ABSTRACT
Old‐growth tropical forests are being extensively deforested and fragmented worldwide. Yet forest recovery through succession has led to an expansion of secondary forests in human‐modified ...tropical landscapes (HMTLs). Secondary forests thus emerge as a potential repository for tropical biodiversity, and also as a source of essential ecosystem functions and services in HMTLs. Such critical roles are controversial, however, as they depend on successional, landscape and socio‐economic dynamics, which can vary widely within and across landscapes and regions. Understanding the main drivers of successional pathways of disturbed tropical forests is critically needed for improving management, conservation, and restoration strategies. Here, we combine emerging knowledge from tropical forest succession, forest fragmentation and landscape ecology research to identify the main driving forces shaping successional pathways at different spatial scales. We also explore causal connections between land‐use dynamics and the level of predictability of successional pathways, and examine potential implications of such connections to determine the importance of secondary forests for biodiversity conservation in HMTLs. We show that secondary succession (SS) in tropical landscapes is a multifactorial phenomenon affected by a myriad of forces operating at multiple spatio‐temporal scales. SS is relatively fast and more predictable in recently modified landscapes and where well‐preserved biodiversity‐rich native forests are still present in the landscape. Yet the increasing variation in landscape spatial configuration and matrix heterogeneity in landscapes with intermediate levels of disturbance increases the uncertainty of successional pathways. In landscapes that have suffered extensive and intensive human disturbances, however, succession can be slow or arrested, with impoverished assemblages and reduced potential to deliver ecosystem functions and services. We conclude that: (i) succession must be examined using more comprehensive explanatory models, providing information about the forces affecting not only the presence but also the persistence of species and ecological groups, particularly of those taxa expected to be extirpated from HMTLs; (ii) SS research should integrate new aspects from forest fragmentation and landscape ecology research to address accurately the potential of secondary forests to serve as biodiversity repositories; and (iii) secondary forest stands, as a dynamic component of HMTLs, must be incorporated as key elements of conservation planning; i.e. secondary forest stands must be actively managed (e.g. using assisted forest restoration) according to conservation goals at broad spatial scales.
Light is a key resource for tree performance and hence, tree species partition spatial and temporal gradients in light availability. Although light distribution drives tree performance and species ...replacement during secondary forest succession, we yet lack understanding how light distribution changes with tropical forest development.
This study aims to evaluate how changes in forest structure lead to changes in vertical and horizontal light heterogeneity during tropical forest succession.
We described successional patterns in light using a chronosequence approach in which we compared 14 Mexican secondary forest stands that differ in age (8–32 years) since agricultural abandonment. For each stand, we measured vertical light profiles in 16 grid cells, and structural parameters (diameter at breast height, height and crown dimensions) for each tree.
During succession, we found a rapid increase in stand size (basal area, crown area and length) and stand differentiation (i.e. a gradual leaf distribution along the forest profile), which leads to fast changes in light conditions and more light heterogeneity. The inflection points of the vertical light gradient (i.e. the absolute height at which 50% relative light intensity is attained) rapidly moved towards higher heights in the first 20 years, indicating that larger amounts of light are intercepted by canopy trees. Light attenuation rate (i.e. the rate of light extinction) decreased during succession due to slower accumulation of the crown area with height. Understorey light intensity and heterogeneity slightly decreased during succession because of an increase in crown size and a decrease in lateral gap frequency. Understorey relative light intensity was 1.56% at 32 years after abandonment.
Synthesis. During succession, light conditions changed linearly, which should lead to a continuous and constant replacement of species. Especially in later successional stages, stronger vertical light gradients can limit the regeneration of light‐demanding pioneer species and increase the proportion of shade‐tolerant late‐successional species under the canopy. These changes in light conditions were largely driven by the successional changes in forest structure, as basal area strongly determined the height where most light is absorbed, whereas crown area, and to a lesser extent crown length, determined light distribution.
Secondary forest succession is driven by changes in forest structure and light environment, but few studies have quantified both factors. During succession in a Mexican tropical rainforest, light in the understory became more homogeneously distributed, whereas vertical light heterogeneity increased. Stand basal area determined the height where most light is absorbed, whereas crown architecture determined horizontal and vertical light distribution.
