Trees adjust nutrient acquisition strategies across tropical forest secondary succession.

Nutrient limitation may constrain the ability of recovering and mature tropical forests to serve as a carbon sink. However, it is unclear to what extent trees can utilize nutrient acquisition strategies - especially root phosphatase enzymes and mycorrhizal symbioses - to overcome low nutrient availability across secondary succession. Using a large-scale, full factorial nitrogen and phosphorus fertilization experiment of 76 plots along a secondary successional gradient in lowland wet tropical forests of Panama, we tested the extent to which root phosphatase enzyme activity and mycorrhizal colonization are flexible, and if investment shifts over succession, reflective of changing nutrient limitation. We also conducted a meta-analysis to test how tropical trees adjust these strategies in response to nutrient additions and across succession. We find that tropical trees are dynamic, adjusting investment in strategies - particularly root phosphatase - in response to changing nutrient conditions through succession. These changes reflect a shift from strong nitrogen to weak phosphorus limitation over succession. Our meta-analysis findings were consistent with our field study; we found more predictable responses of root phosphatase than mycorrhizal colonization to nutrient availability. Our findings suggest that nutrient acquisition strategies respond to nutrient availability and demand in tropical forests, likely critical for alleviating nutrient limitation.

Leaf and twig traits predict habitat adaptation and demographic strategies in tropical freshwater swamp forest trees.

Differences in demographic and environmental niches facilitate plant species coexistence in tropical forests. However, the adaptations that enable species to achieve higher demographic rates (e.g. growth or survival) or occupy unique environmental niches (e.g. waterlogged conditions) remain poorly understood. Anatomical traits may better predict plant environmental and demographic strategies because they are direct measurements of structures involved in these adaptations. We collected 18 leaf and twig traits from 29 tree species in a tropical freshwater swamp forest in Singapore. We estimated demographic parameters of the 29 species from growth and survival models, and degree of association toward swamp habitats. We examined pairwise trait-trait, trait-demography and trait-environment links while controlling for phylogeny. Leaf and twig anatomical traits were better predictors of all demographic parameters than other commonly measured leaf and wood traits. Plants with wider vessels had faster growth rates but lower survival rates. Leaf and spongy mesophyll thickness predicted swamp association. These findings demonstrate the utility of anatomical traits as indicators of plant hydraulic strategies and their links to growth-mortality trade-offs and waterlogging stress tolerance that underlie species coexistence mechanisms in tropical forest trees.

Global dominance of lianas over trees is driven by forest disturbance, climate and topography.

Growing evidence suggests that liana competition with trees is threatening the global carbon sink by slowing the recovery of forests following disturbance. A recent theory based on local and regional evidence further proposes that the competitive success of lianas over trees is driven by interactions between forest disturbance and climate. We present the first global assessment of liana-tree relative performance in response to forest disturbance and climate drivers. Using an unprecedented dataset, we analysed 651 vegetation samples representing 26,538 lianas and 82,802 trees from 556 unique locations worldwide, derived from 83 publications. Results show that lianas perform better relative to trees (increasing liana-to-tree ratio) when forests are disturbed, under warmer temperatures and lower precipitation and towards the tropical lowlands. We also found that lianas can be a critical factor hindering forest recovery in disturbed forests experiencing liana-favourable climates, as chronosequence data show that high competitive success of lianas over trees can persist for decades following disturbances, especially when the annual mean temperature exceeds 27.8°C, precipitation is less than 1614 mm and climatic water deficit is more than 829 mm. These findings reveal that degraded tropical forests with environmental conditions favouring lianas are disproportionately more vulnerable to liana dominance and thus can potentially stall succession, with important implications for the global carbon sink, and hence should be the highest priority to consider for restoration management.

