Phylogenetic structure of angiosperm communities during tropical forest succession

The phylogenetic structure of ecological communities can shed light on assembly processes, but the focus of phylogenetic structure research thus far has been on mature ecosystems. Here, I present the first investigation of phylogenetic community structure during succession. In a replicated chronosequence of 30 sites in northeastern Costa Rica, I found strong phylogenetic overdispersion at multiple scales: species present at local sites were a non-random assemblage, more distantly related than chance would predict. Phylogenetic overdispersion was evident when comparing the species present at each site with the regional species pool, the species pool found in each age category to the regional pool or the species present at each site to the pool of species found in sites of that age category. Comparing stem size classes within each age category, I found that during early succession, phylogenetic overdispersion is strongest in small stems. Overdispersion strengthens and spreads into larger size classes as succession proceeds, corroborating an existing model of forest succession. This study is the first evidence that succession leaves a distinct signature in the phylogenetic structure of communities.     

Functional diversity and redundancy of tropical forest mammals over time

Globally, tropical rain forests comprise some of the most diverse and functionally rich ecosystems but are increasingly degraded by human impacts. Protected areas have been shown to conserve species diversity, but their effectiveness at maintaining functional diversity over time is less well known, despite the fact that functional diversity likely reveals more ecological information than taxonomic diversity. By extension, the degree to which species loss decreases functional diversity within protected areas is also unknown; functional redundancy may buffer communities from loss of functional diversity from some local extinctions. Using eight years of camera trap data, we quantified annual functional dispersion of the large mammal community in the Volcán Barva region of Costa Rica and tested for changes in functional dispersion over time in response to environmental and anthropogenic predictors. We quantified functional redundancy based on simulated declines in functional dispersion with species loss. Observed functional dispersion did not change significantly over time and was not associated with measured environmental or anthropogenic predictors. Quantitative modeling of observed functional traits over time did not identify significant changes. We did however find qualitative trends in relative trait proportions, which could be indicative of functional change in the future. We found high functional redundancy, with average simulated functional dispersion declining significantly only after 9 out of 21 large mammal species were lost from the community. We cautiously suggest that protected tropical rain forests can conserve functional diversity over the course of a decade even in heavily fragmented landscapes. Abstract in Spanish is available with online material.     

Litter dynamics recover faster than arthropod biodiversity during tropical forest succession

Litterfall and litter decomposition are key elements of nutrient cycling in tropical forests, a process in which decomposer communities such as macro-arthropods play a critical role. Understanding the rate and extent to which ecosystem function and biodiversity recover during succession is useful to managing the growing area of tropical successional forest globally. Using a replicated chronosequence of forest succession (5–15, 15–30, 30–45 years, and primary forest) on abandoned pastures in lowland tropical wet forest, we examined litterfall, litter chemistry, and effects of macro-arthropod exclusion on decomposition of two litter types (primary and 5- to 15-years-old secondary forest). Further, we assessed macro-arthropod diversity and community composition across the chronosequence. Overstory cover, litterfall, and litter nutrients reached levels similar to primary forest within 15–30 years. Young secondary forest litter (5–15 years) had lower initial N and P content, higher C:N, and decayed 60 percent faster than primary forest litter. The presence of macro-arthropods strongly mediated decomposition and nutrient release rates, increasing litter mass loss by 35–44 percent, N released by 53 percent, and P release by 84 percent. Forest age had no effect on soil nutrients, rates of litter decomposition, nutrient release, or macro-arthropod influence. In contrast, abundance and community composition of macro-arthropods remained significantly lower and distinct in all ages of secondary compared with primary forest. Order richness was lower in 5–15 years of secondary compared with primary forest. Our results suggest that in highly productive tropical wet forest, functional recovery of litter dynamics precedes recovery of decomposer community structure and biodiversity.     

The relationship between tree biodiversity and biomass dynamics changes with tropical forest succession

Theory predicts shifts in the magnitude and direction of biodiversity effects on ecosystem function (BEF) over succession, but this theory remains largely untested. We studied the relationship between aboveground tree biomass dynamics (Δbiomass) and multiple dimensions of biodiversity over 8–16 years in eight successional rainforests. We tested whether successional changes in diversity–Δbiomass correlations reflect predictions of niche theories. Diversity–Δbiomass correlations were positive early but weak later in succession, suggesting saturation of niche space with increasing diversity. Early in succession, phylogenetic diversity and functional diversity in two leaf traits exhibited the strongest positive correlations with Δbiomass, indicating complementarity or positive selection effects. In mid‐successional stands, high biodiversity was associated with greater mortality‐driven biomass loss, i.e. negative selection effects, suggesting successional niche trade‐offs and loss of fast‐growing pioneer species. Our results demonstrate that BEF relationships are dynamic across succession, thus successional context is essential to understanding BEF in a given system.     

