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Naturally regenerating forests or secondary forests (SFs) are a promising strategy for restoring large expanses of tropical forests at low cost and with high environmental benefits. This expectation is supported by the high resilience of tropical forests after natural disturbances, yet this resilience can be severely reduced by human impacts. Assessing the characteristics of SFs and their ecological integrity (EI) is essential to evaluating their role for conservation, restoration, and provisioning of ecosystem services. In this study, we aim to propose a concept and indicators that allow the assessment and classification of the EI of SFs. To this end, we review the literature to assess how EI has been addressed in different ecosystems and which indicators of EI are most commonly used for tropical forests. Building upon this knowledge we propose a modification of the concept of EI to embrace SFs and suggest indicators of EI that can be applied to different successional stages or stand ages. Additionally, we relate these indicators to ecosystem service provision in order to support the practical application of the theory. EI is generally defined as the ability of ecosystems to support and maintain composition, structure and function similar to the reference conditions of an undisturbed ecosystem. This definition does not consider the temporal dynamics of recovering ecosystems, such as SFs. Therefore, we suggest incorporation of an optimal successional trajectory as a reference in addition to the old‐growth forest reference. The optimal successional trajectory represents the maximum EI that can be attained at each successional stage in a given region and enables the evaluation of EI at any given age class. We further suggest a list of indicators, the main ones being: compositional indicators (species diversity/richness and indicator species); structural indicators (basal area, heterogeneity of basal area and canopy cover); function indicators (tree growth and mortality); and landscape proxies (landscape heterogeneity, landscape connectivity). Finally, we discuss how this approach can assist in defining the value of SF patches to provide ecosystem services, restore forests and contribute to ecosystem conservation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Questions Secondary succession in the tropics can follow alternative pathways. Land-use history is known to engender alternative successional communities, but the underlying mechanisms driving and sustaining divergence remain unclear. In this study we aim to answer the following questions: (1) does previous land use act as a filter for species composition in secondary forests; and (2) what are the relative roles of management practices, soil properties and landscape composition in determining species composition? Location Central Amazon, Brazil. Methods We sampled trees, shrubs and palms (≥1cm diameter) in 38 early secondary forests (5 yr after abandonment) located along gradients of land-use intensity in five shifting cultivation landscapes. We measured the diameter and height of each sampled plant, identified it to species or morpho-species level and checked if it was resprouting or not. At each secondary forest we also collected soil samples for chemical and physical analyses and estimated the amount of old-growth forest surrounding it (landscape composition). Results We found that previous land-use intensity determined species composition. With increasing land-use intensity, management practices of cut-and-burn and associated reduction in soil quality filtered out seed-dependent species and favoured strong sprouters and species that can cope with low nutrient availability. Landscape composition had a weak effect on species assemblages. We found specific species assemblages and indicator species associated with different levels of previous land-use intensity. As a consequence of these local filters, species α- and β-diversity decreased and therefore early successional communities became more similar to each other. Conclusion Species composition of successional forests is strongly determined by different land-use intensities. Dispersal limitation has a limited effect on determining the composition of the dominant species. Filtering effects of management practices and soil quality determine the species dominating the canopy at early stages of succession and narrow down the range of species able to colonize and establish. This study highlights how land use shapes successional communities and suggests that alternative successional pathways are determined at early stages of succession. Therefore, accounting for land-use history is crucial to improve the understanding of tropical secondary succession. We present a list of indicator species for different levels of previous land-use intensity that can be used to support conservation and restoration decisions in the Amazon. [ABSTRACT FROM AUTHOR]

We developed a method to reconstruct land use history from Landsat images time-series. The method uses a breakpoint detection framework derived from the econometrics field and applicable to time-series regression models. The Breaks For Additive Season and Trend (BFAST) framework is used for defining the time-series regression models which may contain trend and phenology, hence appropriately modelling vegetation intra and inter-annual dynamics. All available Landsat data are used for a selected study area, and the time-series are partitioned into segments delimited by breakpoints. Segments can be associated to land use regimes, while the breakpoints then correspond to shifts in land use regimes. In order to further characterize these shifts, we classified the unlabelled breakpoints returned by the algorithm into their corresponding processes. We used a Random Forest classifier, trained from a set of visually interpreted time-series profiles to infer the processes and assign labels to the breakpoints. The whole approach was applied to quantifying the number of cultivation cycles in a swidden agriculture system in Brazil (state of Amazonas). Number and frequency of cultivation cycles is of particular ecological relevance in these systems since they largely affect the capacity of the forest to regenerate after land abandonment. We applied the method to a Landsat time-series of Normalized Difference Moisture Index (NDMI) spanning the 1984–2015 period and derived from it the number of cultivation cycles during that period at the individual field scale level. Agricultural fields boundaries used to apply the method were derived using a multi-temporal segmentation approach. We validated the number of cultivation cycles predicted by the method against in-situ information collected from farmers interviews, resulting in a Normalized Residual Mean Squared Error (NRMSE) of 0.25. Overall the method performed well, producing maps with coherent spatial patterns. We identified various sources of error in the approach, including low data availability in the 90s and sub-object mixture of land uses. We conclude that the method holds great promise for land use history mapping in the tropics and beyond. [ABSTRACT FROM AUTHOR]

