Field observed relationships between biodiversity and ecosystem functioning during secondary succession in a tropical lowland rainforest

Trait-based community assembly and functional strategies across three subtropical karst forests, Southwestern China

Background

Variations in community-level plant functional traits are widely used to elucidate vegetation adaptation strategies across different environmental gradients. Moreover, studying functional variation among different forest types aids in understanding the mechanisms by which environmental factors and functional strategies shift community structure.

Methods

Based on five plant functional traits, including four leaf and one wood trait, for 150 woody species, we analyzed shifts in the community-weighted mean trait values across three forest types in a karst forest landscape: deciduous, mixed, and evergreen forests. We also assessed the relative contributions of stochastic processes, environmental filtering, and niche differentiation to drive community structure using a trait-based null model approach.

Results

We found marked changes in functional strategy, from resource acquisition on dry, fertile soil plots in deciduous forests to resource conservation on moist, infertile soil conditions in evergreen forests. The trait-based null model showed strong evidence of environmental filtering and convergent patterns in traits across three forest types, as well as low niche differentiation in most functional traits. Some evidence of overdispersion of LDMC and LT occurred to partially support the recent theory of Scheffer and Van Nes that competition could result in a clumped pattern of species along a niche axis.

Discussion

Our findings suggest a change in environmental gradient from deciduous to evergreen forest, together with a shift from acquisitive to conservative traits. Environmental filtering, stochastic processes, niche differentiation, and overdispersion mechanisms together drive community assembly in karst forest landscapes. These findings will contribute to a deeper understanding of the changes in functional traits among karst plants and their adaptive strategies, with important implications for understanding other community assemblies in subtropical forest systems.

Shifts in Community Vegetative Organs and Their Dissimilar Trade-Off Patterns in a Tropical Coastal Secondary Forest, Hainan Island, Southern China

The ecology of functional features highlights the importance of the leaf economic spectrum (LES) in understanding plant trade-offs between conservative and commercial resource use. However, it is still unclear whether changes in the plant attributes of various vegetative organs can be altered and whether the plant economic spectrum (PES) is categorized by multiple vegetative organs. We investigated a total of 12 functional features of 174 woody tree species, with leaf and stem attributes, on Hainan Island. We used principal component analysis (PCA) to analyze the changes in attributes and connections to understand how the plant trade-offs differ. We detected that stem organic matter (SOM) and stem organic carbon (SOC) contributed most to the first principal component, followed by leaf organic matter (LOM) and leaf organic carbon (LOC). Using Spearman correlation analysis, we determined that leaf total nitrogen (LTN) and specific leaf area (SLA), LTN and leaf total phosphorus (LTP), and finally stem total nitrogen (STN) and stem total phosphorus (STP) were positively significantly correlated. These significant variations in the traits of nutrients are regulated, while the morphological traits of aboveground vegetative organs are diverse. The coexistence of species and community assembly can increase our knowledge on the tropical coastal secondary forests. Furthermore, our outcomes can help us to better understand the restoration of habitats and green infrastructure design, suggesting that selecting different species across multiple trait axes can help ensure functionality at the maximum level.

Differing Trade-Off Patterns of Tree Vegetative Organs in a Tropical Cloud Forest

Functional trait ecology demonstrates the significance of the leaf economics spectrum in understanding plants' trade-off between acquisitive and conservative resource utilization. However, whether trait variations of different vegetative organs are coordinated and whether the plant economics spectrum is characterized by more than one vegetative organ remain controversial. To gain insights into these questions, within a tropical cloud forest in Hainan Island, a total of 13 functional traits of 84 tree species were analyzed here, including leaf, stem and root traits. By using standardized major axis (SMA) regression and principal components analysis, we examined the trait variations and correlations for deciphering plants' trade-off pattern. We found decreases of leaf phosphorus content, leaf nitrogen content and specific leaf area and increases of leaf mass per unit area (LMA), wood density and leaf thickness along the first principal component, while there were decreases of specific root length and specific root area and increases of root tissue density along the second principal component. Root phosphorus and nitrogen contents were significantly positively associated with the phosphorus and nitrogen contents of both stem and leaf. Wood density was significantly positively associated with LMA and leaf thickness, but negatively associated with leaf thickness and specific leaf area. Our results indicate that, in the tropical cloud forest, there is a "fast-slow" economic spectrum characterized by leaf and stem. Changes of nutrient trait are coordinated, whereas the relationships of morphological traits varied independently between plant above- and below-ground parts, while root nutrient traits are decoupled from root morphological traits. Our findings can provide an insight into the species coexistence and community assembly in high-altitude tropical forests.

