Plant trait responses to the environment and effects on ecosystem properties

A combined analysis of plant trait responses to the environment, and their effects on ecosystem properties has recently been proposed. In this study, we related the trait composition of plant communities to soil nutrients and disturbance as environmental drivers and to productivity, decomposition and soil carbon as ecosystem properties. We surveyed two sites, one comprising intensively grazed and fertilized grasslands, the other consisting of semi-natural grassland and open heathland. Species abundance and trait values of 49 species were recorded in 69 plots, as well as parameters describing soil resources, land-use disturbances, and ecosystem properties. Our main goal was to test whether the average or the diversity of the trait values of the vegetation had stronger effects on ecosystem properties (mass ratio vs. diversity hypothesis). Structural equation modeling was used to perform a simultaneous analysis of trait responses and effects. Specific leaf area and leaf nutrient contents were always negatively correlated with stem dry matter content and canopy height, indicating greater investments in supportive and nutrient-conserving tissue as plants increased in size. In the agricultural site, disturbance was the single most important factor decreasing plant height, while leaf traits such as specific leaf area and leaf nutrient contents increased with soil resources in heathlands. Productivity was directly or indirectly driven by leaf traits, and investments in structural tissue increased standing biomass and soil carbon. Different environmental drivers in the two sites produced opposing leaf trait effects on litter decomposition. Ecosystem properties were explained by the community mean trait value as predicted by the mass ratio hypothesis. Evidence for effects of functional diversity on productivity and other ecosystem properties was not detected, suggesting that diversity–productivity relationships depend on the length of the investigated environmental gradients. We conclude that changes in community composition and dominance hierarchies deserve the most attention when ecosystem properties must be maintained.     

Interaction of increasing atmospheric carbon dioxide and soil nitrogen on the carbon balance of tundra microcosms

Summary Natural cores of vegetation and soils of arctic tundra were collected in frozen condition in winter near Barrow, Alaska (71°20N). These cores were used as microcosms in a phytotron experiment to measure the interactions, if any, between increasing atmospheric CO₂ concentration and fertilization by ammonium nitrate on net ecosystem CO₂ exchange and net yield of tundra vegetation. Increased soil N significantly enhanced net ecosystem CO₂ uptake. The effect of increased CO₂ concentration had little or no effect on mean net ecosystem carbon balance of the tundra microcosms. Added N significantly increased leaf area and phytomass of vascular plants in the microcosms while increased atmospheric CO₂ had no effect on these parameters. We conclude that atmospheric CO₂ is not now limiting net ecosystem production in the tundra and that its direct effects will be slight even at double the present concentration. the most probable effects of carbon dioxide in the coastal tundra will be through its indirect effects on temperature, water table, peat decomposition, and the availability of soil nutrients.     

The role of plant species in biomass production and response to elevated CO2 and N

In an experiment that factorially manipulated plant diversity, CO₂, and N, we quantified the effects of the presence of species on assemblage biomass over 10 time points distributed over 5 years. Thirteen of the 16 species planted had statistically significant effects on aboveground and/or belowground biomass. Species differed dramatically in their effects on biomass without any relationship between aboveground and below‐ground effects. Temporal complementarity among species in their effects seasonally, successionally, and in response to a dry summer maintained the diversity–biomass relationships over time and may be the cause behind higher diversity plots having less variation in biomass over time. The response of plant biomass to elevated N, but not CO₂, was at times entirely dependent on the presence of a single species.     

Macrofauna assemblage composition and soil moisture interact to affect soil ecosystem functions

