Do temperate tree species diversity and identity influence soil microbial community function and composition?

Studies of biodiversity–ecosystem function in treed ecosystems have generally focused on aboveground functions. This study investigates intertrophic links between tree diversity and soil microbial community function and composition. We examined how microbial communities in surface mineral soil responded to experimental gradients of tree species richness (SR ), functional diversity (FD ), community‐weighted mean trait value (CWM ), and tree identity. The site was a 4‐year‐old common garden experiment near Montreal, Canada, consisting of deciduous and evergreen tree species mixtures. Microbial community composition, community‐level physiological profiles, and respiration were evaluated using phospholipid fatty acid (PLFA ) analysis and the MicroResp^? system, respectively. The relationship between tree species richness and glucose‐induced respiration (GIR ), basal respiration (BR ), metabolic quotient (qCO ₂) followed a positive but saturating shape. Microbial communities associated with species mixtures were more active (basal respiration [BR ]), with higher biomass (glucose‐induced respiration [GIR ]), and used a greater number of carbon sources than monocultures. Communities associated with deciduous tree species used a greater number of carbon sources than those associated with evergreen species, suggesting a greater soil carbon storage capacity. There were no differences in microbial composition (PLFA ) between monocultures and SR mixtures. The FD and the CWM of several functional traits affected both BR and GIR . In general, the CWM of traits had stronger effects than did FD , suggesting that certain traits of dominant species have more effect on ecosystem processes than does FD . Both the functions of GIR and BR were positively related to aboveground tree community productivity. Both tree diversity (SR ) and identity (species and functional identity—leaf habit) affected soil microbial community respiration, biomass, and composition. For the first time, we identified functional traits related to life‐history strategy, as well as root traits that influence another trophic level, soil microbial community function, via effects on BR and GIR .     

Plant density affects measures of biodiversity effects

Aim s: We tested for the effect of final sowing plant density (i.e. density of established seedlings) on the values of biodiversity effects [transgressive overyielding, net effect, complementarity effect (CE) and selection effect (SE), trait-dependent complementarity and dominance effect] in a glasshouse pot experiment.Methods: We conducted a single-season (4 months) glasshouse experiment. Species monocultures and mixtures containing up to four common meadow species from different functional groups were sown and subsequently thinned to five density levels (8–128 individuals per pot, i.e. 200–3200 individuals m ?2). Community functioning was characterized by yield (both living and dead biomass) of all constituent species.Important Findings: Our results show that plant density (final sowing density in our case, but this finding can be generalized) affects the yields of both monocultures and mixtures. As these and their relationships are the basis for calculation of biodiversity effects, these effects also varied along the density gradient. Net biodiversity effect, CE and SE all increased with density. The net biodiversity effect and the CE switched from negative to quite positive in the four-species mixture. Using Fox's tripartite partitioning, trait-dependent complementarity was minor in comparison to the dominance effect. One of our experimental species did not follow the density-productivity relationship, called constant final yield (CFY), which was reflected in the biodiversity measures. The shape of the density-productivity relationship for experimental species affects also the values of biodiversity indices, particularly when species do not follow the CFY relationship. According to our data and recent simulation experiments, the values of commonly used biodiversity effects can be rather misleading if a species has, e.g. a unimodal dependence of yield for the density gradient and the density level used in the experiment is higher than the peak density.     

Effects of plant species richness on stand structure and productivity

Aims Aboveground biomass production commonly increases with species richness in plant biodiversity experiments. Little is known about the direct mechanisms that cause this result. We tested if by occupying different heights and depths above and below ground, and by optimizing the vertical distribution of leaf nitrogen, species in mixtures can contribute to increased resource uptake and, thus, increased productivity of the community in comparison with monocultures.Methods We grew 24 grassland plant species, grouped into four nonoverlapping species pools, in monoculture and 3- and 6-species mixture in spatially heterogeneous and uniform soil nutrient conditions. Layered harvests of above- and belowground biomass, as well as leaf nitrogen and light measurements, were taken to assess vertical canopy and root space structure.Important findings The distribution of leaf mass was shifted toward greater heights and light absorption was correspondingly enhanced in mixtures. However, only some mixtures had leaf nitrogen concentration profiles predicted to optimize whole-community carbon gain, whereas in other mixtures species seemed to behave more 'selfish'. Nevertheless, even in these communities, biomass production increased with species richness. The distribution of root biomass below ground did not change from monocultures to three- and six-species mixtures and there was also no indication that mixtures were better than monocultures at extracting heterogeneously as compared to homogeneously distributed soil resources. We conclude that positive biodiversity effect on aboveground biomass production cannot easily be explained by a single or few common mechanisms of differential space use. Rather, it seems that mechanisms vary with the particular set of species combined in a community.     

