Tropical Forest Succession Research Articles

Tropical carbon sink accelerated by symbiotic dinitrogen fixation

A major uncertainty in the land carbon cycle is whether symbiotic nitrogen fixation acts to enhance the tropical forest carbon sink. Nitrogen-fixing trees can supply vital quantities of the growth-limiting nutrient nitrogen, but the extent to which the resulting carbon–nitrogen feedback safeguards ecosystem carbon sequestration remains unclear. We combine (i) field observations from 112 plots spanning 300 years of succession in Panamanian tropical forests, and (ii) a new model that resolves nitrogen and light competition at the scale of individual trees. Fixation doubled carbon accumulation in early succession and enhanced total carbon in mature forests by ~10% (~12MgC ha−1) through two mechanisms: (i) a direct fixation effect on tree growth, and (ii) an indirect effect on the successional sequence of non-fixing trees. We estimate that including nitrogen-fixing trees in Neotropical reforestation projects could safeguard the sequestration of 6.7 Gt CO2 over the next 20 years. Our results highlight the connection between functional diversity of plant communities and the critical ecosystem service of carbon sequestration for mitigating climate change.

Do lianas shape ant communities in an early successional tropical forest?

AbstractAlmost half of lowland tropical forests are at various stages of regeneration following deforestation or fragmentation. Changes in tree communities along successional gradients have predictable bottom‐up effects on consumers. Liana (woody vine) assemblages also change with succession, but their effects on animal succession remain unexplored. Here we used a large‐scale liana removal experiment across a forest successional chronosequence (7–31 years) to determine the importance of lianas to ant community structure. We conducted 1,088 surveys of ants foraging on and living in trees using tree trunk baiting and hand‐collecting techniques at 34 paired forest plots, half of which had all lianas removed. Ant species composition, β‐diversity, and species richness were not affected by liana removal; however, ant species co‐occurrence (the coexistence of two or more species in a single tree) was more frequent in control plots, where lianas were present, versus removal plots. Forest stand age had a larger effect on ant community structure than the presence of lianas. Mean ant species richness in a forest plot increased by ca. 10% with increasing forest age across the 31‐year chronosequence. Ant surveys from forest >20 years old included more canopy specialists and fewer ground‐nesting ant species versus those from forests <20 years old. Consequently, lianas had a minimal effect on arboreal ant communities in this early successional forest, where rapidly changing tree community structure was more important to ant species richness and composition.

Wood anatomy of dominant species with contrasting ecological performance in tropical dry forest succession

Environmental restrictions faced by successional species (those occurring in recovering vegetation) change gradually between early and late succession. In tropical dry forest (TDF), legume species dominate early succession, that is, accumulate more biomass than coexisting species, but later are gradually replaced by late successional species. Stem anatomical traits are involved in water conduction efficiency and safety, and thus biomass accumulation. We hypothesized that anatomical characteristics of dominant species may explain their status in their respective successional community. We analyzed the wood anatomy of TDF dominant successional species: Mimosa eurycarpa and M. tenuiflora (early successional), Lysiloma divaricatum (late successional), and Euphorbia schlechtendalii (successionally ambiguous). Anatomical variables and indices were estimated and compared between species through linear modeling. Unexpectedly, Mimosa species differed in porosity type, which is one characteristic linked to water stress tolerance. M. eurycarpa and L. divaricatum are ring-porous (an uncommon feature in tropical species), whereas M. tenuiflora and E. schlechtendalii displayed diffuse porosity. M. eurycarpa had the lowest vulnerability index (VI) and the highest relative hydraulic conductivity. Within-individual differences in relative water conduction in ring-porous species, driven by high vessel density in earlywood and low VI in latewood, likely represent a highly advantageous strategy in water-stressed successional environments.

The accumulation of species and recovery of species composition along a 70 year succession in a tropical secondary forest