Changing climatic conditions and habitat fragmentation are predicted to alter the soil moisture conditions of temperate forests. It is not well understood how the soil macrofauna community will respond to changes in soil moisture, and how changes to species diversity and community composition may affect ecosystem functions, such as litter decomposition and soil fluxes. Moreover, few studies have considered the interactions between the abiotic and biotic factors that regulate soil processes. Here we attempt to disentangle the interactive effects of two of the main factors that regulate soil processes at small scales - moisture and macrofauna assemblage composition. The response of assemblages of three common temperate soil invertebrates (Glomeris marginata Villers, Porcellio scaber Latreille and Philoscia muscorum Scopoli) to two contrasting soil moisture levels was examined in a series of laboratory mesocosm experiments. The contribution of the invertebrates to the leaf litter mass loss of two common temperate tree species of contrasting litter quality (easily decomposing Fraxinus excelsior L. and recalcitrant Quercus robur L.) and to soil CO₂ fluxes were measured. Both moisture conditions and litter type influenced the functioning of the invertebrate assemblages, which was greater in high moisture conditions compared with low moisture conditions and on good quality vs. recalcitrant litter. In high moisture conditions, all macrofauna assemblages functioned at equal rates, whereas in low moisture conditions there were pronounced differences in litter mass loss among the assemblages. This indicates that species identity and assemblage composition are more important when moisture is limited. We suggest that complementarity between macrofauna species may mitigate the reduced functioning of some species, highlighting the importance of maintaining macrofauna species richness.     

Patterns in Species Persistence and Biomass Production in Soil Microcosms Recovering from a Disturbance Reject a Neutral Hypothesis for Bacterial Community Assembly

The neutral theory of biodiversity has emerged as a major null hypothesis in community ecology. The neutral theory may sufficiently well explain the structuring of microbial communities as the extremely high microbial diversity has led to an expectation of high ecological equivalence among species. To address this possibility, we worked with microcosms of two soils; the microcosms were either exposed, or not, to a dilution disturbance which reduces community sizes and removes some very rare species. After incubation for recovery, changes in bacterial species composition in microcosms compared with the source soils were assessed by pyrosequencing of bacterial 16S rRNA genes. Our assays could detect species with a proportional abundance ≥ 0.0001 in each community, and changes in the abundances of these species should have occurred during the recovery growth, but not be caused by the disturbance per se. The undisturbed microcosms showed slight changes in bacterial species diversity and composition, with a small number of initially low-abundance species going extinct. In microcosms recovering from the disturbance, however, species diversity decreased dramatically (by > 50%); and in most cases there was not a positive relationship between species initial abundance and their chance of persistence. Furthermore, a positive relationship between species richness and community biomass was observed in microcosms of one soil, but not in those of the other soil. The results are not consistent with a neutral hypothesis that predicts a positive abundance-persistence relationship and a null effect of diversity on ecosystem functioning. Adaptation mechanisms, in particular those associated with species interactions including facilitation and predation, may provide better explanations.     

Differential effects of functional traits on aboveground biomass in semi-natural grasslands

Despite increasing evidence on the importance of species functional characteristics for ecosystem processes, two major hypotheses suggest different mechanisms: the ‘mass ratio hypothesis’ assumes that functional traits of the dominant species determine ecosystem processes, while the ‘complementarity hypothesis’ predicts that resource niches may be used more completely when a community is functionally more diverse. Here, we present a method which uses two different groups of biotic predictor variables being (1) abundance‐weighted mean (=aggregated) trait values and (2) functional trait diversity based on Rao's quadratic diversity (FD_Q) to test the competing hypotheses on biodiversity–ecosystem functioning relationships after accounting for co‐varying abiotic factors. We applied this method to data recorded on biodiversity–biomass relationships and environmental variables in 35 semi‐natural temperate grasslands and used a literature‐based matrix of fourteen plant functional traits to assess the explanatory power of models including different sets of predictor variables. Aboveground community biomass did not correlate with species richness. Abiotic factors, in particular soil nitrogen concentration, explained about 50% of variability in aboveground biomass. The best model incorporating functional trait diversity explained only about 30%, while the best model based on aggregated trait values explained about 54% of variability in aboveground biomass. The inclusion of all predictor variable groups in a combined model increased the predictive power to about 75%. This model comprised soil nitrogen concentration as abiotic factor, aggregated traits being indicative for species competitive dominance (rooting depth, leaf distribution, specific leaf area, perennial life cycle) and functional trait diversity in vegetative plant height, leaf area and life cycle. Our study strongly suggests that abiotic factors, trait values of the dominant species and functional trait diversity in combination may best explain differences in aboveground community biomass in natural ecosystems and that their isolated consideration may be misleading.     

