and under what conditions.

Strong floristic distinctiveness across Neotropical successional forests.

Forests that regrow naturally on abandoned fields are important for restoring biodiversity and ecosystem services, but can they also preserve the distinct regional tree floras? Using the floristic composition of 1215 early successional forests (≤20 years) in 75 human-modified landscapes across the Neotropic realm, we identified 14 distinct floristic groups, with a between-group dissimilarity of 0.97. Floristic groups were associated with location, bioregions, soil pH, temperature seasonality, and water availability. Hence, there is large continental-scale variation in the species composition of early successional forests, which is mainly associated with biogeographic and environmental factors but not with human disturbance indicators. This floristic distinctiveness is partially driven by regionally restricted species belonging to widespread genera. Early secondary forests contribute therefore to restoring and conserving the distinctiveness of bioregions across the Neotropical realm, and forest restoration initiatives should use local species to assure that these distinct floras are maintained.

The role of land-use history in driving successional pathways and its implications for the restoration of tropical forests.

Secondary forests are increasingly important components of human-modified landscapes in the tropics. Successional pathways, however, can vary enormously across and within landscapes, with divergent regrowth rates, vegetation structure and species composition. While climatic and edaphic conditions drive variations across regions, land-use history plays a central role in driving alternative successional pathways within human-modified landscapes. How land use affects succession depends on its intensity, spatial extent, frequency, duration and management practices, and is mediated by a complex combination of mechanisms acting on different ecosystem components and at different spatial and temporal scales. We review the literature aiming to provide a comprehensive understanding of the mechanisms underlying the long-lasting effects of land use on tropical forest succession and to discuss its implications for forest restoration. We organize it following a framework based on the hierarchical model of succession and ecological filtering theory. This review shows that our knowledge is mostly derived from studies in Neotropical forests regenerating after abandonment of shifting cultivation or pasture systems. Vegetation is the ecological component assessed most often. Little is known regarding how the recovery of belowground processes and microbiota communities is affected by previous land-use history. In published studies, land-use history has been mostly characterized by type, without discrimination of intensity, extent, duration or frequency. We compile and discuss the metrics used to describe land-use history, aiming to facilitate future studies. The literature shows that (i) species availability to succession is affected by transformations in the landscape that affect dispersal, and by management practices and seed predation, which affect the composition and diversity of propagules on site. Once a species successfully reaches an abandoned field, its establishment and performance are dependent on resistance to management practices, tolerance to (modified) soil conditions, herbivory, competition with weeds and invasive species, and facilitation by remnant trees. (ii) Structural and compositional divergences at early stages of succession remain for decades, suggesting that early communities play an important role in governing further ecosystem functioning and processes during succession. Management interventions at early stages could help enhance recovery rates and manipulate successional pathways. (iii) The combination of local and landscape conditions defines the limitations to succession and therefore the potential for natural regeneration to restore ecosystem properties effectively. The knowledge summarized here could enable the identification of conditions in which natural regeneration could efficiently promote forest restoration, and where specific management practices are required to foster succession. Finally, characterization of the landscape context and previous land-use history is essential to understand the limitations to succession and therefore to define cost-effective restoration strategies. Advancing knowledge on these two aspects is key for finding generalizable relations that will increase the predictability of succession and the efficiency of forest restoration under different landscape contexts.

Eighty-four per cent of all Amazonian arboreal plant individuals are useful to humans.

Plants have been used in Amazonian forests for millennia and some of these plants are disproportionally abundant (hyperdominant). At local scales, people generally use the most abundant plants, which may be abundant as the result of management of indigenous peoples and local communities. However, it is unknown whether plant use is also associated with abundance at larger scales. We used the population sizes of 4,454 arboreal species (trees and palms) estimated from 1946 forest plots and compiled information about uses from 29 Amazonian ethnobotany books and articles published between 1926 and 2013 to investigate the relationship between species usefulness and their population sizes, and how this relationship is influenced by the degree of domestication of arboreal species across Amazonia. We found that half of the arboreal species (2,253) are useful to humans, which represents 84% of the estimated individuals in Amazonian forests. Useful species have mean populations sizes six times larger than non-useful species, and their abundance is related with the probability of usefulness. Incipiently domesticated species are the most abundant. Population size was weakly related to specific uses, but strongly related with the multiplicity of uses. This study highlights the enormous usefulness of Amazonian arboreal species for local peoples. Our findings support the hypothesis that the most abundant plant species have a greater chance to be useful at both local and larger scales, and suggest that although people use the most abundant plants, indigenous people and local communities have contributed to plant abundance through long-term management.

The importance of wood traits and hydraulic conductance for the performance and life history strategies of 42 rainforest tree species.

