Summary
Biodiversity–ecosystem functioning (BEF) experiments address ecosystem‐level consequences of species loss by comparing communities of high species richness with communities from which species ...have been gradually eliminated. BEF experiments originally started with microcosms in the laboratory and with grassland ecosystems. A new frontier in experimental BEF research is manipulating tree diversity in forest ecosystems, compelling researchers to think big and comprehensively.
We present and discuss some of the major issues to be considered in the design of BEF experiments with trees and illustrate these with a new forest biodiversity experiment established in subtropical China (Xingangshan, Jiangxi Province) in 2009/2010. Using a pool of 40 tree species, extinction scenarios were simulated with tree richness levels of 1, 2, 4, 8 and 16 species on a total of 566 plots of 25·8 × 25·8 m each.
The goal of this experiment is to estimate effects of tree and shrub species richness on carbon storage and soil erosion; therefore, the experiment was established on sloped terrain. The following important design choices were made: (i) establishing many small rather than fewer larger plots, (ii) using high planting density and random mixing of species rather than lower planting density and patchwise mixing of species, (iii) establishing a map of the initial ‘ecoscape’ to characterize site heterogeneity before the onset of biodiversity effects and (iv) manipulating tree species richness not only in random but also in trait‐oriented extinction scenarios.
Data management and analysis are particularly challenging in BEF experiments with their hierarchical designs nesting individuals within‐species populations within plots within‐species compositions. Statistical analysis best proceeds by partitioning these random terms into fixed‐term contrasts, for example, species composition into contrasts for species richness and the presence of particular functional groups, which can then be tested against the remaining random variation among compositions.
We conclude that forest BEF experiments provide exciting and timely research options. They especially require careful thinking to allow multiple disciplines to measure and analyse data jointly and effectively. Achieving specific research goals and synergy with previous experiments involves trade‐offs between different designs and requires manifold design decisions.
Understanding how species diversity affects plant performance is a central question in biodiversity–ecosystem functioning (BEF) research. At the community level, functional trait means and trait ...dissimilarities have been used to explain biodiversity effects, but with mixed success. To disentangle how functional traits explain community growth and underpin biodiversity–ecosystem functioning relationships, we should elucidate how plant traits affect individual growth across species richness levels, because the role of functional traits on growth depends on the ecological context of the individual.
We addressed this topic by using detailed data of twelve functional traits and annual growth for 529 individual trees of 31 species at five species‐richness levels in a large forest biodiversity experiment in south‐east China from 4 to 9 years after planting.
Our analyses show first that individual trait values can change with species richness, indicating that trait values can change due to the biotic context. Secondly, we show that early tree growth is more strongly affected by traits than by species richness. Finally, our data show that growth–trait relationships can change with species richness and with forest age. Trait effects on growth are more pronounced at higher richness levels, indicating that measuring traits on individual trees across richness levels can improve growth predictions and inference of BEF relationships that are shaped by functional traits.
Synthesis. This study shows that functional trait values and their effect on individual tree growth depend on species richness. Our data support the notion that to elucidate how functional traits shape biodiversity–ecosystem functioning relationships, an important step is to consider the biotic context of individual trees within a community. We have made an initial step by analysing how functional traits affect individual‐tree growth in a diversity‐dependent manner and future research should continue by elucidating the role of traits on tree–tree interactions across diversity levels.
in Chinese
物种多样性对植物在群落中性能的影响是当前生物多样性与生态系统功能（BEF）研究中的核心问题之一。尽管在群落水平上, 常用功能性状均值和相异性来解释生物多样性效应, 但至今未能得出一致结论。这主要是因为功能性状对群落动态的影响主要取决于群落中个体所处的生态环境。因此厘清功能性状对群落动态的影响及其对生物多样性‐生态系统功能关系的贡献, 应首先阐明不同物种多样性水平下植物功能性状对个体动态的影响机制。
本研究以中国东南部亚热带地区大型森林多样性与生态系统功能实验中所种植的31个物种529株乔木为研究对象, 通过对其在五个多样性水平下栽种4‐9年后生长的连续监测及测取12种植物功能性状开展了相关研究。
研究结果表明植物个体功能性状随着群落多样性的变化而变化, 说明生物环境能够影响功能性状；其次, 功能性状对树木早期生长的影响要强于物种多样性；结果还表明功能性状与生长的关系随着群落多样性和林龄的不同而变化。在物种丰富度更高的群落, 功能性状对生长的影响更明显。这说明在不同多样性水平的群落中, 测取个体水平的功能性状能够提高对生长的预测和对功能性状影响的BEF关系的推断。
综合以上, 研究结果显示植物功能性状对木本植物个体的影响取决于其所在群落的物种丰富度。因此我们认为阐明功能性状对BEF关系的影响机制, 必须要将群落中个体所处的生物环境考虑在内。这是首次关于功能性状如何依赖于多样性而影响树木个体生长的研究, 并提出未来应着重于研究不同多样性水平下功能性状在种间相互作用中的贡献。
Our study made an initial step to show trait effects on growth are more pronounced at higher richness levels, indicating that to elucidate how functional traits shape biodiversity–ecosystem functioning relationships, an important step is to consider the biotic context of individual trees within a community.