Traits of dominant plant species drive normalized difference vegetation index in grasslands globally

Aim

Theoretical, experimental and observational studies have shown that biodiversity–ecosystem functioning (BEF) relationships are influenced by functional community structure through two mutually non-exclusive mechanisms: (1) the dominance effect (which relates to the traits of the dominant species); and (2) the niche partitioning effect [which relates to functional diversity (FD)]. Although both mechanisms have been studied in plant communities and experiments at small spatial extents, it remains unclear whether evidence from small-extent case studies translates into a generalizable macroecological pattern. Here, we evaluate dominance and niche partitioning effects simultaneously in grassland systems world-wide.

Location

Two thousand nine hundred and forty-one grassland plots globally.

Time period

2000–2014.

Major taxa studied

Vascular plants.

Methods

We obtained plot-based data on functional community structure from the global vegetation plot database “sPlot”, which combines species composition with plant trait data from the “TRY” database. We used data on the community-weighted mean (CWM) and FD for 18 ecologically relevant plant traits. As an indicator of primary productivity, we extracted the satellite-derived normalized difference vegetation index (NDVI) from MODIS. Using generalized additive models and deviation partitioning, we estimated the contributions of trait CWM and FD to the variation in annual maximum NDVI, while controlling for climatic variables and spatial structure.

Results

Grassland communities dominated by relatively tall species with acquisitive traits had higher NDVI values, suggesting the prevalence of dominance effects for BEF relationships. We found no support for niche partitioning for the functional traits analysed, because NDVI remained unaffected by FD. Most of the predictive power of traits was shared by climatic predictors and spatial coordinates. This highlights the importance of community assembly processes for BEF relationships in natural communities.

Main conclusions

Our analysis provides empirical evidence that plant functional community structure and global patterns in primary productivity are linked through the resource economics and size traits of the dominant species. This is an important test of the hypotheses underlying BEF relationships at the global scale.     

Disentangling community functional components in a litter‐macrodetritivore model system reveals the predominance of the mass ratio hypothesis

Recent investigations have shown that two components of community trait composition are important for key ecosystem processes: (i) the community‐weighted mean trait value (CWM), related to the mass ratio hypothesis and dominant trait values in the community, and (ii) functional diversity (FD), related to the complementarity hypothesis and the divergence of trait values. However, no experiments controlling for the inherent dependence between CWM and FD have been conducted so far. We used a novel experimental framework to disentangle the unique and shared effects of CWM and FD in a leaf litter‐macrodetritivore model system. We manipulated isopod assemblages varying in species number, CWM and FD of litter consumption rate to test the relative contribution of these community parameters in the decomposition process. We showed that CWM, but also the combination of CWM and FD, is a main factor controlling litter decomposition. When we tested individual biodiversity components separately, CWM of litter consumption rate showed a significant effect on decomposition, while FD and species richness alone did not. Our study demonstrated that (i) trait composition rather than species diversity drives litter decomposition, (ii) dominant trait values in the community (CWM) play a chief role in driving ecosystem processes, corroborating the mass ratio hypothesis, and (iii) trait dissimilarity can contribute in modulating the overall biodiversity effects. Future challenge is to assess whether the generality of our finding, that is, that dominant trait values (CWM) predominate over trait dissimilarity (FD), holds for other ecosystem processes, environmental conditions and different spatial and temporal scales.     