Abstract The majority of global forests are secondary and are at different stages of succession. Assessing the dynamics of species richness and similarity, and quantifying the importance of environmental filtering, dispersal limitation and other stochastic processes are essential to understanding the mechanisms of succession for forest restoration. In this article, we explored the accumulation of species, the relative importance of spatial distance, environmental factors and stand age in determining species composition of a tropical secondary forest succession in Hong Kong. Twenty-eight plots with median age of 7, 20, 39, 61 and over 70 years in the secondary forest were established and surveyed, and the indicator species for each age class were identified. Species composition shows large variation both within and between age classes, while species richness in the old growth forest (>70 yr) was significantly lower than in the mid-age classes. Rarefied species richness showed a rapid accumulation during early succession, but species richness levelled off from 20 yrs onwards. Variation partitioning indicated that spatial distance alone explained 33% of the variation in species composition, followed by environmental distance (8%) and stand age (1%). The results of Nonmetric Multidimensional Scaling suggested that idiosyncratic successional pathways and alternative stable states might be prevalent. Our results suggested dispersal limitation was the main limiting factor in explaining the turnover of species during forest succession, while environmental filtering played a lesser role in shaping species distributions. Our results highlight the importance of active restoration in overcoming the barriers of succession in secondary vegetation in the tropics.

Shade tolerance within the context of the successional process in tropical rain forests

Shade tolerance (the capacity to survive and grow over long periods under shade) is a key component of plant fitness and the foundation of current theories of forest succession in tropical rain forests. It serves as a paradigm to understand the optimal allocation of limited resources under dynamic light regimes. I analyze how tropical rain forest succession influences the expression of ecophysiological mechanisms leading to shade tolerance, and identify future areas that will increase our understanding of the ecological and evolutionary consequences of this phenomenon. Shade tolerance is a multivariate, continuous functional trait reflecting the growth-mortality trade-off of investing resources under limited light vs. exploiting high light conditions. I propose the life cycle successional trajectory model of Gómez-Pompa & Vázquez-Yanes as an integrative tool to understand tropical rain forest succession. This model shows how species distribute along the successional environmental gradient based on their degree of shade tolerance and represents a more integrative paradigm to understand the interface between different aspects of species diversity (ontogenetic variation and functional diversity) throughout succession. It proposes that different trait combinations determining shade tolerance are expressed at different stages of the life cycle, which affects how and when plants enter the successional trajectory. Models explaining the expression of shade tolerance (resource availability, carbon gain, CSR, resource competition) are based on whole-plant economics and are not mutually exclusive. The analysis of shade tolerance is biased towards tree seedlings in the understory of mature forests. Other life stages (juvenile and adult trees), life forms, and microhabitats throughout the forest profile are almost always excluded from these analyses. More integrative explanations based on the distribution of functional traits among species, ontogenetic stages, and the nature of the environmental gradient are being developed based on long-term data and chronosequence comparisons. In summary, shade-tolerance is a complex phenomenon, is determined by multiple characters that change ontogenetically over space and time and entails considerable plasticity. Current methods do not account for this plasticity. Understanding the nature of shade tolerance and its functional basis is critical to comprehending plant performance and improving the management, restoration and conservation of tropical rain forests given the combined threats of global warming and habitat loss

Observed variation in soil properties can drive large variation in modelled forest functioning and composition during tropical forest secondary succession.

Censuses of tropical forest plots reveal large variation in biomass and plant composition. This paper evaluates whether such variation can emerge solely from realistic variation in a set of commonly measured soil chemical and physical properties. Controlled simulations were performed using a mechanistic model that includes forest dynamics, microbe-mediated biogeochemistry, and competition for nitrogen and phosphorus. Observations from 18 forest inventory plots in Guanacaste, Costa Rica were used to determine realistic variation in soil properties. In simulations of secondary succession, the across-plot range in plant biomass reached 30% of the mean and was attributable primarily to nutrient limitation and secondarily to soil texture differences that affected water availability. The contributions of different plant functional types to total biomass varied widely across plots and depended on soil nutrient status. In Central America, soil-induced variation in plant biomass increased with mean annual precipitation because of changes in nutrient limitation. In Central America, large variation in plant biomass and ecosystem composition arises mechanistically from realistic variation in soil properties. The degree of biomass and compositional variation is climate sensitive. In general, model predictions can be improved through better representation of soil nutrient processes, including their spatial variation.