Des preuves de plus en plus nombreuses suggèrent que la competition entre lianes et les arbres menace le puits de carbone mondial en ralentissant la récupération des forêts après une perturbation. Une théorie récente, fondée sur des observations locales et régionales, propose en outre que le succès compétitif des lianes sur les arbres est dû aux interactions entre la perturbation forestière et le climat. Nous présentons la première évaluation mondiale de la performance relative des lianes par rapport aux arbres en réponse aux perturbations forestières et aux facteurs climatiques. En utilisant un ensemble de données sans précédent, nous avons analysé 651 échantillons de végétation représentant 26,538 lianes et 82,802 arbres, issus de 556 emplacements uniques dans le monde entier, tirés de 83 publications. Les résultats montrent que les lianes ont de meilleure performances par rapport aux arbres (augmentation du ratio liane-arbre) lorsque les forêts sont perturbées, sous des zones chaudes aves précipitations faibles, et vers les basses altitudes tropicales. Nous avons également constaté que les lianes peuvent être un facteur critique entravant la récupération des forêts dans les forêts perturbées connaissant des climats favorables aux lianes, car les données de chronoséquence montrent que le succès compétitif élevé des lianes sur les arbres peut persister pendant des décennies après les perturbations, surtout lorsque la température annuelle moyenne dépasse 27.8°C, que les précipitations sont inférieures à 1614 mm et que le déficit hydrique climatique est supérieur à 829 mm. Ces découvertes révèlent que les forêts tropicales dégradées avec des conditions environnementales favorables aux lianes sont disproportionnellement plus vulnérables à la dominance des lianes, et peuvent ainsi potentiellement entraver la succession, avec d'importantes implications pour le puits de carbone mondial et devraient donc être la plus haute priorité à considérer pour la gestion de la restauration.

Legume-microbiome interactions unlock mineral nutrients in regrowing tropical forests.

Legume trees form an abundant and functionally important component of tropical forests worldwide with N2-fixing symbioses linked to enhanced growth and recruitment in early secondary succession. However, it remains unclear how N2-fixers meet the high demands for inorganic nutrients imposed by rapid biomass accumulation on nutrient-poor tropical soils. Here, we show that N2-fixing trees in secondary Neotropical forests triggered twofold higher in situ weathering of fresh primary silicates compared to non-N2-fixing trees and induced locally enhanced nutrient cycling by the soil microbiome community. Shotgun metagenomic data from weathered minerals support the role of enhanced nitrogen and carbon cycling in increasing acidity and weathering. Metagenomic and marker gene analyses further revealed increased microbial potential beneath N2-fixers for anaerobic iron reduction, a process regulating the pool of phosphorus bound to iron-bearing soil minerals. We find that the Fe(III)-reducing gene pool in soil is dominated by acidophilic Acidobacteria, including a highly abundant genus of previously undescribed bacteria, Candidatus Acidoferrum, genus novus. The resulting dependence of the Fe-cycling gene pool to pH determines the high iron-reducing potential encoded in the metagenome of the more acidic soils of N2-fixers and their nonfixing neighbors. We infer that by promoting the activities of a specialized local microbiome through changes in soil pH and C:N ratios, N2-fixing trees can influence the wider biogeochemical functioning of tropical forest ecosystems in a manner that enhances their ability to assimilate and store atmospheric carbon.

Tallo: A global tree allometry and crown architecture database.

Data capturing multiple axes of tree size and shape, such as a tree's stem diameter, height and crown size, underpin a wide range of ecological research-from developing and testing theory on forest structure and dynamics, to estimating forest carbon stocks and their uncertainties, and integrating remote sensing imagery into forest monitoring programmes. However, these data can be surprisingly hard to come by, particularly for certain regions of the world and for specific taxonomic groups, posing a real barrier to progress in these fields. To overcome this challenge, we developed the Tallo database, a collection of 498,838 georeferenced and taxonomically standardized records of individual trees for which stem diameter, height and/or crown radius have been measured. These data were collected at 61,856 globally distributed sites, spanning all major forested and non-forested biomes. The majority of trees in the database are identified to species (88%), and collectively Tallo includes data for 5163 species distributed across 1453 genera and 187 plant families. The database is publicly archived under a CC-BY 4.0 licence and can be access from: https://doi.org/10.5281/zenodo.6637599. To demonstrate its value, here we present three case studies that highlight how the Tallo database can be used to address a range of theoretical and applied questions in ecology-from testing the predictions of metabolic scaling theory, to exploring the limits of tree allometric plasticity along environmental gradients and modelling global variation in maximum attainable tree height. In doing so, we provide a key resource for field ecologists, remote sensing researchers and the modelling community working together to better understand the role that trees play in regulating the terrestrial carbon cycle.

Successional syndromes of saplings in tropical secondary forests emerge from environment-dependent trait-demography relationships.