Swidden cultivation is one of the most widespread agricultural systems in the tropics. Due to socio-economic changes, swiddens are either abandoned, substituted for other agricultural systems, or intensified. In the region of the middle Amazon river, Brazil, the high market demand for cassava flour ( farinha ) combined with land scarcity is inducing agricultural intensification. We define agricultural intensification as an increase in the frequency of swidden–fallow cycles and a decrease in the fallow period. In this study, we evaluate the consequences of agricultural intensification for management practices and swidden productivity in one of the main cassava producing areas of the Brazilian Amazon. We used ethnographic and biophysical surveys to characterize the current management practices and to evaluate the effect of repeated swidden–fallow cycles within a short fallow period regime on swidden size, weed infestation and life-form composition, weeding effort and cassava productivity. Our results show that with repeated swidden–fallow cycles cassava yield decreases, weed cover increases and weed composition changes from a tree-dominated to a graminoid-dominated community. Such changes in the weed community result in increased weeding effort, to which farmers respond by cultivating smaller swiddens. Therefore, the ongoing agricultural intensification leads to lower swidden productivity and household income without ensuing clear benefits for farmers. Limited access to fertilizers, herbicides and technical assistance combined with the market demand for a single product hinders adaptation. Broadening market opportunities and improving technical assistance to farmers could raise the diversification of production and sources of income and guarantee higher resilience to the system. [ABSTRACT FROM AUTHOR]

1. Understanding how land-use intensification affects forest resilience is a key for elucidating the mechanisms underlying regeneration processes and for planning more sustainable land-use systems. Here, we evaluate how the intensification of a swidden cultivation system affects secondary-forest resilience in the Amazon. 2. Along a gradient of land-use intensity, we analysed the relative role of management intensity, soil properties and landscape configuration in determining the resilience of early secondary forests (SFs). We assessed resilience as the recovery level of forest structure and species diversity achieved by SFs 5 years after abandonment. We used as a reference the recovery level achieved by SFs subjected to the lowest intensity of use, given that these SFs are part of a dynamic system and may not develop to old-growth forests. Therefore, we interpreted a deviation from this reference level as a change in forest resilience. 3. The recovery of forest structure was determined by management intensity, while the recovery of species diversity was driven by landscape configuration. With increasing number of cycles and weeding frequency along with decreasing fallow period and patch area, SF basal area and canopy height decreased, regeneration shifted from a seed- to sprout-dependent strategy, and liana infestation on trees increased. With decreasing area covered by old-growth forest, species richness and Shannon diversity decreased. 4. Secondary-forest resilience decreased with land-use intensification, mainly mediated by the effect of management intensity upon regeneration strategies. Our findings demonstrate the - many times overlooked - importance of previous management intensity in determining the structure of SFs and highlight the importance of regeneration strategy for forest resilience. 5. Synthesis. Swidden cultivation supports people's livelihoods and transforms landscapes in the tropics. The sustainability of this system depends on ecosystem services provided by SFs that develop during the fallow period. Land-use intensification reduces the resilience of SFs and ultimately may drive the system towards an arrested succession state that holds a lower potential to deliver ecosystem services to the Amazonian people. Under an intensification scenario, the adaptation of management practices is needed to guarantee the resilience of swidden cultivation systems. [ABSTRACT FROM AUTHOR]