Opposing mechanisms affect taxonomic convergence between tree assemblages during tropical forest succession

Whether successional forests converge towards an equilibrium in species composition remains an elusive question, hampered by high idiosyncrasy in successional dynamics. Based on long‐term tree monitoring in second‐growth (SG) and old‐growth (OG) forests in Costa Rica, we show that patterns of convergence between pairs of forest stands depend upon the relative abundance of species exhibiting distinct responses to the successional gradient. For instance, forest generalists contributed to convergence between SG and OG forests, whereas rare species and old‐growth specialists were a source of divergence. Overall, opposing trends in taxonomic similarity among different subsets of species nullified each other, producing a net outcome of stasis over time. Our results offer an explanation for the limited convergence observed between pairwise communities and suggest that rare species and old‐growth specialists may be prone to dispersal limitation, while the dynamics of generalists and second‐growth specialists are more predictable, enhancing resilience in tropical secondary forests.     

Small-scale patterning in the recruitment of forest species during succession in tropical dry forest,Mozambique

Within a few years of field abandonment on the coastal dunes, a considerable number of species of trees and woody lianes of the climax evergreen forest enter the succession. At a relatively large-scale of analysis (a 100 m² plot), there is no evidence of soil development during the early succession, with no accumulation of soil organic matter nor nutrients being detected. At this scale there is, therefore, little support for the facilitation model of succession. However, when smaller scales of analysis are employed (e.g. a plot equivalent to the canopy area of a sapling) a very different picture emerges; most recruitment of forest woody species occurs beneath saplings found in early succession. Nutrient concentrations were higher in soils under sapling canopies than in soils beyond sapling canopies. With increasing sapling size, an increasing number of seedlings of forest woody species were found and there is progressive soil amelioration. Thus, during the early succession, there is considerable small-scale patterning in soil development and recruitment. This pattern is largely established through bird dispersal, with the intensity of recruitment being dependent on the distance to seed sources.     

Pathways, mechanisms and predictability of vegetation change during tropical dry forest succession

The development of forest succession theory has been based on studies in temperate and tropical wet forests. As rates and pathways of succession vary with the environment, advances in successional theory and study approaches are challenged by controversies derived from such variation and by the scarcity of studies in other ecosystems. During five years, we studied development pathways and dynamics in a chronosequence spanning from very early to late successional stages (ca. 1–60 years) in a tropical dry forest of Mexico. We (1) contrasted dynamic pathways of change in structure, diversity, and species composition with static, chronosequence-based trends, (2) examined how structure and successional dynamics of guilds of trees shape community change, and (3) assessed the predictability of succession in this system. Forest diversity and structure increased with time but tree density stabilized early in succession. Dynamic pathways matched chronosequence trends. Succession consisted of two tree-dominated phases characterized by the development and dynamics of a pioneer and a mature forest species guild, respectively. Pioneer species dominated early recruitment (until ca. 10 years after abandonment), and declined before slower growing mature-forest species became dominant or reached maximum development rates (after 40–45 years). Pioneers promoted their replacement early in succession, while mature-forest species recruited and grew constantly throughout the process, with their lowest mortality coinciding with the peak of pioneer abundance. In contrast to prevailing stochastic views, we observed an orderly, community driven series of changes in this dry forest secondary succession. Chronosequences thus represent a valuable approach for revealing system-specific successional pathways, formulating hypotheses on causes and mechanisms and, in combination with repeated sampling, evaluating the effects of vegetation dynamics in pathway variation.     

The role of frugivorous bats in tropical forest succession

Discussion of successional change has traditionally focused on plants. The role of animals in producing and responding to successional change has received far less attention. Dispersal of plant propagules by animals is a fundamental part of successional change in the tropics. Here we review the role played by frugivorous bats in successional change in tropical forests. We explore the similarities and differences of this ecological service provided by New and Old World seed-dispersing bats and conclude with a discussion of their current economic and conservation implications. Our review suggests that frugivorous New World phyllostomid bats play a more important role in early plant succession than their Old World pteropodid counterparts. We propose that phyllostomid bats have shared a long evolutionary history with small-seeded early successional shrubs and treelets while pteropodid bats are principally dispersers of the seeds of later successional canopy fruits. When species of figs (Ficus) are involved in the early stages of primary succession (e.g. in the river meander system in Amazonia and on Krakatau, Indonesia), both groups of bats are important contributors of propagules. Because they disperse and sometimes pollinate canopy trees, pteropodid bats have a considerable impact on the economic value of Old World tropical forests; phyllostomid bats appear to make a more modest direct contribution to the economic value of New World tropical forests. Nonetheless, because they critically influence forest regeneration, phyllostomid bats make an important indirect contribution to the economic value of these forests. Overall, fruit-eating bats play important roles in forest regeneration throughout the tropics, making their conservation highly desirable.     