General trait relationships in stems: a study on the performance and interrelationships of several functional and structural parameters involved in corticular photosynthesis

We addressed corticular photosynthesis, focusing on parameters of underlying dark and light reactions as well as structural differentiation. To unveil general stem traits and underlying principles that may be valid across several tree species, CO₂ exchange rates and chlorophyll‐fluorescence parameters were measured in current‐year to 3‐year‐old stems of five deciduous tree species (including climax and pioneer species). Across species, dark CO₂ efflux rates (R_d) of stems exhibited a common regression relationship with photosynthetic rates (A) and light‐adapted quantum efficiency of photosystem II (PSII) (ΔF/Fm′), a pattern analogous to leaf trait correlations. Furthermore, A and ΔF/Fm′ were closely interrelated to each other. Consistent correlations of stem structure and function were also assessed among species. Changes in tissue structure during ageing significantly affected several stem functional parameters. Stem CO₂ efflux during the dark and corticular photosynthetic rates declined with increasing stem age as well as light‐adapted quantum efficiency of PSII. Furthermore, a strong relationship between stem R_d and peridermal PFD‐transmittance (T) as well as between R_d and total bark chlorophyll was evident. Consistent results were found for the relationships between corticular photosynthesis (or primary photosynthetic reactions like ΔF/Fm′) and selected structural traits. The found correlation patterns among functional and/or structural traits of stems and their concordance with leaf trait relationships may aid in identifying underlying mechanisms and scaling relationships that link traits to plant and ecosystem function.     

Biogeographic constraints on the world‐wide leaf economics spectrum

Aim The world‐wide leaf economic spectrum (LES) describes tight coordination of leaf traits across global floras, reported to date as being largely independent of phylogeny and biogeography. Here, we present and test an alternative, historical perspective that predicts that biogeography places significant constraints on global trait evolution. These hypothesized constraints could lead to important deviations in leaf trait relationships between isolated floras that were influenced by different magnitudes of genetic constraint and selection. Location Global, including floristic regions of the Northern and Southern Hemispheres, eastern North America, East Asia (EAS), the Hawaiian Islands and tropical mainland floras. Methods We use a large leaf‐trait database (GLOPNET) and species native distribution data to test for variation in leaf trait relationships modulated by floristic region, controlling for climatic differences. Standardized major axis analyses were used to evaluate biogeographic effects on bivariate relationships between LES traits, including relationships of photosynthetic capacity and dark respiration rate (A_mass–R_d‐mass), leaf lifespan and mass per area ratio (LL–LMA), and photosynthetic capacity and nitrogen content (A_mass–N_mass). Results Independent of climate or biome, floras of different evolutionary histories exhibited different leaf trait allometries. Floras of the Northern Hemisphere exhibited greater rates of return on resource investment (steeper slopes for the trait relationships analysed), and the more diverse temperate EAS flora exhibited greater slopes or intercepts in leaf trait relationships, with the exception of the A_mass–N_mass relationship. In contrast to our hypothesis, plants of the floristically isolated Hawaiian Islands exhibited a similar A_mass–N_mass relationship to those of mainland tropical regions. Main conclusions Differences in leaf trait allometries among global floristic regions support a historical perspective in understanding leaf trait relationships and suggest that independent floras can exhibit different tradeoffs in resource capture strategies.     

Competitive displacement alters top-down effects on carbon dioxide concentrations in a freshwater ecosystem

Climate change and invasive species have the potential to alter species diversity, creating novel species interactions. Interspecific competition and facilitation between predators may either enhance or dampen trophic cascades, ultimately influencing total predator effects on communities and biogeochemical cycling of ecosystems. However, previous studies have only investigated the effects of a single predator species on CO₂ flux of aquatic ecosystems. In this study, we measured and compared the individual and joint effects of predatory damselfly larvae and diving beetles on total prey biomass, leaf litter processing, and dissolved CO₂ concentrations of experimental bromeliad ecosystems. Damselfly larvae created strong trophic cascades that reduced CO₂ concentrations by ~46 % relative to no-predator treatments. Conversely, the effects of diving beetles on prey biomass, leaf litter processing, and dissolved CO₂ were not statistically different to no-predator treatments. Relative to multiplicative null models, the presence of damselfly larvae and diving beetles together resulted in antagonistic relations that eliminated trophic cascades and top-down influences on CO₂ concentrations. Furthermore, we showed that the antagonistic interactions between predators occurred due to a tactile response that culminated in competitive displacement of damselfly larvae. Our results demonstrate that predator identity and predator–predator interactions can influence CO₂ concentrations of an aquatic ecosystem. We suggest that predator effects on CO₂ fluxes may depend on the particular predator species removed or added to the ecosystem and their interactions with other predators.     