Environmental legislation requires well‐defined concepts, criteria, and indicators to support its effective implementation and provide legal security to the diverse stakeholders involved. The Native Vegetation Protection Law (NVPL) establishes the obligation to restore native vegetation in protected areas within rural properties. This federal law is regulated and implemented by each state through its own Environmental Compliance Program (Programa de Regularização Ambiental [PRA], in Portuguese). Landowners must register their land, indicate whether native vegetation needs to be restored (i.e. if there is an environmental debt), and submit a restoration plan to the state government for analysis, validation, and approval. Due to the high cost of forest restoration, natural regeneration has most likely become a viable strategy for Amazonian landowners to meet their environmental debts. However, according to disturbance intensity, frequency, and duration, natural regeneration will not always succeed. In this context, we analyzed the PRAs of the seven main Amazonian states to assess the status of their implementation, identify how natural regeneration is defined and approached in the law, and evaluate whether improvements in the law are needed to ensure effective restoration to accomplish the NVPL. [ABSTRACT FROM AUTHOR]

Blue carbon in mangroves represents one of highest values of carbon stocks per hectare, and could play an important role in climate change mitigation. In this study we estimated the carbon prices needed to promote mangrove conservation and restoration under mechanisms of payment for ecosystem services (PES). We mapped the remaining and deforested mangroves across the globe in 2017, and crossed this information with carbon stocks in the biomass and soil and with land opportunity and restoration costs. In accordance with previous studies we found that Southeast Asia holds the largest opportunities for blue carbon programs to support conservation and restoration. Conserving remaining mangroves would avoid the release of up to 15.51 PgCO 2 to the atmosphere, and could be achieved at carbon prices between 3.0 and 13.0 US$ per tCO 2 for 90% of remaining mangroves. Restoring mangroves can sequester up to 0.32 PgCO 2 globally. Carbon prices between 4.5 and 18.0 US$ per tCO 2 could support the restoration of 90% of deforested mangroves. Such prices, however, may not apply to contexts of high-profit alternative land-uses. In such contexts, the valuation of co-benefits and the combination of carbon-based mechanisms and sustainable management may be a viable pathway. [ABSTRACT FROM AUTHOR]

Aim: Successional changes in functional diversity provide insights into community assembly by indicating how species are filtered into local communities based on their traits. Here, we assess successional changes in taxonomic and functional richness, evenness and redundancy along gradients of climate, soil pH and forest cover. Location: Neotropics. Time period: Last 0–100 years. Major taxa studied: Trees. Methods: We used 22 forest chronosequence studies and 676 plots across the Neotropics to analyse successional changes in Hill's taxonomic and functional diversity of trees, and how these successional changes vary with continental‐scale gradients in precipitation, soil pH and surrounding forest cover. Results: Taxonomic and functional richness and functional redundancy increased, while taxonomic and functional evenness decreased over time. Functional richness and evenness changed strongly when not accounting for taxonomic richness, but changed more weakly after statistically accounting for taxonomic richness, indicating that changes in functional diversity are largely driven by taxonomic richness. Nevertheless, the successional increases in functional richness when correcting for taxonomic richness may indicate that environmental heterogeneity and limiting similarity increase during succession. The taxonomically‐independent successional decreases in functional evenness may indicate that stronger filtering and competition select for dominant species with similar trait values, while many rare species and traits are added to the community. Such filtering and competition may also lead to increased functional redundancy. The changes in taxonomically‐independent functional diversity varied with resource availability and were stronger in harsh, resource‐poor environments, but weak in benign, productive environments. Hence, in resource‐poor environments, environmental filtering and facilitation are important, whereas in productive environments, weaker abiotic filtering allows for high initial functional diversity and weak successional changes. Main conclusion: We found that taxonomic and functional richness and functional redundancy increased and taxonomic and functional evenness decreased during succession, mainly caused by the increasing number of rare species and traits due to the arrival of new species and due to changing (a)biotic filters. [ABSTRACT FROM AUTHOR]

Secondary tropical forests play an increasingly important role in carbon budgets and biodiversity conservation. Understanding successional trajectories is therefore imperative for guiding forest restoration and climate change mitigation efforts. Forest succession is driven by the demographic strategies—combinations of growth, mortality and recruitment rates—of the tree species in the community. However, our understanding of demographic diversity in tropical tree species stems almost exclusively from old‐growth forests. Here, we assembled demographic information from repeated forest inventories along chronosequences in two wet (Costa Rica, Panama) and two dry (Mexico) Neotropical forests to assess whether the ranges of demographic strategies present in a community shift across succession. We calculated demographic rates for >500 tree species while controlling for canopy status to compare demographic diversity (i.e., the ranges of demographic strategies) in early successional (0–30 years), late successional (30–120 years) and old‐growth forests using two‐dimensional hypervolumes of pairs of demographic rates. Ranges of demographic strategies largely overlapped across successional stages, and early successional stages already covered the full spectrum of demographic strategies found in old‐growth forests. An exception was a group of species characterized by exceptionally high mortality rates that was confined to early successional stages in the two wet forests. The range of demographic strategies did not expand with succession. Our results suggest that studies of long‐term forest monitoring plots in old‐growth forests, from which most of our current understanding of demographic strategies of tropical tree species is derived, are surprisingly representative of demographic diversity in general, but do not replace the need for further studies in secondary forests. [ABSTRACT FROM AUTHOR]