Viewing tropical forest succession as a three-dimensional dynamical system

As tropical forests are complex systems, they tend to be modelled either roughly via scaling relationships or in a detailed manner as high-dimensional systems with many variables. We propose an approach which lies between the two whereby succession in a tropical forest is viewed as a trajectory in the configuration space of a dynamical system with just three dependent variables, namely, the mean leaf-area index (LAI) and its standard deviation (SD) or coefficient of variation along a transect, and the mean diameter at breast height (DBH) of trees above the 90th percentile of the distribution of tree DBHs near the transect. Four stages in this forest succession are identified: (I) naturally afforesting grassland: the initial stage with scattered trees in grassland; (II) very young forest: mostly covered by trees with a few remaining gaps; (III) young smooth forest: almost complete cover by trees of mostly similar age resulting in a low SD; and (IV) old growth or mature forest: the attracting region in configuration space characterized by fluctuating SD from tree deaths and regrowth. High-resolution LAI measurements and other field data from Khao Yai National Park, Thailand show how the system passes through these stages in configuration space, as do simple considerations and a crude cellular automaton model.     

Change in community phylogenetic structure during tropical forest succession: evidence from New Guinea

Ecologists have recently interpreted patterns of phylogenetic distance among coexisting species as indicative of processes affecting community assembly during forest succession. We investigated plant community phylogenetic structure along a successional gradient in New Guinean lowland rain forest. We surveyed all trees with diameter at breast height ≥ 5 cm in nineteen 0.25 ha plots representing younger secondary, older secondary, and primary forest. We estimated plant community phylogeny from rbcL gene sequences to quantify change in phylogenetic structure during succession. Mean phylogenetic distance among co‐occurring trees increased with total basal area per plot, a proxy for forest age. Significant phylogenetic clustering was detected in secondary forest whereas primary forest was significantly over‐dispersed relative to null expectations. We examined the sensitivity of these patterns to various methods of branch length estimation and phylogenetic uncertainty. Power to detect community phylogenetic patterns when equal branch lengths were assumed was weak in comparison to direct molecular and time‐calibrated measures of divergence. Inferred change during forest succession was also robust to phylogenetic uncertainty so long as temporal information was incorporated in estimates of divergence. The observed patterns are consistent with processes of environmental filtering during tropical forest succession giving way to other processes in primary forests including density‐dependence.     

The role of remnant forest trees in tropical secondary succession

This paper emphasizes the contribution of remnant trees to the establishment of woody species during succession on abandoned fields and pastures in the Mexican rain forest area, Los Tuxtlas, Veracruz. Remnant trees, original large forest trees left in the clearings by traditional farmers will become natural perching sites for both passing and resident birds. Frugivorous birds drop or regurgitate seeds and fruits which fall under the canopies of remnant trees during their stay, thus contributing to an accumulation of species, which make these remnant trees into ‘regeneration nuclei’. The species transported into these sites belong chiefly to older stages of successional development and reach these otherwise isolated areas, counteracting the depauperization of tropical land, brought about by both intensive and extensive clearing. In a study of seven remnant trees, 29 woody species and two climbers were found, 86% of which are bird dispersed. The total number of species per tree varied from 6 to 15 and was higher under remnant trees with fruits attractive to birds. Floristic variations of the understorey as detected by detrended correspondence analysis was correlated with the relative amount of shade-tolerant primary and late secondary trees versus light dependent pioneers and early successional trees.     

Removal of cattle accelerates tropical dry forest succession in Northwestern Mexico

Domestic livestock influence patterns of secondary succession across forest ecosystems. However, the effects of cattle on the regeneration of tropical dry forests (TDF) in Mexico are poorly understood, largely because it is difficult to locate forests that are not grazed by cattle or other livestock. We describe changes in forest composition and structure along a successional chronosequence of TDF stands with and without cattle (chronic grazing or exclusion from grazing for ~ 8 year). Forest stands were grouped into five successional stages, ranging from recently abandoned to mature forest, for a total of 2.7 ha of the sampled area. The absence of cattle increased woody plant (tree and shrub) density and species richness, particularly in mid-successional and mature forest stands. Species diversity and evenness were generally greater in sites where cattle were removed and cattle grazing in early successional stands reduced establishment and/or recruitment of new individuals and species. Removal of cattle from forest stands undergoing succession appears to facilitate a progressive and non-linear change of forest structure and compositional attributes associated with rapid recovery, while cattle browsing acts as a chronic disturbance factor that compromises the resilience and structural and functional integrity of the TDF in northwestern Mexico. These results are important for the conservation, management, and restoration of Neotropical dry forests.     