Identifying generalisable processes that underpin population dynamics is crucial for understanding successional patterns. While longitudinal or chronosequence data are powerful tools for doing so, the traditional focus on community-level shifts in taxonomic and functional composition rather than species-level trait-demography relationships has made generalisation difficult. Using joint species distribution models, we demonstrate how three traits-photosynthetic rate, adult stature, and seed mass-moderate recruitment and sapling mortality rates of 46 woody species during secondary succession. We show that the pioneer syndrome emerges from higher photosynthetic rates, shorter adult statures and lighter seeds that facilitate exploitation of light in younger secondary forests, while 'long-lived pioneer' and 'late successional' syndromes are associated with trait values that enable species to persist in the understory or reach the upper canopy in older secondary forests. Our study highlights the context dependency of trait-demography relationships, which drive successional shifts in sapling's species composition in secondary forests.

Lianas do not reduce tree biomass accumulation in young successional tropical dry forests.

Young successional tropical forests are crucial in the global carbon cycle because they can quickly sequester large quantities of atmospheric carbon. However, lianas (woody vines) can significantly decrease biomass accumulation in young regenerating forests. Lianas are abundant in tropical dry forests, and thus we hypothesized that lianas reduce biomass accretion in dry forests. Lianas may be particularly detrimental to the growth of young trees, which are vulnerable to competition from lianas. Alternatively, lianas may have a stronger negative effect on the largest trees because lianas seek the high-light environment at the top of the forest canopy. We tested these hypotheses using a liana-removal experiment in 13 dry forest stands that ranged from 1 to 70 years in southwestern Panama. We measured biomass accumulation annually for more than 10,000 stems from 2013 to 2017. Contrary to our expectations, liana removal had no effect on tree biomass accumulation across our successional forests and throughout our study period. Liana removal did not benefit smaller trees or larger trees. Lianas did not increase biomass accumulation on recruits, and did not increase biomass loss due to mortality. Surprisingly, removing lianas had a negative effect on three out of 41 tree species. Lianas had no effect on biomass accumulation and loss, possibly because: (1) trees allocated resources to roots instead of stems, (2) trees and lianas partitioned water, (3) higher irradiance after liana removal reduced soil moisture, or (4) low water availability might have been such a strong stressor that it reduced plant-plant competition.

Key role of symbiotic dinitrogen fixation in tropical forest secondary succession.

Forests contribute a significant portion of the land carbon sink, but their ability to sequester CO2 may be constrained by nitrogen, a major plant-limiting nutrient. Many tropical forests possess tree species capable of fixing atmospheric dinitrogen (N2), but it is unclear whether this functional group can supply the nitrogen needed as forests recover from disturbance or previous land use, or expand in response to rising CO2 (refs 6, 8). Here we identify a powerful feedback mechanism in which N2 fixation can overcome ecosystem-scale deficiencies in nitrogen that emerge during periods of rapid biomass accumulation in tropical forests. Over a 300-year chronosequence in Panama, N2-fixing tree species accumulated carbon up to nine times faster per individual than their non-fixing neighbours (greatest difference in youngest forests), and showed species-specific differences in the amount and timing of fixation. As a result of fast growth and high fixation, fixers provided a large fraction of the nitrogen needed to support net forest growth (50,000 kg carbon per hectare) in the first 12 years. A key element of ecosystem functional diversity was ensured by the presence of different N2-fixing tree species across the entire forest age sequence. These findings show that symbiotic N2 fixation can have a central role in nitrogen cycling during tropical forest stand development, with potentially important implications for the ability of tropical forests to sequester CO2.

Phosphatase activity and nitrogen fixation reflect species differences, not nutrient trading or nutrient balance, across tropical rainforest trees.

A fundamental biogeochemical paradox is that nitrogen-rich tropical forests contain abundant nitrogen-fixing trees, which support a globally significant tropical carbon sink. One explanation for this pattern holds that nitrogen-fixing trees can overcome phosphorus limitation in tropical forests by synthesizing phosphatase enzymes to acquire soil organic phosphorus, but empirical evidence remains scarce. We evaluated whether nitrogen fixation and phosphatase activity are linked across 97 trees from seven species, and tested two hypotheses for explaining investment in nutrient strategies: trading nitrogen-for-phosphorus or balancing nutrient demand. Both strategies varied across species but were not explained by nitrogen-for-phosphorus trading or nutrient balance. This indicates that (1) studies of these nutrient strategies require broad sampling within and across species, (2) factors other than nutrient trading must be invoked to resolve the paradox of tropical nitrogen fixation, and (3) nitrogen-fixing trees cannot provide a positive nitrogen-phosphorus-carbon feedback to alleviate nutrient limitation of the tropical carbon sink.