Recovery of amphibian, reptile, bird and mammal diversity during secondary forest succession in the tropics

In tropical regions, many studies have focused on how vegetation and ecosystem processes recover following the abandonment of anthropogenic activities, but less attention has been given to the recovery patterns of vertebrates. Here we conduct a meta‐analysis (n = 147 studies) of amphibian, reptile, bird and mammal recovery during tropical secondary forest succession (i.e. natural regeneration). For each taxonomic group, we compared changes in species richness and compositional similarity during natural secondary succession to reference forests (mature or old growth forest). In addition, we evaluated the response of forest specialists and the change in bird and mammal functional groups during natural secondary succession in the tropical moist forest biome. Overall, species richness of all groups reached levels of the reference forests during natural secondary succession, but this was not the case for species compositional similarity. The delay in recovery of forest specialists may be the reason for the delay in recovery of species compositional similarity. Overall, vertebrate recovery increased with successional stage, but other potential predictors of diversity recovery, such as, the geographical setting (amphibian and reptile species compositional similarity recovered more rapidly on islands), rainfall (mammal species richness and compositional similarity recovered faster in regions of low rainfall), and the landscape context (amphibian, reptile and mammal species compositional similarity recovered faster in regions with more forest patches) influenced vertebrate recovery. These results demonstrate the important role of secondary forests in providing habitat for many vertebrates, but the slow recovery of species compositional similarity, forest specialists and some functional groups (e.g. insectivorous birds) highlighted the challenge of secondary forest persistence, and strongly argues for the continued protection of old growth/mature forest as habitat for forest specialists and as sources for secondary forest sites.

Long‐term recovery of the functional community assembly and carbon pools in an African tropical forest succession

AbstractOn the African continent, the population is expected to expand fourfold in the next century, which will increasingly impact the global carbon cycle and biodiversity conservation. Therefore, it is of vital importance to understand how carbon stocks and community assembly recover after slash‐and‐burn events in tropical second growth forests. We inventoried a chronosequence of 15 1‐ha plots in lowland tropical forest of the central Congo Basin and evaluated changes in aboveground and soil organic carbon stocks and in tree species diversity, functional composition, and community‐weighted functional traits with succession. We aimed to track long‐term recovery trajectories of species and carbon stocks in secondary forests, comparing 5 to 200 + year old secondary forest with reference primary forest. Along the successional gradient, the functional composition followed a trajectory from resource acquisition to resource conservation, except for nitrogen‐related leaf traits. Despite a fast, initial recovery of species diversity and functional composition, there were still important structural and carbon stock differences between old growth secondary and pristine forest, which suggests that a full recovery of secondary forests might take much longer than currently shown. As such, the aboveground carbon stocks of 200 + year old forest were only 57% of those in the pristine reference forest, which suggests a slow recovery of aboveground carbon stocks, although more research is needed to confirm this observation. The results of this study highlight the need for more in‐depth studies on forest recovery in Central Africa, to gain insight into the processes that control biodiversity and carbon stock recovery.

Trait‐based community assembly pattern along a forest succession gradient in a seasonally dry tropical forest

AbstractWe aimed to determine the important functional dimensions that may drive forest succession and community assembly patterns in dry tropical forests. We investigated whether there were patterns in specific functional strategies during succession in the dry tropical forests of the Florida Keys, whose unique physical setting includes nutrient‐stressed, salt‐stressed, and water‐limited environments. The study, which focused on ten traits, determined the leading trait dimensions by which species differentiate from one another in the study area. The general patterns of trait covariation at individual sites and among species were analyzed using principal component analysis. Trait niche overlap indices were calculated for all species sampled across all plots. Evidence for/against likely community assembly processes was tested using the coefficient of heterogeneity to determine whether variation within and among five key traits was clustered, random, or evenly distributed across young, old, or all measured forest stands. A combination of plant architecture, wood density, and three leaf traits (specific leaf area, leaf phosphorus, and leaf nitrogen) comprised a key set of functional traits that are important for understanding the community assembly process in dry tropical forest. Older forest stands were dominated by species with low specific area, low leaf nitrogen content, dense wood, and deeper and narrower canopies. Trees of old forests had leaves with lighter carbon isotope composition, suggesting that such individuals were making more efficient use of scarce water. Tests of trait distributions showed significant clustering across forests of all ages. When individual trait distributions in old stands were tested, they displayed either randomly or evenly distributed traits across trait niche space, indicating that resource partitioning was predominant in shaping community composition. Physical traits of trees in young communities are associated with resource acquisitive strategies, while old communities are dominated by species with traits that enhance survival in environments defined by competition or chronic resource scarcity.

Do the distribution patterns of plant functional traits change during early secondary succession in tropical montane cloud forests?