In closed‐canopy systems globally, plants exhibit intense competition for light, prioritizing vertical growth to attain elevated positions within the canopy. Light competition is especially intense ...in tropical rainforests because of their dense shaded stands, and during forest succession because of concomitant changes in vertical light profiles. We evaluated how the height growth of individual tree differs among forest light strata (canopy, sub‐canopy and understorey) and successional guilds (early, mid‐ and late successional species) during secondary succession in a Mexican rainforest. Fourteen secondary forest stands differing in time since agricultural abandonment (1–25 years) were monitored for seven consecutive years. For each stand and census year we estimated relative light intensity (RLI) for each height and categorized trees into forest light strata: understorey (RLI ≦ 33.3%), sub‐canopy (33.3% ≦ RLI ≦ 66.6%) and canopy (RLI ≧ 66.6%), and into successional guilds based on the literature. We estimated two measures of height growth: absolute height growth (HGabs, cm year−1) calculated as the difference in tree height between two consecutive censuses, and biomass partitioning to height growth (HGbp, in kg kg−1 × 100) calculated as the percentage of total aboveground biomass growth partitioned to height growth. Earlier in succession, trees for all strata had greater HGabs and HGbp, resulting in rapid vertical forest development. HGabs was fastest for canopy trees, followed by sub‐canopy and understorey trees. These differences in HGabs among strata, combined with their inter‐specific variation and continuous recruitment of small individuals, lead to a rapid differentiation in tree sizes and increase stand structural heterogeneity. HGbp was greater for understorey and sub‐canopy trees than for canopy trees, reflecting ontogenetic changes in the light competition strategy from growth to persistence. With succession, both HGabs and HGbp decreased, most strongly for canopy trees, probably because of an increased exposure to drought stress. These successional changes stabilize stand size structure and reduce the rate of development.
Successional theories Poorter, Lourens; Amissah, Lucy; Bongers, Frans ...
Biological reviews of the Cambridge Philosophical Society,
12/2023, Letnik:
98, Številka:
6
Journal Article
Recenzirano
Odprti dostop
Succession is a fundamental concept in ecology because it indicates how species populations, communities, and ecosystems change over time on new substrate or after a disturbance. A mechanistic ...understanding of succession is needed to predict how ecosystems will respond to land-use change and to design effective ecosystem restoration strategies. Yet, despite a century of conceptual advances a comprehensive successional theory is lacking. Here we provide an overview of 19 successional theories ('models') and their key points, group them based on conceptual similarity, explain conceptual development in successional ideas and provide suggestions how to move forward. Four groups of models can be recognised. The first group (patch & plants) focuses on plants at the patch level and consists of three subgroups that originated in the early 20th century. One subgroup focuses on the processes (dispersal, establishment, and performance) that operate sequentially during succession. Another subgroup emphasises individualistic species responses during succession, and how this is driven by species traits. A last subgroup focuses on how vegetation structure and underlying demographic processes change during succession. A second group of models (ecosystems) provides a more holistic view of succession by considering the ecosystem, its biota, interactions, diversity, and ecosystem structure and processes. The third group (landscape) considers a larger spatial scale and includes the effect of the surrounding landscape matrix on succession as the distance to neighbouring vegetation patches determines the potential for seed dispersal, and the quality of the neighbouring patches determines the abundance and composition of seed sources and biotic dispersal vectors. A fourth group (socio-ecological systems) includes the human component by focusing on socio-ecological systems where management practices have long-lasting legacies on successional pathways and where regrowing vegetations deliver a range of ecosystem services to local and global stakeholders. The four groups of models differ in spatial scale (patch, landscape) or organisational level (plant species, ecosystem, socio-ecological system), increase in scale and scope, and reflect the increasingly broader perspective on succession over time. They coincide approximately with four periods that reflect the prevailing view of succession of that time, although all views still coexist. The four successional views are: succession of plants (from 1910 onwards) where succession was seen through the lens of species replacement; succession of communities and ecosystems (from 1965 onwards) when there was a more holistic view of succession; succession in landscapes (from 2000 onwards) when it was realised that the structure and composition of landscapes strongly impact successional pathways, and increased remote-sensing technology allowed for a better quantification of the landscape context; and succession with people (from 2015 onwards) when it was realised that people and societal drivers have strong effects on successional pathways, that ecosystem processes and services are important for human well-being, and that restoration is most successful when it is done by and for local people. Our review suggests that the hierarchical successional framework of Pickett is the best starting point to move forward as this framework already includes several factors, and because it is flexible, enabling application to different systems. The framework focuses mainly on species replacement and could be improved by focusing on succession occurring at different hierarchical scales (population, community, ecosystem, socio-ecological system), and by integrating it with more recent developments and other successional models: by considering different spatial scales (landscape, region), temporal scales (ecosystem processes occurring over centuries, and evolution), and by taking the effects of the surrounding landscape (landscape integrity and composition, the disperser community) and societal factors (previous and current land-use intensity) into account. Such a new, comprehensive framework could be tested using a combination of empirical research, experiments, process-based modelling and novel tools. Applying the framework to seres across broadscale environmental and disturbance gradients allows a better insight into what successional processes matter and under what conditions.