Diversity effects under different nutrient addition and cutting frequency environments in experimental plant communities

The effects of biodiversity on productivity have been well studied in the past decades. However, the responses of these biodiversity effects to modern grassland managements have not been explicitly tested. By establishing a five years diversity-manipulated experiment with different cutting frequency and nutrient addition levels, we explored the changes of biodiversity effects and the underlying mechanisms under these managements. Our results showed that community biomass increased with species richness. The correlations were observed under all management regimes, but their strengths varied with management intensity. The net biodiversity effects (NE) increased with nutrient supply, but reduced with frequent cutting. These two factors also interactively influenced NE. Importantly, their influences could last 5 years or longer. The NE changes mainly resulted from the variations of complementarity effects (CE), i.e., the aboveground space partitioning of our species. However, the selection effects (SE) were minimally influenced by nutrient addition and cutting frequency, indicating that under these conditions our species had comparably competitive strength. Especially, CE increased over time in highly cutting subplots, suggesting that this relationship was condition–dependent. We conclude that biodiversity is vitally important for ecosystem functioning even when the ecosystems are disturbed by human activities, and is most effective in enhancing biomass productivity under nutrient supply and low cutting frequency conditions. Field studies with species that come from other functional groups are needed to draw a more general conclusion.     

Detecting the role of individual species for overyielding in experimental grassland communities composed of potentially dominant species

Several studies have shown that the contribution of individual species to the positive relationship between species richness and community biomass production cannot be easily predicted from species monocultures. Here, we used a biodiversity experiment with a pool of nine potentially dominant grassland species to relate the species richness–productivity relationship to responses in density, size and aboveground allocation patterns of individual species. Aboveground community biomass increased strongly with the transition from monocultures to two-species mixtures but only slightly with the transition from two- to nine-species mixtures. Tripartite partitioning showed that the strong increase shown by the former was due to trait-independent complementarity effects, while the slight increase shown by the latter was due to dominance effects. Trait-dependent complementarity effects depended on species composition. Relative yield total (RYT) was greater than 1 (RYT > 1) in mixtures but did not increase with species richness, which is consistent with the constant complementarity effect. The relative yield (RY) of only one species, Arrhenatherum elatius, continually increased with species richness, while those of the other species studied decreased with species richness or varied among different species compositions within richness levels. High observed/expected RYs (RYo/RYe > 1) of individual species were mainly due to increased module densities, whereas low observed/expected RYs (RYo/RYe < 1) were due to more pronounced decreases in module density (species with stoloniferous or creeping growth) or module size (species with clearly-defined plant individuals). The trade-off between module density and size, typical for plant populations under the law of constant final yield, was compensated among species. The positive trait-independent complementarity effect could be explained by an increase in community module density, which reached a maximum at low species richness. In contrast, the increasing dominance effect was attributable to the species-specific ability, in particular that of A. elatius, to increase module size, while intrinsic growth limitations led to a suppression of the remaining species in many mixtures. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Complementarity among species in horizontal versus vertical rooting space

Aims: Many experiments have shown a positive effect of species richnesson productivity in grassland plant communities. However, itis poorly understood how environmental conditions affect thisrelationship. We aimed to test whether deep soil and limitingnutrient conditions increase the complementarity effect (CE)of species richness due to enhanced potential for resource partitioning. Methods: We grew monocultures and mixtures of four common grassland speciesin pots on shallow and deep soil, factorially combined withtwo nutrient levels. Soil volume was kept constant to avoidconfounding soil depth and volume. Using an additive partitioningmethod, we separated biodiversity effects on plant productivityinto components due to species complementarity and dominance. Important findings: Net biodiversity and complementarity effects were consistentlyhigher in shallow pots, which was unexpected, and at the lownutrient level. These two results suggest that although belowgroundpartitioning of resources was important, especially under lownutrient conditions, it was not due to differences in rootingdepths. We conclude that in our experiment (i) horizontal rootsegregation might have been more important than the partitioningof rooting depths and (ii) that the positive effects of deepsoil found in other studies were due to the combination of deepersoil with larger soil volume.     

Can root trait diversity explain complementarity effects in a grassland biodiversity experiment?