Successional dynamics in Neotropical forests are as uncertain as they are predictable.

Although forest succession has traditionally been approached as a deterministic process, successional trajectories of vegetation change vary widely, even among nearby stands with similar environmental conditions and disturbance histories. Here, we provide the first attempt, to our knowledge, to quantify predictability and uncertainty during succession based on the most extensive long-term datasets ever assembled for Neotropical forests. We develop a novel approach that integrates deterministic and stochastic components into different candidate models describing the dynamical interactions among three widely used and interrelated forest attributes--stem density, basal area, and species density. Within each of the seven study sites, successional trajectories were highly idiosyncratic, even when controlling for prior land use, environment, and initial conditions in these attributes. Plot factors were far more important than stand age in explaining successional trajectories. For each site, the best-fit model was able to capture the complete set of time series in certain attributes only when both the deterministic and stochastic components were set to similar magnitudes. Surprisingly, predictability of stem density, basal area, and species density did not show consistent trends across attributes, study sites, or land use history, and was independent of plot size and time series length. The model developed here represents the best approach, to date, for characterizing autogenic successional dynamics and demonstrates the low predictability of successional trajectories. These high levels of uncertainty suggest that the impacts of allogenic factors on rates of change during tropical forest succession are far more pervasive than previously thought, challenging the way ecologists view and investigate forest regeneration.

Liana effects on biomass dynamics strengthen during secondary forest succession.

Secondary forests are important carbon sinks, but their biomass dynamics vary markedly within and across landscapes. The biotic and abiotic drivers of this variation are still not well understood. We tested the effects of soil resource availability and competition by lianas on the biomass dynamics of young secondary tropical forests in Panama and assessed the extent to which liana effects were mediated by soil resource availability. Over a five-year period, growth, mortality, and recruitment of woody plants of ≥1 cm diameter were monitored in 84 plots in 3-30-year-old secondary forests across the Agua Salud site in central Panama. Biomass dynamics and the effects of lianas and soil resources were examined using (generalized) linear mixed-effect models and a model averaging approach. There was strong spatial and temporal variation in liana biomass within and across the plots. The relative biomass of lianas had a strong negative effect on overall tree growth, growth of understory trees decreased with soil fertility and dry season soil water content, and the effect of lianas on tree mortality varied with soil fertility. Tree recruitment was not associated with any of the predictor variables. Our model indicates that tree biomass growth across our landscape was reduced with 22% due to competition with lianas, and that the effect of lianas increased during succession, from 19% after five years to 32% after 30 years. The projected liana-induced growth reduction after 60 years was 47%, which was consistent with data from a nearby site. Our study shows that the observed liana proliferation across tropical forests may reduce the sequestration and storage of carbon in young secondary forests, with important implications for the carbon balance of tropical forest landscapes and consequently for global climate change. Our study highlights the need to incorporate lianas and soil variables in research on the biomass dynamics of secondary forest across tropical landscapes, and the need for well-replicated longitudinal studies to cover landscape-level variability in the relevant abiotic and biotic components.

Demographic drivers of functional composition dynamics.

Mechanisms of community assembly and ecosystem function are often analyzed using community-weighted mean trait values (CWMs). We present a novel conceptual framework to quantify the contribution of demographic processes (i.e., growth, recruitment, and mortality) to temporal changes in CWMs. We used this framework to analyze mechanisms of secondary succession in wet tropical forests in Mexico. Seed size increased over time, reflecting a trade-off between colonization by small seeds early in succession, to establishment by large seeds later in succession. Specific leaf area (SLA) and leaf phosphorus content decreased over time, reflecting a trade-off between fast growth early in succession vs. high survival late in succession. On average, CWM shifts were driven mainly (70%) by growth of surviving trees that comprise the bulk of standing biomass, then mortality (25%), and weakly by recruitment (5%). Trait shifts of growing and recruiting trees mirrored the CWM trait shifts, and traits of dying trees did not change during succession, indicating that these traits are important for recruitment and growth, but not for mortality, during the first 30 yr of succession. Identifying the demographic drivers of functional composition change links population dynamics to community change, and enhances insights into mechanisms of succession.

A hyperspectral image can predict tropical tree growth rates in single-species stands.