Effects of thinning on the demography and functional community structure of a secondary tropical lowland rain forest

Tropical secondary forests play a prominent role in conserving biodiversity and providing ecosystem services, but their recovery can be slow and their succession trajectory is distinct from old-growth forests. Thinning is an essential silvicultural approach to enhance the recovery rate and timber production of forests. However, the selection of trees to thin has been mainly based on size class rather than on species identity. There is little empirical or experimental evidence of species-focused thinning with the goal of altering species composition. We examined the effects of thinning on community structure, demographic rates, species richness and functional diversity of woody plants in a detailed investigation of 60-year-old secondary tropical lowland rain forest on Hainan Island, South China. The density and basal area of trees ≥5 cm DBH (diameter at breast height) increased significantly after five years' recovery with no significant change for saplings (DBH < 5 cm). Species composition after thinning changed significantly and mid-to late-successional species of both saplings and trees were more abundant after five years' recovery. The relative growth rates (RGR) and recruitment rates were significantly higher in thinning plots for both saplings and trees, and RGRs increased by 127% and 48%, respectively. The mortality rate decreased by 13% for trees and increased by 47% for saplings in thinning plots compared to the control. The community weighted mean (CWM) of the specific leaf area (SLA) of saplings showed a significantly decreasing trend while CWMs of wood density (WD) and mean maximum height (Hmax) of saplings increased after thinning. By contrast, CWMs of SLA and Hmax of trees were significantly higher, but WD was significantly lower- in thinning plots than the control. RGR and recruitment rate of saplings and trees increased significantly as thinning intensity increased. However, the thinning intensity had a weak or nil effect on the mortality rate. Our results support the selective removal and girdling of pioneer and mid-successional species in a way that could accelerate recovery and improve the growth and recruitment of late-successional species in tropical secondary forests. Thinning at a relatively low intensity can maintain species diversity and alter species functional composition. This outcome shows promise for improved future management of tropical forests in human-modified tropical forest landscapes.

Plant traits alone are poor predictors of ecosystem properties and long-term ecosystem functioning

Earth is home to over 350,000 vascular plant species that differ in their traits in innumerable ways. A key challenge is to predict how natural or anthropogenically driven changes in the identity, abundance and diversity of co-occurring plant species drive important ecosystem-level properties such as biomass production or carbon storage. Here, we analyse the extent to which 42 different ecosystem properties can be predicted by 41 plant traits in 78 experimentally manipulated grassland plots over 10 years. Despite the unprecedented number of traits analysed, the average percentage of variation in ecosystem properties jointly explained was only moderate (32.6%) within individual years, and even much lower (12.7%) across years. Most other studies linking ecosystem properties to plant traits analysed no more than six traits and, when including only six traits in our analysis, the average percentage of variation explained in across-year levels of ecosystem properties dropped to 4.8%. Furthermore, we found on average only 12.2% overlap in significant predictors among ecosystem properties, indicating that a small set of key traits able to explain multiple ecosystem properties does not exist. Our results therefore suggest that there are specific limits to the extent to which traits per se can predict the long-term functional consequences of biodiversity change, so that data on additional drivers, such as interacting abiotic factors, may be required to improve predictions of ecosystem property levels.

Disentangling how management affects biomass stock and productivity of tropical secondary forests fallows

A better understanding of biomass production in secondary forests after cultivation is essential for assessing the resilience of slash and burn systems and their capacity to deliver ecosystem services. Biomass production is influenced by management legacies, landscape configuration and soil, but these drivers are rarely studied simultaneously, nor is the role of changes in vegetation properties in linking them to biomass production. We assessed how management legacies affect biomass in secondary forests created by slash and burn in the central Congo Basin, and tested whether changes in productivity could be attributed to changes in stem density, functional diversity, functional identity or soil. Using data from 6452 trees in 96 fallow plots nested in 3 study sites, we looked for the main determinants of aboveground biomass (AGB) of woody vegetation in fallow systems. Next, using a subset of 58 plots in fallow fields aged 5 to 10 years, we used confirmatory path analysis to explore the relations between management history, soil, vegetation properties and biomass productivity. The sampled fallow fields had, on average, 58.4 (±46.2) Mg ha^-1 AGB. AGB was positively related to both fallow age and to the proportion of remnant trees in AGB and negatively related to the number of previous cultivation cycles. Biomass productivity varied with the number of previous slash-and-burn cycles, with notable declines in the fourth cycle. The effect of management history was mainly through a reduction in the dominance of fast growing tree species and in the number of regenerating stems, which were also indirectly affected by an increase in C. odorata cover. Soil fertility status and the biomass of remnant trees also modified biomass productivity. Our findings suggest that under the current management intensity the capacity of the slash and burn system to provide important ecosystem functions, such as carbon sequestration, is declining.