Tropical forests disappear rapidly because of deforestation, yet they have the potential to regrow naturally on abandoned lands. We analyze how 12 forest attributes recover during secondary succession and how their recovery is interrelated using 77 sites across the tropics. Tropical forests are highly resilient to low-intensity land use; after 20 years, forest attributes attain 78% (33 to 100%) of their old-growth values. Recovery to 90% of old-growth values is fastest for soil (<1 decade) and plant functioning (<2.5 decades), intermediate for structure and species diversity (2.5 to 6 decades), and slowest for biomass and species composition (’12 decades). Network analysis shows three independent clusters of attribute recovery, related to structure, species diversity, and species composition. Secondary forests should be embraced as a low-cost, natural solution for ecosystem restoration, climate change mitigation, and biodiversity conservation. [ABSTRACT FROM AUTHOR]

Background: Tropical forests are threatened by intensifying natural and anthropogenic disturbance regimes. Disturbances reduce tree cover and leave the organic topsoil vulnerable to erosion processes, but when resources are still abundant forests usually recover. Scope: Across the tropics, variation in rainfall erosivity – a measure of potential soil exposure to water erosion – indicates that soils in the wetter regions would experience high erosion rates if they were not protected by tree cover. However, twenty-first-century global land cover data reveal that in wet South America tropical tree cover is decreasing and bare soil area is increasing. Here we address the role of soil erosion in a positive feedback mechanism that may persistently alter the functioning of disturbed tropical forests. Conclusions: Based on an extensive literature review, we propose a conceptual model in which soil erosion reinforces disturbance effects on tropical forests, reducing their resilience with time and increasing their likelihood of being trapped in an alternative vegetation state that is persistently vulnerable to erosion. We present supporting field evidence from two distinct forests in central Amazonia that have been repeatedly disturbed. Overall, the strength of the erosion feedback depends on disturbance types and regimes, as well as on local environmental conditions, such as topography, flooding, and soil fertility. As disturbances intensify in tropical landscapes, we argue that the erosion feedback may help to explain why certain forests persist in a degraded state and often undergo critical functional shifts. [ABSTRACT FROM AUTHOR]

Aim: Tropical forest succession and associated changes in community composition are driven by species demographic rates, but how demographic strategies shift during succession remains unclear. Our goal was to identify generalities in demographic trade‐offs and successional shifts in demographic strategies across Neotropical forests that cover a large rainfall gradient and to test whether the current conceptual model of tropical forest succession applies to wet and dry forests. Location: Mexico and Central America. Time period: 1985–2018. Major taxa studied: Trees. Methods: We used repeated forest inventory data from two wet and two dry forests to quantify demographic rates of 781 tree species. For each forest, we explored the main demographic trade‐offs and assigned tree species to five demographic groups by performing a weighted principal components analysis to account for differences in sample size. We aggregated the basal area and abundance across demographic groups to identify successional shifts in demographic strategies over the entire successional gradient from very young (<5 years) to old‐growth forests. Results: Across all forests, we found two demographic trade‐offs, namely the growth–survival trade‐off and the stature–recruitment trade‐off, enabling the data‐driven assignment of species to five demographic strategies. Fast species dominated early in succession and were then replaced by long‐lived pioneers in three forests. Intermediate and slow species increased in basal area over succession in all forests, but, in contrast to the current conceptual model, long‐lived pioneers continued to dominate until the old‐growth stage in all forests. The basal area of short‐lived breeders was low across all successional stages. Main conclusions: The current conceptual model of Neotropical forest succession should be revised to incorporate the dominance of long‐lived pioneers in late‐successional and old‐growth forests. Moreover, the definition of consistent demographic strategies that show clear dominance shifts across succession substantially improves the mechanistic understanding and predictability of Neotropical forest succession. [ABSTRACT FROM AUTHOR]