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Research about biodiversity-productivity relationships has focused on herbaceous ecosystems, with results from tree field studies only recently beginning to emerge. Also, the latter are concentrated ...largely in the temperate zone. Tree species diversity generally is much higher in subtropical and tropical than in temperate or boreal forests, with reasons not fully understood. Niche overlap and thus complementarity in the use of resources that support productivity may be lower in forests than in herbaceous ecosystems, suggesting weaker productivity responses to diversity change in forests. We studied stand basal area, vertical structure, leaf area, and their relationship with tree species richness in a subtropical forest in south-east China. Permanent forest plots of 30 x 30 m were selected to span largely independent gradients in tree species richness and secondary successional age. Plots with higher tree species richness had a higher stand basal area. Also, stand basal area increases over a 4-year census interval were larger at high than at low diversity. These effects translated into increased carbon stocks in aboveground phytomass (estimated using allometric equations). A higher variability in tree height in more diverse plots suggested that these effects were facilitated by denser canopy packing due to architectural complementarity between species. In contrast, leaf area was not or even negatively affected by tree diversity, indicating a decoupling of carbon accumulation from leaf area. Alternatively, the same community leaf area might have assimilated more C per time interval in more than in less diverse plots because of differences in leaf turnover and productivity or because of differences in the display of leaves in vertical and horizontal space. Overall, our study suggests that in species-rich forests niche-based processes support a positive diversity-productivity relationship and that this translates into increased carbon storage in long-lived woody structures. Given the high growth rates of these forests during secondary succession, our results further indicate that a forest management promoting tree diversity after disturbance may accelerate CO2 sequestration from the atmosphere and thus be relevant in a climate-change context.
Ecosystem functioning and human well-being critically depend on numerous species interactions above- and belowground. However, unraveling the structure of multitrophic interaction webs at the ...ecosystem level is challenging for biodiverse ecosystems. Attempts to identify major relationships between trophic levels usually rely on simplified proxies, such as species diversity. Here, we propose to consider the full information on species composition across trophic levels, using Procrustes correlation and structural equation models. We show that species composition data of a highly diverse subtropical forest-with 5,716 taxa across 25 trophic groups- reveal strong interrelationships among plants, arthropods, and microorganisms, indicating complex multitrophic interactions. We found substantial support for top-down effects of microorganisms belowground, indicating important feedbacks of microbial symbionts, pathogens, and decomposers on plant communities. In contrast, aboveground pathways were characterized by bottom-up control of plants on arthropods, including many non-trophic links. Additional analyses based on diversity patterns revealed much weaker interrelationships. Our study suggests that multitrophic communities in our forest system are structured via top-down effects of belowground biota on plants, which in turn affect aboveground arthropod communities across trophic levels. Moreover, the study shows that the consequences of species loss will be more complex than indicated by studies based solely on diversity.
Afforestation is globally increasing to produce timber and pulp wood, but also to enhance ecosystem services such as carbon sequestration, nutrient retention or groundwater recharge. In China, large ...areas have been and will be afforested in order to compensate for the negative impacts of former clear‐cuttings and to make use of the ecosystem services associated with afforestation. In order to further optimize these services with regard to balanced nutrient (particularly nitrogen) cycles, it is important to know whether the use of mixtures of native tree species in afforestation projects promotes the acquisition and retention of nitrogen compared with the currently established large‐scale monocultures. To test the effect of species richness on system N retention and tree sapling N uptake, we conducted a ¹⁵N tracer experiment in a young tree plantation. To this end, saplings of four abundant early successional tree species were planted in monocultures, in two‐ and four‐species mixtures and as single trees. Nitrogen retention increased with higher species richness due to enhanced N pools in sapling biomass. These species richness effects strengthened over time. Species‐specific differences in ¹⁵N recoveries over time revealed below‐ground niche differentiation with regard to N uptake, which is likely to result in complementary resource use among coexisting species. Synthesis and applications. This study provides evidence that mixed afforestation promotes N retention from the sapling stage. To further improve ecosystem services associated with afforestation, we strongly suggest the use of mixtures of native tree species instead of monocultures. Mixtures of four species may reduce system N losses and thus may lessen groundwater contamination due to N leaching. We encourage further investigations to find optimal species combinations that promote a wide range of ecosystem services related to more closed nutrient cycles and minimized soil erosion. In our study, the plantations' capability to retain N could be optimized by means of both increasing tree species richness and by choosing the optimal species combinations.