Several ecological processes intervene in the assembling of plant communities, such as environmental filtering and biotic interactions (e.g., competition, facilitation). The analysis of the distribution of the functional traits of plants helps to identify which of these processes is involved in the succession of a plant community assembly. We analyzed the distribution patterns of two groups of functional traits in a chronosequence spanning the first 40 years of secondary succession (SS) of a plant community of the tropical montane cloud forest (TMCF). One group of traits (leaf area, leaf thickness, leaf phosphorous and leaf nitrogen) is associated to the resource-acquisition strategy, and the second group to the conservation strategy (wood density, leaf dry mass per area unit, leaf density and leaf carbon). For each group, two multi-trait indices were quantified, namely functional divergence and dispersion. The number of individuals per plot was randomized in order to produce a null model and analyze the simulated index-distribution patterns. Then, the standardized size effect (SES) was calculated. Our results showed that both, divergence and dispersion, decreased towards the end of the successional gradient, leading to convergent patterns in the conservation traits group. The SES acquired negative values, which suggests that convergent patterns may be driven by environmental filtering. Functional diversity showed a tendency to decrease even when a species turnover between successional stages had occurred (ANOSIM, R Global = 0.64, P = 0.01). The dominance of Oreomunnea mexicana, at the end of the chronosequence, may have a particularly important role in the observed functional convergence.

Relationship between soil properties and leaf functional traits in early secondary succession of tropical montane cloud forest

AbstractRecognizing soil properties associated with the plant strategy in relation to use and acquisition of resources during early secondary succession (ESS) is an important issue in plant ecology. We analyzed the relationship between 10 leaf functional traits (LFTs) and 17 soil properties including soil nutrients, transformation flows and pH along a plant chronosequence spanning first 40 years of ESS in a tropical mountain cloud forest. To recognize the multivariate variation of soil properties, a principal component analysis (PCA) was performed. The first axis of PCA was mainly associated with soil pH, P, Ca, Mg, K, C:N and N transformation flow, and this first axis was tightly associated with vegetation ages. The community weighted mean of the LFTs was also associated with vegetation ages. In particular, leaf P declined while leaf density, C, C:P and N:P increased with vegetation ages. Our analyses suggest associations between soil properties (the first axis of the PCA) and the LFTs (leaf area, leaf thickness, leaf density, leaf C, leaf C:P and leaf N:P) over the plant chronosequence. A fast acquisition strategy was observed in the younger sites and a resource conservation strategy in the older sites. Our results highlight the possible importance of changes in soil properties along secondary succession on plant functional traits in tropical cloud forest.

Análisis funcional del secuestro de carbono en un bosque seco tropical interandino

Los cambios en los rasgos funcionales relacionados con el secuestro de carbono durante la sucesión secundaria del Bosque Seco Tropical (bs-T) en Colombia son poco conocidos. Aquí analizamos cómo varía el secuestro de carbono a lo largo de tres etapas sucesionales: pastizales abandonados, arbustales y bosques secundarios; siguiendo un enfoque basado en rasgos a nivel de especie y comunidad. Medimos siete rasgos funcionales en 41 especies leñosas. Identificamos cuatro tipos funcionales de plantas (TFPs) relacionados con estrategias conservativas y adquisitivas. Tanto el área basal como la abundancia relativa de los TFPs adquisitivos disminuyeron a lo largo de las etapas de sucesión. Por el contrario, los patrones de cambio para los TFPs conservativos fueron menos consistentes. La Media Ponderada de la Comunidad (MPC) del área foliar, contenido foliar de materia seca, densidad de madera y altura máxima fueron significativamente mayores en los bosques secundarios respecto a los estados sucesionales más tempranos. Estos hallazgos ayudan a ampliar la comprensión del secuestro de carbono durante la sucesión del bs-T.

How does avian seed dispersal shape the structure of early successional tropical forests?