Abandonment of agricultural lands promotes the global expansion of secondary forests, which are critical for preserving biodiversity and ecosystem functions and services. Such roles largely depend, however, on two essential successional attributes, trajectory and recovery rate, which are expected to depend on landscape-scale forest cover in nonlinear ways. Using a multi-scale approach and a large vegetation dataset (843 plots, 3511 tree species) from 22 secondary forest chronosequences distributed across the Neotropics, we show that successional trajectories of woody plant species richness, stem density and basal area are less predictable in landscapes (4 km radius) with intermediate (40–60%) forest cover than in landscapes with high (greater than 60%) forest cover. This supports theory suggesting that high spatial and environmental heterogeneity in intermediately deforested landscapes can increase the variation of key ecological factors for forest recovery (e.g. seed dispersal and seedling recruitment), increasing the uncertainty of successional trajectories. Regarding the recovery rate, only species richness is positively related to forest cover in relatively small (1 km radius) landscapes. These findings highlight the importance of using a spatially explicit landscape approach in restoration initiatives and suggest that these initiatives can be more effective in more forested landscapes, especially if implemented across spatial extents of 1–4 km radius. [ABSTRACT FROM AUTHOR]

Mining contributes importantly to tropical deforestation and land degradation. To mitigate these effects, mining companies are increasingly obliged to restore abandoned mine lands, but factors driving restoration success are hardly evaluated. Here, we investigate the influence of ecological factors (restoration age, soil properties and surrounding forest area) and management factors (diversity and density of planted species, mine zone) on the recovery rate of forest structure and tree diversity on 40 post-mining restoration areas in Southern Amazonia, Brazil, using a 9-year annual monitoring dataset consisting of over 25 000 trees.We found that recovery of forest structure was closely associated with interactions between soil quality and the planted tree communities, and that tree diversity recovery was positively associated with the amount of surrounding forests. We also observed that forest structure and diversity recover more slowly in mine tailings compared to pit surroundings. Our study confirms the complexity of mine land restoration but also reveals that planting design and soil improvement can increase restoration success. For resource-efficient mine restoration, we recommend the focusing of efforts on tailings, which are hardest to restore, and reducing efforts in pit surroundings and areas close to surrounding forest because of their potential for restoration by natural regeneration. [ABSTRACT FROM AUTHOR]

In the original version of this article, the following text must be added in the acknowledgement. M.H., B.M.F. and R.S.O. acknowledge the project grant from Instituto Serrapilheira/Serra-1709–18983. [ABSTRACT FROM AUTHOR]

Techniques for forest restoration have been widely developed over the past decades, allowing the reestablishment of vegetation in extreme cases such as surface mining. However, there are still issues related to management and monitoring that require further understanding, especially concerning comparisons with reference ecosystems. In this study, hierarchical agglomerative clustering (HAC) with uncertainty estimation is proposed as a methodology for forest restoration assessment. For this purpose, analysis was made of phytosociological variables for 27 plots located in reforested closed mines and in the Amazon forest reference ecosystem. The technique grouped the reference ecosystem separately from the reclamation sites. The HAC was affected by dependency among the analyzed variables, and heterogeneity was observed for all the phytosociological parameters in the cluster groups formed by the mining locations. However, each group showed specific characteristics related to the different environmental conditions and the forest restoration performance. The results demonstrated that HAC with uncertainty estimation was more suitable for defining groups, compared to the classical approach, offering a promising methodology for evaluation of the outcomes of forest restoration and for guiding management actions in disturbed tropical forests. [ABSTRACT FROM AUTHOR]

The massive forests of central Amazonia are often considered relatively resilient against climatic variation, but this view is challenged by the wildfires invoked by recent droughts. The impact of such fires that spread from pervasive sources of ignition may reveal where forests are less likely to persist in a drier future. Here we combine field observations with remotely sensed information for the whole Amazon to show that the annually inundated lowland forests that run through the heart of the system may be trapped relatively easily into a fire-dominated savanna state. This lower forest resilience on floodplains is suggested by patterns of tree cover distribution across the basin, and supported by our field and remote sensing studies showing that floodplain fires have a stronger and longer-lasting impact on forest structure as well as soil fertility. Although floodplains cover only 14% of the Amazon basin, their fires can have substantial cascading effects because forests and peatlands may release large amounts of carbon, and wildfires can spread to adjacent uplands. Floodplains are thus an Achilles’ heel of the Amazon system when it comes to the risk of large-scale climatedriven transitions. [ABSTRACT FROM AUTHOR]