Remote sensing is increasingly needed to meet the critical demand for estimates of forest structure and composition at landscape to continental scales. Hyperspectral images can detect tree canopy properties, including species identity, leaf chemistry and disease. Tree growth rates are related to these measurable canopy properties but whether growth can be directly predicted from hyperspectral data remains unknown. We used a single hyperspectral image and light detection and ranging-derived elevation to predict growth rates for 20 tropical tree species planted in experimental plots. We asked whether a consistent relationship between spectral data and growth rates exists across all species and which spectral regions, associated with different canopy chemical and structural properties, are important for predicting growth rates. We found that a linear combination of narrowband indices and elevation is correlated with standardized growth rates across all 20 tree species (R2  = 53.70%). Although wavelengths from the entire visible-to-shortwave infrared spectrum were involved in our analysis, results point to relatively greater importance of visible and near-infrared regions for relating canopy reflectance to tree growth data. Overall, we demonstrate the potential for hyperspectral data to quantify tree demography over a much larger area than possible with field-based methods in forest inventory plots.

Changing gears during succession: shifting functional strategies in young tropical secondary forests.

Adaptations to resource availability strongly shape patterns of community composition along successional gradients in environmental conditions. In the present study, we examined the extent to which variation in functional composition explains shifts in trait-based functional strategies in young tropical secondary forests during the most dynamic stage of succession (0-20 years). Functional composition of two size classes in 51 secondary forest plots was determined using community-weighted means of seven functional traits, which were intensively measured on 55 woody plant species (n = 875-1,761 individuals). Along the successional gradient in forest structure, there was a significant and consistent shift in functional strategies from resource acquisition to resource conservation. Leaf toughness and adult plant size increased significantly, while net photosynthetic capacity (A(mass)) decreased significantly during succession. Shifts in functional strategies within size classes for A(mass) and wood density also support the hypothesis that changes in functional composition are shaped by environmental conditions along successional gradients. In general, 'hard' functional traits, e.g., A(mass) and leaf toughness, linked to different facets of plant performance exhibited greater sensitivity to successional changes in forest structure than 'soft' traits, such as leaf mass area and leaf dry matter content. Our results also suggested that stochastic processes related to previous land-use history, dispersal limitation, and abiotic factors explained variation in functional composition beyond that attributed to deterministic shifts in functional strategies. Further data on seed dispersal vectors and distance and landscape configuration are needed to improve current mechanistic models of succession in tropical secondary forests.

Feedback loops drive ecological succession: towards a unified conceptual framework.

The core principle shared by most theories and models of succession is that, following a major disturbance, plant-environment feedback dynamics drive a directional change in the plant community. The most commonly studied feedback loops are those in which the regrowth of the plant community causes changes to the abiotic (e.g. soil nutrients) or biotic (e.g. dispersers) environment, which differentially affect species availability or performance. This, in turn, leads to shifts in the species composition of the plant community. However, there are many other PE feedback loops that potentially drive succession, each of which can be considered a model of succession. While plant-environment feedback loops in principle generate predictable successional trajectories, succession is generally observed to be highly variable. Factors contributing to this variability are the stochastic processes involved in feedback dynamics, such as individual mortality and seed dispersal, and extrinsic causes of succession, which are not affected by changes in the plant community but do affect species performance or availability. Both can lead to variation in the identity of dominant species within communities. This, in turn, leads to further contingencies if these species differ in their effect on their environment (priority effects). Predictability and variability are thus intrinsically linked features of ecological succession. We present a new conceptual framework of ecological succession that integrates the propositions discussed above. This framework defines seven general causes: landscape context, disturbance and land-use, biotic factors, abiotic factors, species availability, species performance, and the plant community. When involved in a feedback loop, these general causes drive succession and when not, they are extrinsic causes that create variability in successional trajectories and dynamics. The proposed framework provides a guide for linking these general causes into causal pathways that represent specific models of succession. Our framework represents a systematic approach to identifying the main feedback processes and causes of variation at different successional stages. It can be used for systematic comparisons among study sites and along environmental gradients, to conceptualise studies, and to guide the formulation of research questions and design of field studies. Mapping an extensive field study onto our conceptual framework revealed that the pathways representing the study's empirical outcomes and conceptual model had important differences, underlining the need to move beyond the conceptual models that currently dominate in specific fields and to find ways to examine the importance of and interactions among alternative causal pathways of succession. To further this aim, we argue for integrating long-term studies across environmental and anthropogenic gradients, combined with controlled experiments and dynamic modelling.