The Janzen-Connell hypothesis is among the most important theories put forward to explain species coexistence in species-rich communities. However, the relative importance of Janzen-Connell effects ...with respect to other prominent mechanisms of community assembly, such as dispersal limitation, self-thinning due to competition, or habitat association, is largely unresolved. Here we use data from a 24-ha Gutianshan subtropical forest to address it. First we tested for significant associations of adults, juveniles, and saplings with environmental variables. Second we evaluated if aggregation decreased with life stage. In a third analysis we approximately factored out the effect of habitat association and comprehensively analyzed the spatial associations of intraspecific adults and offspring (saplings, juveniles) of 46 common species at continuous neighborhood distances. We found i) that, except for one, all species were associated with at least one environmental variable during at least one of their life stages, but the frequency of significant habitat associations declined with increasing life stage; ii) a decline in aggregation with increasing life stage that was strongest from juveniles to adults; and iii) intraspecific adult-offspring associations were dominated by positive relationships at neighborhood distances up to 10 m. Our results suggest that Janzen-Connell effects were not the dominant mechanisms in structuring the spatial patterns of established trees in the subtropical Gutianshan forest. The spatial patterns may rather reflect the joint effects of size-dependent self-thinning, dispersal limitation and habitat associations. Our findings contribute to a more comprehensive understanding of the relative importance of Janzen-Connell effects in influencing plant community structure under strong topographic heterogeneity.
Precise estimation of root biomass is important for understanding carbon stocks and dynamics in forests. Traditionally, biomass estimates are based on allometric scaling relationships between stem ...diameter and coarse root biomass calculated using linear regression (LR) on log-transformed data. Recently, it has been suggested that nonlinear regression (NLR) is a preferable fitting method for scaling relationships. But while this claim has been contested on both theoretical and empirical grounds, and statistical methods have been developed to aid in choosing between the two methods in particular cases, few studies have examined the ramifications of erroneously applying NLR. Here, we use direct measurements of 159 trees belonging to three locally dominant species in east China to compare the LR and NLR models of diameter-root biomass allometry. We then contrast model predictions by estimating stand coarse root biomass based on census data from the nearby 24-ha Gutianshan forest plot and by testing the ability of the models to predict known root biomass values measured on multiple tropical species at the Pasoh Forest Reserve in Malaysia. Based on likelihood estimates for model error distributions, as well as the accuracy of extrapolative predictions, we find that LR on log-transformed data is superior to NLR for fitting diameter-root biomass scaling models. More importantly, inappropriately using NLR leads to grossly inaccurate stand biomass estimates, especially for stands dominated by smaller trees.
Subtropical broad-leaved forests in southeastern China support a high diversity of woody plants. Using a comparative study design with 30 ×× 30 m plots (
n
== 27) from five successional stages (<20, ...<40, <60, <80, and ≥≥80 yr), we investigated how the gradient in species composition reflects underlying processes of community assembly. In particular, we tested whether species richness of adult trees and shrubs decreased or increased and assessed to which degree this pattern was caused by negative density dependence or continuous immigration over time. Furthermore, we tested whether rare species were increasingly enriched and the species composition of adult trees and shrubs became more similar to species composition of seedlings during the course of succession. We counted the individuals of all adult species and shrubs >1 m in height in each plot and counted all woody recruits (bank of all seedlings ≤≤1 m in height) in each central 10 ×× 10 m quadrant of each plot. In addition, we measured a number of environmental variables (elevation, slope, aspect, soil moisture, pH, C, N, and C/N ratio) and biotic structural variables (height and cover of layers). Adult species richness varied from 25 to 69 species per plot, and in total 148 woody species from 46 families were recorded. There was a clear successional gradient in species composition as revealed by nonmetric multidimensional scaling (NMDS), but only a poor differentiation of different successional stages with respect to particular species. Adult richness per 100 individuals (rarefaction method) increased with successional stage. None of the measured abiotic variables were significantly correlated with adult species richness. We found no evidence that rare species were responsible for the increasing adult species richness, as richness of rare species among both adults and recruits was independent of the successional stage. Furthermore, the similarity between established adults and recruits did not increase with successional stage. There was a constant number of recruit species and also of exclusive recruit species, i.e., those that had not been present as adult individuals, across all successional stages, suggesting a continuous random immigration over time.
Aims: With the aim of understanding why some of the world's forests exhibit higher tree beta diversity values than others, we asked: (1) what is the contribution of environmentally related variation ...versus pure spatial and local stochastic variation to tree beta diversity assessed at the forest plot scale; (2) at what resolution are these beta-diversity components more apparent; and (3) what determines the variation in tree beta diversity observed across regions/continents? Location: World-wide. Methods: We compiled an unprecedented data set of 10 large-scale stem-mapping forest plots differing in latitude, tree species richness and topographic variability. We assessed the tree beta diversity found within each forest plot separately. The non-directional variation in tree species composition among cells of the plot was our measure of beta diversity. We compared the beta diversity of each plot with the value expected under a null model. We also apportioned the beta diversity into four components: pure topographic, spatially structured topographic, pure spatial and unexplained. We used linear mixed models to interpret the variation of beta diversity values across the plots. Results: Total tree beta diversity within a forest plot decreased with increasing cell size, and increased with tree species richness and the amount of topographic variability of the plot. The topography-related component of beta diversity was correlated with the amount of topographic variability but was unrelated to its species richness. The unexplained variation was correlated with the beta diversity expected under the null model and with species richness. Main conclusions: Because different components of beta diversity have different determinants, comparisons of tree beta diversity across regions should quantify not only overall variation in species composition but also its components. Global-scale patterns in tree beta diversity are largely coupled with changes in gamma richness due to the relationship between the latter and the variation generated by local stochastic assembly processes.
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