Stability of above-ground and below-ground processes to extreme drought in model grassland ecosystems: Interactions with plant species diversity and soil nitrogen availability

Extreme drought events have the potential to cause dramatic changes in ecosystem structure and function, but the controls upon ecosystem stability to drought remain poorly understood. Here we used model systems of two commonly occurring, temperate grassland communities to investigate the short-term interactive effects of a simulated 100-year summer drought event, soil nitrogen (N) availability and plant species diversity (low/high) on key ecosystem processes related to carbon (C) and N cycling. Whole ecosystem CO₂ fluxes and leaching losses were recorded during drought and post-rewetting. Litter decomposition and C/N stocks in vegetation, soil and soil microbes were assessed 4 weeks after the end of drought. Experimental drought caused strong reductions in ecosystem respiration and net ecosystem CO₂ exchange, but ecosystem fluxes recovered rapidly following rewetting irrespective of N and species diversity. As expected, root C stocks and litter decomposition were adversely affected by drought across all N and plant diversity treatments. In contrast, drought increased soil water retention, organic nutrient leaching losses and soil fertility. Drought responses of above-ground vegetation C stocks varied depending on plant diversity, with greater stability of above-ground vegetation C to drought in the high versus low diversity treatment. This positive effect of high plant diversity on above-ground vegetation C stability coincided with a decrease in the stability of microbial biomass C. Unlike species diversity, soil N availability had limited effects on the stability of ecosystem processes to extreme drought. Overall, our findings indicate that extreme drought events promote post-drought soil nutrient retention and soil fertility, with cascading effects on ecosystem C fixation rates. Data on above-ground ecosystem processes underline the importance of species diversity for grassland function in a changing environment. Furthermore, our results suggest that plant–soil interactions play a key role for the short-term stability of above-ground vegetation C storage to extreme drought events.     

Microplastics reduce soil microbial network complexity and ecological deterministic selection

Microplastics have been proposed as emerging threats for terrestrial systems as they may potentially alter the physicochemical/biophysical soil environments. Due to the variety of properties of microplastics and soils, the microplastic-induced effects in soil ecosystems are greatly manifold. Here, we studied effects of three polymer microplastics (polyamide-6, polyethylene, and polyethylene terephthalate) on soil properties with four different soil types. The success patterns, interaction relationships, and assembly processes of soil bacterial communities were also studied. Microplastics have the potential to promote CO₂ emissions and enhance the soil humification. Even though microplastics did not significantly alter the diversity and composition of the soil microbial community, the application of microplastics decreased the network complexity and stability, including network size, connectivity, and the number of module and keystone species. The bacterial community assembly was governed by deterministic selection (77.3%–90.9%) in all treatments, while microplastics increased the contribution of stochastic processes from 9.1% in control to 13.6%–22.7%. The neutral model results also indicated most of the bacterial taxa were present in the predicted neutral region (approximately 98%), suggesting the importance of stochastic processes. These findings provided a fundamental insight in understanding the effects of microplastics on soil ecosystems.     

Ecological divergence of <Emphasis Type="Italic">Chaetopteryx rugulosa</Emphasis> species complex (Insecta,Trichoptera) linked to climatic niche diversification