Early pioneer species share life histories enabling them to colonize disturbed sites, but how much they differ demographically and how such differentiation determines pioneer species turnover during ...succession are still open questions. Here, we approached these issues by comparing the demography of dominant pioneer tree species during the secondary succession of tropical rainforest in Southeast Mexico.
We assessed changes in population density, population structure, vital rates and intrinsic population growth rate (r) of the pioneer species Trema micrantha, Cecropia peltata and Trichospermum mexicanum during the first 35 years of succession. For this, we combined chronosequence and long‐term (from 2000 to 2018) data from 14 abandoned cornfields with 0.5–35 years fallow age.
Trema colonized and disappeared first during succession (<15 years), followed by Cecropia (<28) and Trichospermum (>31). All species exhibited hump‐shaped successional trajectories of population density and biomass with Trema reaching a peak first, followed by Cecropia and later Trichospermum. Species exhibited a fast reduction in r with fallow age, with Trema reaching negative growth rates (r < 0) in the third, Cecropia in the fourth, and Trichospermum in the seventh year of succession. Recruitment, growth and mortality rates of seedlings and juveniles defined the period of population increase and the age of succession at which each species reached maximum density and biomass. The mortality rate in mature stages determined how long each species persisted during succession. An important variation in species replacement occurred among study sites. In some sites, one species was abundant and the others were almost absent, while it was the opposite in other sites. We inferred that priority inhibitory effects operated among species during the field colonization.
Synthesis. Although Trema, Cecropia and Trichospermum are considered typical pioneer trees, these species differed importantly in their demographic attributes during succession. The speed at which r declined with age of succession indicated the moment at which each species reached its maximum density and species replacement sequence during succession. However, inter‐specific priority inhibitory effects during field colonization may also be involved in the chance of colonization and replacement between species with similar regeneration strategies.
The replacement among pioneer tropical rainforest tree species along succession in Southern Mexico based on chronosequence (14 plots) and long‐term data (19 years). On average (left‐hand graphs), across plots and years, Trema micrantha (yellow dots) is replaced by Cecropia peltata (blue dost) and C. peltata by Trichospermum mexicanum (red dost). However, such process has a high spatial and temporal variability when considering the population dynamics of each species at the individual plot level (right‐hand graphs; each line represent a plot and the fallow years during which each population was monitored).
Landscape‐level disturbances, such as forest loss, can profoundly alter the functional composition and diversity of biotic assemblages. In fact, the landscape‐moderated functional trait selection ...(LMFTS) hypothesis states that landscape‐level disturbances may act as environmental filters that select a set of species with disturbance‐adapted attributes while causing the loss of species with disturbance‐sensitive attributes, ultimately compromising ecosystem functioning. However, the impact of landscape patterns on the functional composition and diversity of tropical regenerating trees (saplings) is unknown.
Using a multiscale approach to identify the best spatial scale (i.e. the scale of effect), we tested the effect of forest cover, matrix openness and forest patch density (fragmentation) on functional composition and functional diversity of tree saplings in old‐growth forest patches (n = 59) in three Mexican rainforest regions with different degree of deforestation. For 368 species and ~23,000 individuals, we compiled information from global and national databases on six functional traits related to seed dispersal and plant establishment and calculated their community abundance‐weighted mean (CWM) and three complementary functional diversity indices.