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.

Long-term regeneration of a tropical plant community after sand mining

Sandy coastal plant communities in tropical regions have been historically under strong anthropic pressure. In Brazil, these systems shelter communities with highly plastic plant species. However, the potential of these systems to regenerate without human assistance after disturbances has hardly been examined. We determined the natural regeneration of a coastal sandy plain vegetation (restinga) in Brazil, 16 years after the end of sand removal. We inventoried 38 plots: 20 within a sand-mined site and 18 in an adjacent undisturbed site. We expected lower diversity values in the sand-mined site compared to the undisturbed site, but similar species composition between the two sites due to the spatial proximity of the two sites and the high plasticity of restinga species. Species were ranked using abundance and importance value index in both sites, and comparisons were performed using Rényi entropy profiles, rarefaction curves, principal component analysis, and redundancy analysis. Species composition and dominant species differed markedly between the two sites. Bromeliads and Clusia hilariana, well-known nurse plants, dominated the undisturbed site but were almost absent in the regenerating site. Species richness did not differ between both sites, but diversity was higher in the undisturbed site. Within-site composition differences in the mined area were associated with field characteristics. Interestingly, species classified as subordinate or rare in the undisturbed site became dominants in the regenerating site. These newer dominants in the sand-mined site are not those known as nurse plants in other restingas, thus yielding strong implications for restoration.

Early subtropical forest growth is driven by community mean trait values and functional diversity rather than the abiotic environment

While functional diversity (FD) has been shown to be positively related to a number of ecosystem functions including biomass production, it may have a much less pronounced effect than that of environmental factors or species-specific properties. Leaf and wood traits can be considered particularly relevant to tree growth, as they reflect a trade-off between resources invested into growth and persistence. Our study focussed on the degree to which early forest growth was driven by FD, the environment (11 variables characterizing abiotic habitat conditions), and community-weighted mean (CWM) values of species traits in the context of a large-scale tree diversity experiment (BEF-China). Growth rates of trees with respect to crown diameter were aggregated across 231 plots (hosting between one and 23 tree species) and related to environmental variables, FD, and CWM, the latter two of which were based on 41 plant functional traits. The effects of each of the three predictor groups were analyzed separately by mixed model optimization and jointly by variance partitioning. Numerous single traits predicted plot-level tree growth, both in the models based on CWMs and FD, but none of the environmental variables was able to predict tree growth. In the best models, environment and FD explained only 4 and 31% of variation in crown growth rates, respectively, while CWM trait values explained 42%. In total, the best models accounted for 51% of crown growth. The marginal role of the selected environmental variables was unexpected, given the high topographic heterogeneity and large size of the experiment, as was the significant impact of FD, demonstrating that positive diversity effects already occur during the early stages in tree plantations.

Biomass is the main driver of changes in ecosystem process rates during tropical forest succession

Over half of the world's forests are disturbed, and the rate at which ecosystem processes recover after disturbance is important for the services these forests can provide. We analyze the drivers' underlying changes in rates of key ecosystem processes (biomass productivity, litter productivity, actual litter decomposition, and potential litter decomposition) during secondary succession after shifting cultivation in wet tropical forest of Mexico. We test the importance of three alternative drivers of ecosystem processes: vegetation biomass (vegetation quantity hypothesis), community-weighted trait mean (mass ratio hypothesis), and functional diversity (niche complementarity hypothesis) using structural equation modeling. This allows us to infer the relative importance of different mechanisms underlying ecosystem process recovery. Ecosystem process rates changed during succession, and the strongest driver was aboveground biomass for each of the processes. Productivity of aboveground stem biomass and leaf litter as well as actual litter decomposition increased with initial standing vegetation biomass, whereas potential litter decomposition decreased with standing biomass. Additionally, biomass productivity was positively affected by community-weighted mean of specific leaf area, and potential decomposition was positively affected by functional divergence, and negatively by community-weighted mean of leaf dry matter content. Our empirical results show that functional diversity and community-weighted means are of secondary importance for explaining changes in ecosystem process rates during tropical forest succession. Instead, simply, the amount of vegetation in a site is the major driver of changes, perhaps because there is a steep biomass buildup during succession that overrides more subtle effects of community functional properties on ecosystem processes. We recommend future studies in the field of biodiversity and ecosystem functioning to separate the effects of vegetation quality (community-weighted mean trait values and functional diversity) from those of vegetation quantity (biomass) on ecosystem processes and services.