Climate change may affect species diversity and, consequently, ecological processes such as leaf decomposition. We evaluated the effects of increased temperature and carbon dioxide (CO₂) on fungal biomass, leaf breakdown, and on survival and growth of the shredder Phylloicus elektoros. We hypothesized that climatic changes would result in lower survival and growth of shredders and lower leaf consumption by these organisms. On the other hand, we predicted an increase in fungal biomass in response to climatic changes. We conducted an experiment in Manaus, Brazil, using four microcosms that simulate real-time air temperature and CO₂ (control chamber), as well as three other chambers subjected to fixed increases in temperature and CO₂ as compared to the control chamber. The “extreme” condition represented an increase of ~4.5°C in temperature and ~870 ppm in CO₂ in relation to the control chamber. Total and shredder leaf-breakdown rates, fungal biomass, and shredder survival rates were significantly lower in warmer and CO₂ concentrated atmospheres. Shredder growth rate and leaf breakdown by microorganisms were similar among all climatic conditions. With climatic changes, we found an increase in the relative importance of microorganisms on leaf-breakdown rates as compared to shredders. Thus, lower leaf breakdown and a change in the main decomposer due to future climatic conditions may result in major changes in the pathways of organic matter processing and, consequently, in aquatic food webs.     

Diverse mechanisms for CO2 effects on grassland litter decomposition

The ongoing increase in atmospheric CO₂ concentration ([CO₂]) can potentially alter litter decomposition rates by changing: (i) the litter quality of individual species, (ii) allocation patterns of individual species, (iii) the species composition of ecosystems (which could alter ecosystem‐level litter quality and allocation), (iv) patterns of soil moisture, and (v) the composition and size of microbial communities. To determine the relative importance of these mechanisms in a California annual grassland, we created four mixtures of litter that differed in species composition (the annual legume Lotus wrangelianus Fischer & C. Meyer comprised either 10% or 40% of the initial mass) and atmospheric [CO₂] during growth (ambient or double‐ambient). These mixtures decomposed for 33 weeks at three positions (above, on, and below the soil surface) in four types of grassland microcosms (fertilized and unfertilized microcosms exposed to elevated or ambient [CO₂]) and at a common field site. Initially, legume‐rich litter mixtures had higher nitrogen concentrations ([N]) than legume‐poor mixtures. In most positions and environments, the different litter mixtures decomposed at approximately the same rate. Fertilization and CO₂ enrichment of microcosms had no effect on mass loss of litter within them. However, mass loss was strongly related to litter position in both microcosms and the field. Nitrogen dynamics of litter were significantly related to the initial [N] of litter on the soil surface, but not in other positions. We conclude that changes in allocation patterns and species composition are likely to be the dominant mechanisms through which ecosystem‐level decomposition rates respond to increasing atmospheric [CO₂].     

Genotypic variation for condensed tannin production in trembling aspen (POPULUS TREMULOIDES,salicaceae) under elevated CO2 and in high- and low-fertility soil

The carbon/nutrient balance hypothesis suggests that leaf carbon to nitrogen ratios influence the synthesis of secondary compounds such as condensed tannins. We studied the effects of rising atmospheric carbon dioxide on carbon to nitrogen ratios and tannin production. Six genotypes of Populus tremuloides were grown under elevated and ambient CO₂ partial pressure and high- and low-fertility soil in field open-top chambers in northern lower Michigan, USA. During the second year of exposure, leaves were harvested three times (June, August, and September) and analyzed for condensed tannin concentration. The carbon/nutrient balance hypothesis was supported overall, with significantly greater leaf tannin concentration at high CO₂ and low soil fertility compared to ambient CO₂ and high soil fertility. However, some genotypes increased tannin concentration at elevated compared to ambient CO₂, while others showed no CO₂ response. Performance of lepidopteran leaf miner (Phyllonorycter tremuloidiella) larvae feeding on these plants varied across genotypes, CO₂, and fertility treatments. These results suggest that with rising atmospheric CO₂, plant secondary compound production may vary within species. This could have consequences for plant–herbivore and plant–microbe interactions and for the evolutionary response of this species to global climate change.     

Aboveground and belowground responses to quality and heterogeneity of organic inputs to the boreal forest