Forest loss and matrix openness reduced functional richness and evenness, but only in the two most deforested regions. Overall, fragmentation had contrasting effects on functional diversity and composition, but correlated negatively with some functional traits in the most deforested region. Importantly, in the regions with high‐to‐intermediate degree of deforestation, functional composition experienced major changes: maximum height, seed mass, fruit size and wood density decreased, and SLA increased, in forest patches surrounded by open matrices in highly deforested and fragmented landscapes. This caused a shift of community traits towards more disturbed‐adapted attributes.
Synthesis and applications. In agreement with the LMFTS hypothesis, our results confirm that landscape modifications in regions undergoing high and long‐lasting deforestation greatly impoverish the functional composition and diversity of sapling communities. The shift from communities composed mainly by conservative attributes towards communities with a higher prevalence of disturbance‐adapted attributes disrupts the future community structure and jeopardizes critical ecosystem functions. Management practices focused on preventing deforestation, increasing forest cover and promoting treed matrices are necessary to preserve the functionality of these species rich but increasingly threatened rainforests.
Resumen
Algunos disturbios a nivel de paisaje, como la pérdida de hábitat, pueden alterar profundamente la composición y la diversidad funcional de los ensamblajes bióticos. La hipótesis de la selección de rasgos funcionales moderada por el paisaje (LMFTS) afirma que los disturbios a nivel de paisaje pueden actuar como filtros ambientales que seleccionan especies con atributos adaptados al disturbio, a la vez que pueden causar la pérdida de especies con atributos sensibles al disturbio, poniendo en riesgo en última instancia el funcionamiento del ecosistema. No obstante, aún desconocemos el impacto de los patrones del paisaje sobre la composición y la diversidad funcional de los árboles tropicales en regeneración (brinzales).
Usando un enfoque multiescalar para seleccionar la mejor escala espacial (i.e. la escala del efecto), evaluamos el efecto de la cobertura forestal, la apertura de la matriz y la densidad de parches forestales (fragmentación) sobre la composición funcional y la diversidad funcional de los brinzales en parches de bosque maduro (n = 59) en tres regiones tropicales mexicanas con diferentes grados de deforestación. Para 368 especies y ~ 23,000 individuos, reunimos información proveniente de bases de datos globales y nacionales de seis rasgos funcionales relacionados con la dispersión de semillas y el establecimiento de plántulas, y calculamos su media ponderada de la comunidad (CWMs) y tres índices complementarios de diversidad funcional.
La pérdida de bosque y las matrices abiertas redujeron la riqueza y la equitatividad funcional, pero sólo en las dos regiones más deforestadas. En general, la fragmentación tuvo efectos contrastantes sobre la diversidad y la composición funcional, pero se correlacionó negativamente con algunos rasgos funcionales en la región más deforestada. De forma importante, en las regiones con un grado de deforestación de intermedio a alto, la composición funcional experimentó grandes cambios: la altura máxima, el peso de la semilla, el tamaño del fruto y la densidad de madera disminuyeron, y la SLA aumentó, en parches forestales rodeados por matrices abiertas en paisajes fuertemente deforestados y fragmentados. Esto ocasionó un cambio de los rasgos de la comunidad hacia atributos más adaptados al disturbio.
Síntesis y aplicaciones. Nuestros resultados concuerdan con la hipótesis LMFTS y confirman que las modificaciones paisajísticas en regiones con una deforestación fuerte y duradera puede empobrecer profundamente la composición y la diversidad funcional de las comunidades de brinzales. El cambio de comunidades compuestas mayoritariamente por atributos funcionales conservadores a comunidades con una mayor prevalencia de atributos adaptados al disturbio incide sobre la futura estructura de la comunidad y pone en peligro las funciones críticas del ecosistema. Es necesario diseñar prácticas de manejo enfocadas en la prevención de la deforestación, el aumento de la cobertura forestal y la promoción de matrices arboladas para preservar la funcionalidad de estos bosques tropicales muy diversos pero cada vez más amenazados.