Abstract Frugivores shape plant communities via seed dispersal of fleshy‐fruited plant species. However, the structural characteristics that frugivores impart to plant communities are little understood. Evaluating how frugivores structure plant communities via the nonproportional use of available fruit resources is critical to understand the functioning of ecosystems where fleshy‐fruited plant species are dominant, such as tropical forests. We performed a seed‐addition field experiment to investigate how frugivorous birds shape the composition and richness of forests during early stages of secondary succession in cleared areas in Puerto Rico. The experiment tested whether the bird‐generated seed rain and the subsequent early successional plant communities were proportional representations of the fleshy‐fruited species that dominated the surrounding community. Experimental treatments consisted of patches with (a) seed‐additions by wild birds attracted to experimental patches with pole perches, (b) manual seed‐additions proportional to fruit abundance at the local scale (≤50 m from experimental plots) and (c) manual seed‐additions proportional to fruit abundance at the landscape scale (entire study site). Birds' seed‐additions differed in composition and abundance to expectations based on fruit availability at local and landscape scales. Treatments with seeds added by birds had the highest species richness in both the seed rain and the emergence stages despite how, on average, the monthly richness in the landscape‐scale treatment was double that of birds and the local‐scale treatment. This phenomenon was explained by the highest heterogeneity from the bird seed‐addition treatment across months, and the lowest seed per capita emergence rates in landscape treatments. Rather than reflecting relative fruit abundance, birds biased seed rain and per capita emergence towards a non‐random mixture of both small‐ and large‐seeded species, resulting in richer and distinct plant communities. Because frugivory and seed dispersal patterns depart from random encounters between frugivores and plants in communities, successional forests are characterized by an overrepresentation of proportionally rare plant species, and decreases in the dominance of many common species. Thus, for regenerating tropical forests, frugivory can function as mechanism that promotes persistence of rare plant species and their coexistence with more abundant plants. A plain language summary is available for this article.

Rodent community responses to vegetation and landscape changes in early successional stages of tropical dry forest

The extent to which the array of land covers dominating anthropogenic landscapes impacts on biodiversity remains today an open question of great relevance. The characteristics of the animal communities inhabiting tropical forest early successional stages can determine, to a large extent, the course of secondary succession and natural regeneration. In this study, we evaluated the response of terrestrial rodents to variations in vegetation and landscape attributes, in early stages of tropical dry forest succession, at three integration levels: (1) communities, (2) populations, and (3) individuals. Our results reveal a rodent response to the variation in vegetation and landscape attributes, which indeed was largely modulated by seasonality, and determined by the focal spatial scale considered. In general, most of the species were favored by vegetation structure simplification and increase in understory density, probably due to the increase in refuge and trophic resources. Nevertheless, one species, Lyomis pictus, was sensitive to variation in vegetation composition and negatively affected by a reduction in vegetation structural complexity, which could be explained by the high number of tree seeds included in their diet. We found a higher diversity of rodents in sites surrounded by a high coverage and connectivity of dry and riparian forest, probably because large forest patches act as sources of species and the forest connectivity favors the colonization process toward sites under regeneration. Our results showed that preserving remnants of the original vegetation and implementing management actions at different spatial scales in anthropogenic landscapes is critical for preventing: (1) an increase in generalist populations, (2) rapid faunal homogenization, (3) defaunation, and (4) the loss of specialist species; with profound implications for the ecology of forest, ecosystem services and human health.

Quantifying Changes on Forest Succession in a Dry Tropical Forest Using Angular-Hyperspectral Remote Sensing

The tropical dry forest (TDF) is one the most threatened ecosystems in South America, existing on a landscape with different levels of ecological succession. Among satellites dedicated to Earth observation and monitoring ecosystem succession, CHRIS/PROBA is the only satellite that presents quasi-simultaneous multi-angular pointing and hyperspectral imaging. These two characteristics permit the study of structural and compositional differences of TDFs with different levels of succession. In this paper, we use 2008 and 2014 CHRIS/PROBA images from a TDF in Minas Gerais, Brazil to study ecosystem succession after abandonment. Using a −55° angle of observation; several classifiers including spectral angle mapper (SAM), support vector machine (SVM), and decision trees (DT) were used to test how well they discriminate between different successional stages. Our findings suggest that the SAM is the most appropriate method to classify TDFs as a function of succession (accuracies ~80 for % for late stage, ~85% for the intermediate stage, ~70% for early stage, and ~50% for other classes). Although CHRIS/PROBA cannot be used for large-scale/long-term monitoring of tropical forests because of its experimental nature; our results support the potential of using multi-angle hyperspectral data to characterize the structure and composition of TDFs in the near future.