Tree demographic strategies largely overlap across succession in Neotropical wet and dry forest communities.

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.

Mixed success for carbon payments and subsidies in support of forest restoration in the neotropics.

Restoration of forests in low- and middle-income countries (LMICs) has the potential to contribute to international carbon mitigation targets. However, high upfront costs and variable cashflows are obstacles for many landholders. Carbon payments have been promoted as a mechanism to incentivize restoration and economists have suggested cost-sharing by third parties to reduce financial burdens of restoration. Yet empirical evidence to support this theory, based on robust, dynamic field sampling is lacking. Here we use large, long-term datasets from Panama to evaluate the financial prospects of three forest restoration methods under different cost-sharing and carbon payment designs where income is generated through timber harvests. We show some, but not all options are economically viable. Further work combining growth and survival data from field trials with more sophisticated financial analyses is essential to understanding barriers and realizing the potential of forest restoration in LMICs to help meet global carbon mitigation commitments.

A universal airborne LiDAR approach for tropical forest carbon mapping.

Airborne light detection and ranging (LiDAR) is fast turning the corner from demonstration technology to a key tool for assessing carbon stocks in tropical forests. With its ability to penetrate tropical forest canopies and detect three-dimensional forest structure, LiDAR may prove to be a major component of international strategies to measure and account for carbon emissions from and uptake by tropical forests. To date, however, basic ecological information such as height-diameter allometry and stand-level wood density have not been mechanistically incorporated into methods for mapping forest carbon at regional and global scales. A better incorporation of these structural patterns in forests may reduce the considerable time needed to calibrate airborne data with ground-based forest inventory plots, which presently necessitate exhaustive measurements of tree diameters and heights, as well as tree identifications for wood density estimation. Here, we develop a new approach that can facilitate rapid LiDAR calibration with minimal field data. Throughout four tropical regions (Panama, Peru, Madagascar, and Hawaii), we were able to predict aboveground carbon density estimated in field inventory plots using a single universal LiDAR model (r ( 2 ) = 0.80, RMSE = 27.6 Mg C ha(-1)). This model is comparable in predictive power to locally calibrated models, but relies on limited inputs of basal area and wood density information for a given region, rather than on traditional plot inventories. With this approach, we propose to radically decrease the time required to calibrate airborne LiDAR data and thus increase the output of high-resolution carbon maps, supporting tropical forest conservation and climate mitigation policy.

Multidimensional tropical forest recovery.

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.

Lianas reduce biomass accumulation in early successional tropical forests.

Early successional tropical forests could mitigate climate change via rapid accumulation of atmospheric carbon. However, liana (woody vine) abundance and biomass has been increasing in many tropical forests over the past decades, which may slow the speed at which secondary forests accumulate biomass. Lianas decrease biomass accumulation in tropical forests, and may have a particularly strong effect on young forests by stalling tree growth. As forests mature, trees may outgrow or shed lianas, thus escaping some of the negative effects of lianas. Alternatively, lianas may have the strongest effect in older successional forests if the effect of lianas is commensurate with their density, which increases dramatically in the first decades of forest succession. We tested these two hypotheses using a landscape liana-removal experiment in 30 forest stands that ranged from 10 to 35 yr old in Central Panama. We measured tree growth and biomass accumulation in the stands every year from 2014 to 2017. We found that the effect of liana removal on large trees (≥20-cm diameter) decreased with forest age, supporting the hypothesis that lianas have the strongest negative effects on trees, and thus biomass uptake and carbon storage, in very young successional forests. Large trees accumulated more biomass in the absence of lianas in younger forests than in older forests (compared to controls) even after accounting for the effect of canopy completeness and crown illumination, implying that the detrimental effects of lianas go well beyond resource availability and crown health. There was no significant effect of lianas on small trees (1-20-cm diameter), likely because lianas seek light and thus do not deploy their leaves on small trees that are trapped in the forest understory. Our results show that high liana density early in forest succession reduces forest biomass accumulation by negatively impacting large trees, thus decreasing the capacity of young secondary forests to mitigate climate change. Although the negative effects of lianas on forest biomass diminish as forests age, they do not disappear, and thus lianas are an important component of tropical forest carbon budgets throughout succession.