Leaf litter and other organic resources returned to the soil are important regulators of ecological processes in forest ecosystems, and their ecological impacts may be strongly influenced both by their quality and by interactions between coexisting resource types. To date, most studies on effects of resource identity and mixing have only involved leaf litter, despite the fact that other resource types constitute a major input to the soil. We investigated how quality and heterogeneity of organic substrates found in boreal forests affects the activity and community structure of soil microbes, and plant growth. Six organic substrates (wood, charcoal, berries, sporocarps, vertebrate faeces and leaf litter) were added singly or in mixtures of two, three and six resource types to pots containing forest soil (with or without tree seedlings of Betula pendula Roth). The largest positive effects of single substrates on microbial basal respiration (BR), substrate-induced respiration (SIR) and microbial metabolic quotient (qCO₂) were found for nutrient-rich substrates (faeces and sporocarps) or substrates with high sugar-content (berries). Mixing of substrates had no effect on BR or SIR, but decreased qCO₂ or altered the microbial community structure for specific combinations of substrates. In contrast to the niche complementarity hypothesis, microbial catabolic diversity was not stimulated by greater diversity of resources. Seedling growth responses to single substrates were neutral or negative; the inhibition of growth probably resulted largely from microbial competition for nutrients. Substrate mixing enhanced seedling nutrient-uptake and growth for all mixtures containing sporocarps and leaf litter. Overall, plants responded more strongly to resource heterogeneity than microbes, and synergistic effects only occurred when nutrient-rich substrates were present within the substrate mixtures. In particular, our results demonstrate a role for complex and non-additive interactions among previously overlooked resource types returned to the soil in influencing ecosystem functions such as nutrient cycling and plant productivity.     

Interactive effects of elevated CO2, warming,reduced rainfall,and nitrogen on leaf gas exchange in five perennial grassland species

Global changes can interact to affect photosynthesis and thus ecosystem carbon capture, yet few multi-factor field studies exist to examine such interactions. Here, we evaluate leaf gas exchange responses of five perennial grassland species from four functional groups to individual and interactive global changes in an open-air experiment in Minnesota, USA, including elevated CO₂ (eCO₂), warming, reduced rainfall and increased soil nitrogen supply. All four factors influenced leaf net photosynthesis and/or stomatal conductance, but almost all effects were context-dependent, i.e. they differed among species, varied with levels of other treatments and/or depended on environmental conditions. Firstly, the response of photosynthesis to eCO₂ depended on species and nitrogen, became more positive as vapour pressure deficit increased and, for a C₄ grass and a legume, was more positive under reduced rainfall. Secondly, reduced rainfall increased photosynthesis in three functionally distinct species, potentially via acclimation to low soil moisture. Thirdly, warming had positive, neutral or negative effects on photosynthesis depending on species and rainfall. Overall, our results show that interactions among global changes and environmental conditions may complicate predictions based on simple theoretical expectations of main effects, and that the factors and interactions influencing photosynthesis vary among herbaceous species.     

Disentangling species and functional group richness effects on soil N cycling in a grassland ecosystem

Species richness (SR) and functional group richness (FGR) are often confounded in both observational and experimental field studies of biodiversity and ecosystem function. This precludes discernment of their separate influences on ecosystem processes, including nitrogen (N) cycling, and how those influences might be moderated by global change factors. In a 17‐year field study of grassland species, we used two full factorial experiments to independently vary SR (one or four species, with FGR = 1) and FGR (1–4 groups, with SR = 4) to assess SR and FGR effects on ecosystem N cycling and its response to elevated carbon dioxide (CO₂) and N addition. We hypothesized that increased plant diversity (either SR or FGR) and elevated CO₂ would enhance plant N pools because of greater plant N uptake, but decrease soil N cycling rates because of greater soil carbon inputs and microbial N immobilization. In partial support of these hypotheses, increasing SR or FGR (holding the other constant) enhanced total plant N pools and decreased soil nitrate pools, largely through higher root biomass, and increasing FGR strongly reduced mineralization rates, because of lower root N concentrations. In contrast, increasing SR (holding FGR constant and despite increasing total plant C and N pools) did not alter root N concentrations or net N mineralization rates. Elevated CO₂ had minimal effects on plant and soil N metrics and their responses to plant diversity, whereas enriched N increased plant and soil N pools, but not soil N fluxes. These results show that functional diversity had additional effects on both plant N pools and rates of soil N cycling that were independent of those of species richness.     