Recovery of Functional Diversity Following Shifting Cultivation in Tropical Monsoon Forests

The relationship between biodiversity and ecosystem functioning is an important issue in ecology. Plant functional traits and their diversity are key determinants of ecosystem function in changing environments. Understanding the successional dynamics of functional features in forest ecosystems is a first step to their sustainable management. In this study, we tested the changes in functional community composition with succession in tropical monsoon forests in Xishuangbanna, China. We sampled 33 plots at three successional stages—~40-year-old secondary forests, ~60-year-old secondary forests, and old growth forests—following the abandonment of the shifting cultivation land. Community-level functional traits were calculated based on measurements of nine functional traits for 135 woody plant species. The results show that the community structures and species composition of the old-growth forests were significantly different to those of the secondary stands. The species diversity, including species richness (S), the Shannon–Weaver index (H), and Pielou’s evenness (J), significantly increased during the recovery process after shifting cultivation. The seven studied leaf functional traits (deciduousness, specific leaf area, leaf dry matter content, leaf nitrogen content, leaf phosphorus content, leaf potassium content and leaf carbon content) changed from conservative to acquisitive syndromes during the recovery process, whereas wood density showed the opposite pattern, and seed mass showed no significant change, suggesting that leaf traits are more sensitive to environmental changes than wood or seed traits. The functional richness increased during the recovery process, whereas the functional evenness and divergence had the highest values in the 60-year-old secondary communities. Soil nutrients significantly influenced functional traits, but their effects on functional diversity were less obvious during the secondary succession after shifting cultivation. Our study indicates that the recovery of tropical monsoon forests is rather slow; secondary stands recover far less than the old growth stands in terms of community structure and species and functional diversity, even after about half a century of recovery, highlighting the importance of the conservation of old growth tropical monsoon forest ecosystems.

Effects of Habitat Structure, Plant Cover, and Successional Stage on the Bat Assemblage of a Tropical Dry Forest at Different Spatial Scales

Bats play a fundamental role in ecosystem functioning since they are responsible for several ecological services such as seed dispersal and pollination. Therefore, assessing the effects of habitat structure at different scales on the bat assemblage is extremely important for supporting conservation strategies. The objective of the present study was to investigate the effects of habitat structure at multiple spatial scales on the bat assemblages and their variation along a gradient of secondary succession in a Brazilian tropical dry forest. Our results suggest that bat abundance is higher in areas close to mature forests, which shows the important role of those habitats as refuges for the regional bat fauna (in a fragmented landscape) and for the maintenance of ecosystem services provided by this group in tropical dry forests in a landscape context. In addition, bat abundance was lower in protected areas whose surroundings were better preserved (greater forest extension). This unexpected finding could result from an altered behavior in areas under a strong influence of a fruit crop matrix. Finally, we showed that the effects of the surroundings depend on the successional stage of the area under analysis. Late forests are more susceptible to variations in the forest cover in their surroundings, which show the higher fragility of these environments.

Object-based, multi-sensor habitat mapping of successional age classes for effective management of a 70-year secondary forest succession

Multi-temporal change detection over decades including the pre-satellite era is challenging due to the different image types available over time, and this explains the scarcity of long-term studies of vegetation succession which can play a pivotal role in the restoration of biodiversity in regenerating forests. This study describes a semi-automated, object-based habitat classification method for change detection of tropical forest succession since 1945. The study uses a set of black and white aerial photographs and high-resolution satellite images which differ in quality and resolution, to investigate forest successional patterns and their implications for informed ecosystem and land rehabilitation management. For optimized habitat boundary delineation from black and white aerial photographs and panchromatic satellite images, three levels of hierarchical image object primitives were created. The minimum object sizes of 50 m2, 500 m2, and 1000 m2 maximized inter-object and minimized intra-object variability according to the scale of habitat patches and imagery used. Object-Based Image Analysis (OBIA) provided additional Grey-Level Co-occurrence Matrix (GLCM) textural features of segmented objects which helped to incorporate knowledge-based rule-sets into the final habitat classification which was done manually. Results show accuracies for grassland greater than 94%, monoculture plantations were distinguished from natural forest with 95% accuracy, and isolated mature stands of natural forest achieved 75% accuracy. Consideration of multi-date images increased the accuracy of distinguishing between mixed plantations and natural forest as well as between shrubland and young secondary forest. The resulting maps of vegetation structure at five time periods from 1945 to present gave new insights into the ecological processes of secondary forest succession. These include the surprising rapid rate of natural forest regeneration, at an annual rate of 7.7% from 1945 to 2014, and an even faster rate of 11% during a period when hill fires were controlled. The last areas to succeed to forest are those which are still, or at some time have been under exotic mono-cultural plantations. This suggests that long term protection from hill fire would be a better option for assisting natural succession in the landscape than plantations, which are both costly, and act as barriers to natural succession. Overall, with more than 92% mapping accuracy, the method can be adapted for other multi-temporal, multi-sensor studies as it enables inclusion of spatial theories by dividing the satellite image into time-consistent geographic entities according to the scale of target objects and image resolution. The accurate maps of forest cover patches at different successional stages can also help in site specific management of the recovering forest, such as introduction of shrub seedlings to bridge bottlenecks in seed dispersal according to shrub density and dispersal distances for forest birds. Late successional tree species can also be introduced in areas where only early successional species are present after 50 years of succession.