In agreement with the LMFTS hypothesis, our results confirm that landscape modifications in regions undergoing high and long‐lasting deforestation greatly impoverish the functional composition and diversity of sapling communities. The shift from communities composed mainly by conservative attributes towards communities with a higher prevalence of disturbance‐adapted attributes disrupts the future community structure and jeopardizes critical ecosystem functions. Management practices focused on preventing deforestation, increasing forest cover and promoting treed matrices are necessary to preserve the functionality of these species rich but increasingly threatened rainforests.
Land‐use change threatens biodiversity in tropical landscapes, but its impact on rainforest regeneration remains poorly known. In fact, the landscape‐scale patterns driving the diversity of ...regenerating plants within forest fragments have been rarely explored, and we are uncertain whether such drivers vary across regions with different land‐use change patterns.
We assessed the effect of landscape composition (forest cover and matrix openness) and configuration (forest patch density) on species diversity of sapling assemblages (trees ≥30 cm height and <1 cm diameter) in old‐growth forest fragments from three Mexican rainforest regions with different disturbance levels (n = 20 landscapes per region). We separately assessed old‐growth forest specialists (OGS) and forest generalist (FG) species to test the hypotheses that: (a) OGS species show recruitment limitation (‘loser’ species), and can therefore be negatively impacted by landscape changes, especially by forest loss and matrix openness in more deforested regions; and (b) FG species can regenerate and even proliferate in more disturbed landscapes (‘winner’ species).
We recorded ~24,000 plants from 415 species. Landscape composition showed stronger effects than landscape configuration. The diversity of OGS species generally decreased in more deforested landscapes dominated by open matrices, and FG species followed the opposite response, especially in the regions with high‐to‐intermediate degree of disturbance. Overall, forest fragmentation (patch density) showed weak or no effects on species diversity, especially after controlling for forest cover effects (i.e. fragmentation per se). In contrast to the fragmentation threshold hypothesis, the effect of fragmentation was independent of the regional context. Moreover, FG species were affected by landscape attributes operating at larger scales than OGS species.
Synthesis. Our findings support our hypotheses, and suggest that forest loss and matrix openness, not fragmentation per se, can cause the recruitment failure of tree assemblages in highly deforested rainforests. This can be related to source and dispersal limitation in more deforested landscapes with treeless matrices. Therefore, to promote the regenerative potential (resilience) of forest patches in human‐modified tropical landscapes, conservation programs should focus on preventing forest loss (even the smallest forest patches) and improving matrix quality with treed elements, particularly in highly deforested tropical regions.
Resumen
El cambio de uso del suelo amenaza la biodiversidad de los paisajes tropicales, pero se sabe poco sobre el impacto de este proceso sobre la regeneración del bosque. De hecho, apenas se han explorado los patrones paisajísticos que determinan la diversidad de las plantas que están en la fase de regeneración en fragmentos de bosque, y no está claro si estos factores varían entre regiones que tienen diferentes patrones de cambio de uso del suelo.
En este estudio evaluamos los efectos de dos atributos del paisaje, la composición (i.e., la cobertura forestal y el grado de abertura de la matriz) y la configuración (i.e., la densidad de parches de bosque), sobre la diversidad de especies en ensambles de brinzales (árboles ≥ 30 cm de altura y < 1 cm diámetro) en fragmentos de bosque maduro en tres regiones de bosque tropical lluvioso en México que tienen diferentes niveles de perturbación (n = 20 paisajes por región). Evaluamos de manera separada a las especies especialistas de bosque maduro (OGS) y a las especies generalistas de bosque (FG) para probar las hipótesis de que: (i) las especies OGS muestran limitación en el reclutamiento (‘especies perdedoras’), y por lo tanto pueden sufrir un impacto negativo por los cambios en el paisaje, sobre todo por la pérdida del bosque y la apertura de la matriz en regiones más deforestadas; y (ii) las especies FG se pueden regenerar e incluso proliferar en paisajes más perturbados (‘especies ganadoras’).