中国西南地区热带森林演替序列碳动态

热带森林的破坏是全球性问题,我国西双版纳森林覆盖率受砍伐、火烧和短期耕种丢荒后,面积不断减少,取而代之的是大面积的不同演替状态的次生林。次生林演替过程中的碳储量和碳平衡的变化目前还鲜有研究,为了进一步揭示我国西南地区热带森林演替对于碳蓄积的影响,并制定更科学的热带森林经营管理措施,以结构复杂、生物多样和生物量巨大的热带森林为研究对象,并利用3个热带次生林的样地的实测数据,探讨了不同演替状态的热带次生林的碳储量变化,以及森林的净碳蓄积,死亡碳损失和更新碳增长等碳动态规律,分析表明:(1)在森林的演替过程中,森林的胸径分布频度从近正态分布逐渐向小径级的偏态分布发展,也就是随着演替的进展,小径级林木所占的比例越来越高。(2)热带次生林在森林固碳方面发挥着不可忽略的作用。(3)小的干扰,会波及森林的碳动态;大的干扰,如火灾和砍伐,将导致森林的次生演替,对森林的碳动态产生不可逆转的改变。(4)干旱事件是影响凋落物的季节和年间动态的原因,也是短时间尺度上影响碳平衡的一个重要因子。(5)不论原生林还是次生林,大树在生态系统碳动态方面皆扮演着重要的角色,因此本研究推荐注重大树的研究。     

Functional and genetic diversity changes through time in a cloud forest ant assemblage

Climate change in the Neotropics is causing upslope range shifts. We used arrays of ant species collected in a cloud forest at 1,500 m in Área de Conservación Guanacaste (ACG), northwestern Costa Rica, collected in two time periods (1998–2000 and 2008–2011) to measure changes in species richness and diversity over a decade. Using metrics of community structure, we found that the species assemblage in the collections from the 1990s was significantly phylogenetically clustered and functionally less diverse as compared to collections from the early 2000s. At both time points, the assemblages were significantly phylogenetically clustered and while the difference in functional diversity between the time points was not significant, the ant assemblage has become lighter in color (on average) over time. When individual species are considered, the overall pattern of replacement is consistent with the cloud forest ant assemblage being colonized by arrivals from lower elevation forests. The invertebrate communities on cloud forested mountain tops are especially vulnerable to a changing climate as there are two factors working together; no higher terrain to which they can move and the invasion of more and more taxa from lower downslope. This vulnerability is already measurable.     

Resilience of tropical dry forests – a meta‐analysis of changes in species diversity and composition during secondary succession

Assessing the recovery of species diversity and composition after major disturbance is key to understanding the resilience of tropical forests through successional processes, and its importance for biodiversity conservation. Despite the specific abiotic environment and ecological processes of tropical dry forests, secondary succession has received less attention in this biome than others and changes in species diversity and composition have never been synthesised in a systematic and quantitative review. This study aims to assess in tropical dry forests 1) the directionality of change in species richness and evenness during secondary succession, 2) the convergence of species composition towards that of old‐growth forest and 3) the importance of the previous land use, precipitation regime and water availability in influencing the direction and rate of change. We conducted meta‐analyses of the rate of change in species richness, evenness and composition indices with succession in 13 tropical dry forest chronosequences. Species richness increased with succession, showing a gradual accumulation of species, as did Shannon evenness index. The similarity in species composition of successional forests with old‐growth forests increased with succession, yet at a low rate. Tropical dry forests therefore do show resilience of species composition but it may never reach that of old‐growth forests. We found no significant differences in rates of change between different previous land uses, precipitation regimes or water availability. Our results show high resilience of tropical dry forests in term of species richness but a slow recovery of species composition. They highlight the need for further research on secondary succession in this biome and better understanding of impacts of previous land‐use and landscape‐scale patterns. Synthesis Secondary forests account for an increasing proportion of remaining tropical forest. Assessing their resilience is key to conservation of their biodiversity. Our study is the first meta‐analysis of species changes during succession focussing on tropical dry forests, a highly threatened yet understudied biome. We show a gradual species accumulation and convergence of composition towards that of old‐growth forests. While secondary tropical dry forests offer good potential for biodiversity conservation, their capacity for recovery at a sufficient rate to match threats is uncertain. Further research on this biome is needed to understand the effect of land use history and landscape processes.