*In a comparative study of 42 rainforest tree species we examined relationships amongst wood traits, diameter growth and survival of large trees in the field, and shade tolerance and adult stature of the species. *The species show two orthogonal axes of trait variation: a primary axis related to the vessel size-number trade-off (reflecting investment in hydraulic conductance vs hydraulic safety) and a secondary axis related to investment in parenchyma vs fibres (storage vs strength). Across species, growth rate was positively related to vessel diameter and potential specific hydraulic conductance (K(p)), and negatively related to wood density. Survival rate was only positively related to wood density. *Light-demanding species were characterized by low wood and vessel density and wide vessels. Tall species were characterized by wide vessels with low density and large K(p). Hydraulic traits were more closely associated with adult stature than with light demand, possibly because tall canopy species experience more drought stress and face a higher cavitation risk. *Vessel traits affect growth and wood density affects growth and survival of large trees in the field. Vessel traits and wood density are therefore important components of the performance and life history strategies of tropical tree species.

Interactive effects of tree size, crown exposure and logging on drought-induced mortality.

Large trees in the tropics are reportedly more vulnerable to droughts than their smaller neighbours. This pattern is of interest due to what it portends for forest structure, timber production, carbon sequestration and multiple other values given that intensified El Niño Southern Oscillation (ENSO) events are expected to increase the frequency and intensity of droughts in the Amazon region. What remains unclear is what characteristics of large trees render them especially vulnerable to drought-induced mortality and how this vulnerability changes with forest degradation. Using a large-scale, long-term silvicultural experiment in a transitional Amazonian forest in Bolivia, we disentangle the effects of stem diameter, tree height, crown exposure and logging-induced degradation on risks of drought-induced mortality during the 2004/2005 ENSO event. Overall, tree mortality increased in response to drought in both logged and unlogged plots. Tree height was a much stronger predictor of mortality than stem diameter. In unlogged plots, tree height but not crown exposure was positively associated with drought-induced mortality, whereas in logged plots, neither tree height nor crown exposure was associated with drought-induced mortality. Our results suggest that, at the scale of a site, hydraulic factors related to tree height, not air humidity, are a cause of elevated drought-induced mortality of large trees in unlogged plots.This article is part of a discussion meeting issue 'The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications'.

Legume abundance along successional and rainfall gradients in Neotropical forests.

The nutrient demands of regrowing tropical forests are partly satisfied by nitrogen-fixing legume trees, but our understanding of the abundance of those species is biased towards wet tropical regions. Here we show how the abundance of Leguminosae is affected by both recovery from disturbance and large-scale rainfall gradients through a synthesis of forest inventory plots from a network of 42 Neotropical forest chronosequences. During the first three decades of natural forest regeneration, legume basal area is twice as high in dry compared with wet secondary forests. The tremendous ecological success of legumes in recently disturbed, water-limited forests is likely to be related to both their reduced leaflet size and ability to fix N2, which together enhance legume drought tolerance and water-use efficiency. Earth system models should incorporate these large-scale successional and climatic patterns of legume dominance to provide more accurate estimates of the maximum potential for natural nitrogen fixation across tropical forests.

Socio-ecological costs of Amazon nut and timber production at community household forests in the Bolivian Amazon.

The Bolivian Amazon holds a complex configuration of people and forested landscapes in which communities hold secure tenure rights over a rich ecosystem offering a range of livelihood income opportunities. A large share of this income is derived from Amazon nut (Bertholletia excelsa). Many communities also have long-standing experience with community timber management plans. However, livelihood needs and desires for better living conditions may continue to place these resources under considerable stress as income needs and opportunities intensify and diversify. We aim to identify the socioeconomic and biophysical factors determining the income from forests, husbandry, off-farm and two keystone forest products (i.e., Amazon nut and timber) in the Bolivian Amazon region. We used structural equation modelling tools to account for the complex inter-relationships between socioeconomic and biophysical factors in predicting each source of income. The potential exists to increase incomes from existing livelihood activities in ways that reduce dependency upon forest resources. For example, changes in off-farm income sources can act to increase or decrease forest incomes. Market accessibility, social, financial, and natural and physical assets determined the amount of income community households could derive from Amazon nut and timber. Factors related to community households' local ecological knowledge, such as the number of non-timber forest products harvested and the number of management practices applied to enhance Amazon nut production, defined the amount of income these households could derive from Amazon nut and timber, respectively. The (inter) relationships found among socioeconomic and biophysical factors over income shed light on ways to improve forest-dependent livelihoods in the Bolivian Amazon. We believe that our analysis could be applicable to other contexts throughout the tropics as well.

Spatial and temporal dynamics of shifting cultivation in the middle-Amazonas river: Expansion and intensification.

Shifting cultivation is the main land-use system transforming landscapes in riverine Amazonia. Increased concentration of the human population around villages and increasing market integration during the last decades may be causing agricultural intensification. Studies have shown that agricultural intensification, i.e. higher number of swidden-fallow cycles and shorter fallow periods, reduces crop productivity of swiddens and the regrowth capacity of fallows, undermining the resilience of the shifting cultivation system as a whole. We investigated the temporal and spatial dynamics of shifting cultivation in Brazilian Amazonia to test the hypotheses that (i) agriculture has become more intensive over time, and (ii) patterns of land-use intensity are related to land accessibility and human population density. We applied a breakpoint-detection algorithm to Landsat time-series spanning three decades (1984-2015) and retrieved the temporal dynamics of shifting cultivation fields, which go through alternating phases of crop production (swidden) and secondary forest regrowth (fallow). We found that fallow-period length has decreased from 6.4 to 5.1 years on average, and that expansion over old-growth forest has slowed down over time. Shorter fallow periods and higher frequency of slash and burn cycles are practiced closer to residences and around larger villages. Our results indicate that shifting cultivation in riverine Amazonia has gone through a process of agricultural intensification in the past three decades. The resulting landscape is predominantly covered by young secondary forests (≤ 12 yrs old), and 20% of it have gone through intensive use. Reversing this trend and avoiding the negative consequences of agricultural intensification requires land use planning that accounts for the constraints of land use in riverine areas.