Nitrogen fixer abundance has no effect on biomass recovery during tropical secondary forest succession

Abstract Nitrogen‐fixing trees (N2 fixers) provide new nitrogen critical for rapid biomass accumulation of tropical forests during early secondary succession, but it remains unclear how the abundance of N2 fixers in the forest community affects the growth of non‐fixers or the primary productivity of the whole forest. On the one hand, N2 fixers may enhance forest productivity by providing a facilitative effect through the provision of plant‐available nitrogen to non‐fixing trees. On the other hand, N2 fixers may suppress the growth of non‐fixers by growing faster and competing more vigorously for light and other resources. A third alternative is that the growth of N2 fixers themselves accumulate biomass rapidly, while having a neutral effect on non‐fixers, leading to an overall increase in forest biomass. We examine these alternative hypotheses using 5‐year tree census data from 88 plots in 44 seasonal tropical moist secondary forests (3–32 years old) across a human‐modified landscape in central Panama. We examined whether N2 fixers accumulated biomass more rapidly than non‐fixers, and how relative biomass of N2 fixers as a functional group and as individual species influenced the growth of non‐fixer and whole stand primary productivity. Surprisingly, we found no evidence for either a net competitive or a facilitative effect of N2 fixers as a functional group or individual species on the biomass recovery in these young forests. N2 fixers did not grow faster than non‐fixers. Individual mortality rates were lower among N2 fixers, but biomass losses due to mortality were similar between the two groups. Overall, we found no relationship between the relative abundance of N2 fixers and stand primary productivity during succession. Synthesis. Nitrogen‐fixing trees may be critical for reducing nitrogen limitation and accelerating biomass growth during tropical secondary forest succession, thereby impacting the global carbon cycle. However, our findings indicate that, in early successional seasonal tropical moist forests, nitrogen fixers provide neither a net competitive nor a facilitative effect on non‐fixing trees or the whole forest stand, likely because tropical nitrogen fixers utilize facultative fixation and hence abundance poorly approximates the ecosystem function of fixation. Our results indicate that models should not simply scale symbiotic fixation and its effects from nitrogen‐fixing tree abundance.

Land-use type strongly shapes community composition, but not always diversity of soil microbes in tropical China

Forest secondary succession and conversion to an agricultural use are rapidly altering tropical forests, and this alteration has consequences on soil microbial communities. Using high-throughput sequencing of 16S rRNA genes and internal transcribed spacer regions, we compared bacteria and fungi community composition and diversity in soils between the three stages of forest succession (primary forest, PF; secondary forest, SF; pioneer plant community dominated by Macaranga denticulate trees, PP) and two agricultural drylands (rubber plantation, RP; Plukenetia volubilis plantation, PV) in Xishuangbanna, a tropical region of China, and identified the factors associated with the shift in community composition across the five land-use types. The results indicated that the most abundant bacterial genera (Bacillus, Kitasatospora, Nitrospira and Streptacidiphilus) were found at differentially relative abundances among the five land-use types. Through the analysis of indicator species, several bacterial species were significantly associated with the PP site; one to four species were significantly associated to the other sites. Strikingly, almost all fungal genera had site-specific characteristic. Soil properties, especially pH and available P and K, were associated with microbial community composition. Across the three stages of forest succession, bacterial and fungal richness and bacterial alpha diversity had the lowest levels in the earliest stage (PP), but there were no significant differences in microbial richness or diversity between the late stages (PF vs. SF). Compared to PF and SF with their similar bacterial and fungal diversity, the agricultural drylands (RP and PV) had higher bacterial richness but lower fungal richness, indicating that both PF and SF can act as reservoirs for the recolonisation of forest-associated microbes. Overall, these results showed a distinct difference in soil microbe taxonomic composition, especially in fungi, among the various land-use types, even at a very small geographical scale (<4 km), but no great difference of microbial diversity was found between the late stages of forest succession and agricultural drylands. Tropical forest succession and forest conversion to agricultural drylands strongly affect the distribution of microbial species, whereas microbial diversity may not always tightly follow the same successional trajectories.