Relationship between fresh leaf traits and leaf litter decomposition of 20 plant species in Kerqin sandy land, China

20 plant species (10 monocots and 10 dicots) grown in Kerqin sandy grassland were incubated under indoor conditions to monitor the amount and rate of CO₂ release from the leaf litter. 11 traits of mature fresh leaves including caloric value, contents of Mg, P, N, K, C, C/N, N/P, specific leaf area, dry matter content and leaf surface area were measured to determine the relationship between CO₂ release and leaf characteristics. All those traits have great variation among the 20 species with over 3 fold differences between the maximum and minimum values, and a few traits such as leaf Mg content reached as high as 9 folds. After 28 d's incubation, the average CO₂ release amount from all the species was (4121 ± 1713) μg kg^?1 dry soil. The highest level from Chenopodium acuminatum was (8767 ± 177) μg kg^?1 dry soil, which was 5 folds higher than the lowest level ((1669 ± 47)μg kg^?1 dry soil) from Digitaria sanguinalis. However, CO₂ release rate showed the same trend in all the 20 species, i.e., the leaf litter decomposed faster initially (0–4 d), and gradually slowed down during extended cultural periods. Comparison between monocots and dicots showed that these two taxonomic groups had significant differences in terms of the amount and rate of CO₂ released from leaf litter, and N and C contents, leaf C/N, and dry matter content of mature leaves. Contents of N, C and dry matter, and C/N of mature leaves are significantly correlated with CO₂ release from leaf litter decomposition, which has been revealed by the Pearson correlation test. It can be concluded that these three traits of mature leaves can be used indirectly to predict decomposition rate of the leaf litter.     

Long-term presence of tree species but not chemical diversity affect litter mixture effects on decomposition in a neotropical rainforest

Plant litter diversity effects on decomposition rates are frequently reported, but with a strong bias towards temperate ecosystems. Altered decomposition and nutrient recycling with changing litter diversity may be particularly important in tree species-rich tropical rainforests on nutrient-poor soils. Using 28 different mixtures of leaf litter from 16 Amazonian rainforest tree species, we tested the hypothesis that litter mixture effects on decomposition increase with increasing functional litter diversity. Litter mixtures and all single litter species were exposed in the field for 9 months using custom-made microcosms with soil fauna access. In order to test the hypothesis that the long-term presence of tree species contributing to the litter mixtures increases mixture effects on decomposition, microcosms were installed in a plantation at sites including the respective tree species composition and in a nearby natural forest where these tree species are absent. We found that mixture decomposition deviated from predictions based on single species, with predominantly synergistic effects. Functional litter diversity, defined as either richness, evenness, or divergence based on a wide range of chemical traits, did not explain the observed litter mixture effects. However, synergistic effects in litter mixtures increased with the long-term presence of tree species contributing to these mixtures as the home field advantage hypothesis assumes. Our data suggest that complementarity effects on mixed litter decomposition may emerge through long-term interactions between aboveground and belowground biota.     

Using functional trait diversity to evaluate the contribution of multiple ecological processes to community assembly during succession

Successional chronosequences provide a unique opportunity to study the effects of multiple ecological processes on plant community assembly. Using a series of 0.5 × 0.5 m² plots (n = 30) from five successional sub‐alpine meadow plant communities (ages 3, 5, 9, 12, and undisturbed) in the Qinghai‐Tibetan Plateau, we investigated whether community assembly is stochastic or deterministic for species and functional traits. We tested directional change in species composition, functional trait composition, and then functional trait diversity measured by Rao's quadratic entropy for four traits – plant height, leaf dry matter content, specific leaf area, and seed mass – along two comparable successional chronosequences. We then evaluated the importance of species interactions, habitat filtering and stochasticity by comparing with random communities and partitioning the environmental and spatial components of Rao's quadratic entropy. We found no directional change in species composition, but clear directionality in functional trait composition. None of the abiotic environmental variables (except P) showed linear change with successional age, but soil moisture and nitrogen were positively related to functional diversity within meadows. Functional trait diversity increased significantly with the increase in successional age. Comparison with random communities showed a significant shift from trait divergence in early stages of succession (3‐ and 5‐yr) to convergence in the later stages of succession 9‐, 12‐yr and undisturbed). The relative importance of abiotic variables and spatial structure for functional trait diversity changed in a predictable manner with successional age. Stochasticity at the species level may indicate dispersal limitation, but deterministic effects on functional trait distributions show the role of both habitat effects and biotic interactions.