Response to Comment on "Persistent effects of pre-Columbian plant domestication on Amazonian forest composition".

McMichael et al state that we overlooked the effects of post-Columbian human activities in shaping current floristic patterns in Amazonian forests. We formally show that post-Columbian human influences on Amazonian forests are indeed important, but they have played a smaller role when compared to the persistent effects of pre-Columbian human activities on current forest composition.

Recovery of forest structure and spectral properties after selective logging in lowland Bolivia.

Effective monitoring of selective logging from remotely sensed data requires an understanding of the spatial and temporal thresholds that constrain the utility of those data, as well as the structural and ecological characteristics of forest disturbances that are responsible for those constraints. Here we assess those thresholds and characteristics within the context of selective logging in the Bolivian Amazon. Our study combined field measurements of the spatial and temporal dynamics of felling gaps and skid trails ranging from <1 to 19 months following reduced-impact logging in a forest in lowland Bolivia with remote-sensing measurements from simultaneous monthly ASTER satellite overpasses. A probabilistic spectral mixture model (AutoMCU) was used to derive per-pixel fractional cover estimates of photosynthetic vegetation (PV), non-photosynthetic vegetation (NPV), and soil. Results were compared with the normalized difference in vegetation index (NDVI). The forest studied had considerably lower basal area and harvest volumes than logged sites in the Brazilian Amazon where similar remote-sensing analyses have been performed. Nonetheless, individual felling-gap area was positively correlated with canopy openness, percentage liana coverage, rates of vegetation regrowth, and height of remnant NPV. Both liana growth and NPV occurred primarily in the crown zone of the felling gap, whereas exposed soil was limited to the trunk zone of the gap. In felling gaps >400 m2, NDVI, and the PV and NPV fractions, were distinguishable from unlogged forest values for up to six months after logging; felling gaps <400 m2 were distinguishable for up to three months after harvest, but we were entirely unable to distinguish skid trails from our analysis of the spectral data.

Carbon recovery dynamics following disturbance by selective logging in Amazonian forests.

When 2 Mha of Amazonian forests are disturbed by selective logging each year, more than 90 Tg of carbon (C) is emitted to the atmosphere. Emissions are then counterbalanced by forest regrowth. With an original modelling approach, calibrated on a network of 133 permanent forest plots (175 ha total) across Amazonia, we link regional differences in climate, soil and initial biomass with survivors' and recruits' C fluxes to provide Amazon-wide predictions of post-logging C recovery. We show that net aboveground C recovery over 10 years is higher in the Guiana Shield and in the west (21 ±3 Mg C ha-1) than in the south (12 ±3 Mg C ha-1) where environmental stress is high (low rainfall, high seasonality). We highlight the key role of survivors in the forest regrowth and elaborate a comprehensive map of post-disturbance C recovery potential in Amazonia.

Carbon sequestration potential of second-growth forest regeneration in the Latin American tropics.

Regrowth of tropical secondary forests following complete or nearly complete removal of forest vegetation actively stores carbon in aboveground biomass, partially counterbalancing carbon emissions from deforestation, forest degradation, burning of fossil fuels, and other anthropogenic sources. We estimate the age and spatial extent of lowland second-growth forests in the Latin American tropics and model their potential aboveground carbon accumulation over four decades. Our model shows that, in 2008, second-growth forests (1 to 60 years old) covered 2.4 million km(2) of land (28.1% of the total study area). Over 40 years, these lands can potentially accumulate a total aboveground carbon stock of 8.48 Pg C (petagrams of carbon) in aboveground biomass via low-cost natural regeneration or assisted regeneration, corresponding to a total CO2 sequestration of 31.09 Pg CO2. This total is equivalent to carbon emissions from fossil fuel use and industrial processes in all of Latin America and the Caribbean from 1993 to 2014. Ten countries account for 95% of this carbon storage potential, led by Brazil, Colombia, Mexico, and Venezuela. We model future land-use scenarios to guide national carbon mitigation policies. Permitting natural regeneration on 40% of lowland pastures potentially stores an additional 2.0 Pg C over 40 years. Our study provides information and maps to guide national-level forest-based carbon mitigation plans on the basis of estimated rates of natural regeneration and pasture abandonment. Coupled with avoided deforestation and sustainable forest management, natural regeneration of second-growth forests provides a low-cost mechanism that yields a high carbon sequestration potential with multiple benefits for biodiversity and ecosystem services.