Registramos ~24,000 plantas pertenecientes a 415 especies. Encontramos efectos más fuertes para la composición del paisaje que para su configuración. Comúnmente, la diversidad de especies OGS decreció en los paisajes más deforestados dominados por matrices abiertas, mientras que las especies FG tuvieron una respuesta opuesta, especialmente en las regiones con niveles intermedios o altos de disturbio. En general, la fragmentación del bosque (densidad de parches) tuvo efectos débiles o inexistentes sobre la diversidad de especies, especialmente cuando se controlaron los efectos de la cobertura de bosque (i.e., la fragmentación per se). En contraste con la hipótesis del umbral de fragmentación, el efecto de la fragmentación fue independiente del contexto regional. Además, las especies FG fueron más afectadas por atributos del paisaje que operan a escalas más grandes que las especies OGS.
Síntesis. Nuestros hallazgos apoyan nuestras hipótesis y sugieren que tanto la pérdida de bosque como la apertura de la matriz, pero no la fragmentación per se, pueden hacer que falle el reclutamiento de ensambles de árboles en áreas muy deforestadas de bosque tropical lluvioso. Esto puede estar relacionado con la limitación en la fuente y en la dispersión en paisajes más deforestados con matrices desprovistas de árboles. Por lo tanto, para promover el potencial de regeneración (la resiliencia) de los parches de bosque que están presentes en paisajes tropicales modificados por la actividad humana, los programas de conservación deben enfocarse en la prevención de la pérdida del bosque (incluyendo a los parches más pequeños de bosque) y en la mejora de la calidad de la matriz con elementos arbolados, particularmente en regiones tropicales muy deforestadas.
Forest loss and open matrices decrease the diversity of saplings of old‐growth forest specialist species in rainforest patches within intermediate to highly deforested landscapes. In contrast, forest generalist species are favoured by these landscape changes. Our results suggest a replacement of old‐growth forest specialist species by forest generalist species in the sapling community, leading to the taxonomic impoverishment of the regenerating community in rainforest remnants in human‐modified tropical landscapes.
ABSTRACT
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.
Significance Although forest succession has been approached as a predictable process, successional trajectories vary widely, even among nearby stands with similar environmental conditions and ...disturbance histories. We quantified predictability and uncertainty during tropical forest succession using dynamical models describing the interactions among stem density, basal area, and species density over time. We showed that the trajectories of these forest attributes were poorly predicted by stand age and varied significantly within and among sites. Our models reproduced the general successional trends observed, but high levels of noise were needed to increase model predictability. These levels of uncertainty call into question the premise that successional processes are consistent over space and time, and challenge the way ecologists view tropical forest regeneration.
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 attributesâstem 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.
Deterministic theories predict that local communities assemble from a regional species pool based on niche differences, thus by plant functional adaptations. We tested whether functional traits can ...also explain patterns in species dominance among the suite of co‐occurring species. We predicted that along a gradient of secondary succession, the main driver of species dominance changes from environmental filtering in the relatively harsh (dry and hot) early successional conditions, towards increased competitive interactions and limiting similarity in later successional conditions (when light is limited). We used the Kurtosis (K) (a measure of peakedness) of the functional trait distribution of secondary forest communities in high‐diversity tropical rain forest in Chiapas, Mexico. The forests ranged 1–25 years in age, and we used eight functional leaf traits related to a plants' carbon, water and heat balance. We calculated the functional trait distribution based on species dominance, where trait values were weighted by species' relative basal area, as well as based on species presence, all species counting once. ‘K‐ratio’ was subsequently computed by dividing kurtosis based on species dominance by kurtosis based on species presence. If the K‐ratio is high, the dominant species are functionally similar and we interpreted this as environmentally driven functional convergence allowing species to become dominant. If the K‐ratio is small, dominant species are a functionally dissimilar subset of the species present and we interpreted this as competitively driven functional divergence allowing species to become dominant. We found that in early succession, dominant species represent a functionally narrow subset of species with similar traits, and in late succession, dominant species increasingly represent a wide subset of the species present. This trend was found for traits that reflect photosynthetic performance and light capture, and indicates increased competition for light with succession. No trend was found for traits that indicate defence against herbivory, suggesting no successional changes in herbivore pressure. Synthesis. This is one of the first studies showing that drivers of species dominance change along a gradient of secondary succession. During the early successional time window we evaluated, the importance of environmental filtering as a driving force fades away rapidly, and the importance of niche partitioning for species dominance starts to emerge.
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