Aims The positive relationship between plant biodiversity and community productivity is well established. However, our knowledge about the mechanisms underlying these positive biodiversity effects is still limited. One of the main hypotheses is that complementarity in resource uptake is responsible for the positive biodiversity effects: plant species differ in resource uptake strategy, which results in a more complete exploitation of the available resources in space and time when plant species are growing together. Recent studies suggest that functional diversity of the community, i.e. the diversity in functional characteristics ('traits') among species, rather than species richness per se, is important for positive biodiversity effects. However, experimental evidence for specific trait combinations underlying resource complementarity is scarce. As the root system is responsible for the uptake of nutrients and water, we hypothesize that diversity in root traits may underlie complementary resource use and contribute to the biodiversity effects.Methods In a common garden experiment, 16 grassland species were grown in monoculture, 4-species mixtures differing in root trait diversity and 16-species mixtures. The 4-species mixtures were designed to cover a gradient in average rooting depth. Above-ground biomass was cut after one growing season and used as a proxy for plant productivity to calculate biodiversity effects.Important findings Overall, plant mixtures showed a significant increase in biomass and complementarity effects, but this varied greatly between communities. However, diversity in root traits (measured in a separate greenhouse experiment and based on literature) could not explain this variation in complementarity effects. Instead, complementarity effects were strongly affected by the presence and competitive interactions of two particular species. The large variation in complementarity effects and significant effect of two species emphasizes the importance of community composition for positive biodiversity effects. Future research should focus on identifying the traits associated with the key role of particular species for complementarity effects. This may increase our understanding of the links between functional trait composition and biodiversity effects as well as the relative importance of resource complementarity and other underlying mechanisms for the positive biodiversity effects.     

Interspecific interactions and biomass allocation among grassland plant species

Although a handful of studies have shown how interspecific interactions may influence plant shoot to root ratios, the issue of how these interactions influence biomass partitioning among coexisting plant species remains largely unexplored. In this study, we determined whether a given plant species could induce other plant species to allocate relative biomass to each of four zones (aboveground, and three soil depth layers) in a different manner to what they would otherwise, and whether this may influence the nature of competitive or facilitative interactions amongst coexisting plant species. We used a glasshouse study in which mixtures and monocultures of ten grassland plant species were grown in cylindrical pots to determine the effects of plant species mixtures versus monocultures on the production of shoots and of roots of other species for each of three soil depths. Across all experiments, stimulation of production in mixtures was far less common than suppression of production. Different plant species shifted their allocation to shoots or roots at different depths, suggesting that interspecific interactions can either: (1) increase the ratio of deep to shallow roots, perhaps because competition reduces root growth in the uppermost part of the soil profile; or (2) decrease this ratio by reducing plant vigour to such an extent that the plant cannot produce roots that can reach deep enough to exploit resources at lower depths. Further, these results suggest that there are instances in which competition may have the potential to enforce resource partitioning between coexisting plant species by inducing different species to root at different depths to each other.     

Focusing on individual plants to understand community scale biodiversity effects: the case of root distribution in grasslands

Spatial resource partitioning between species via differences in rooting depth is one of the main explanations for the positive biodiversity–productivity relationship. However, evidence for the importance of this mechanism is limited. This may be due to the community scale at which these interactions are often investigated. Community measures represent net outcomes of species interactions and may obscure the mechanisms underlying belowground interactions. Here, we assess the performance of ~1700 individual plants and their heterospecific neighbours over three growing seasons in experimental grassland plots containing one, four or sixteen different plant species and tested whether their performance in mixtures compared to monocultures was related to their own rooting depth versus the rooting depth of their heterospecific neighbours. Overall, individuals of deep-rooting species performed better in mixtures and this effect significantly increased when surrounded by more shallow-rooting species. This effect was not apparent for the shallow rooting species. Together, including both deep and shallow rooting species increased mixture performance. Our results show that taking the perspective of the individual rather than the community can elucidate the interactions between species that contribute to positive biodiversity effects, emphasizing the need for studies at different scales to disentangle the myriad interactions that take place in diverse communities.     

Steeply Increasing Growth Differential Between Mixture and Monocultures of Tropical Trees

Studies of biodiversity and terrestrial ecosystem functioning have concentrated almost exclusively on temperate grasslands. To broaden the reach of biodiversity‐functioning research, five fast growing species, comprising three eudicot trees and two congeneric palms (none symbiotic with nitrogen‐fixing microorganisms), were grown for 13 yr in a replacement‐series mixture and monocultures on a fertile soil in a high‐rainfall area of lowland Costa Rica. The mixture accrued more biomass and had greater net productivity than the average, but not the most productive, monoculture. Relative Land Output (a measure of comparative yield) increased steeply. The combined evidence points to an increase in resource partitioning or facilitation among species over time. Spatial partitioning of aboveground space (for light capture) and soil (possibly for retrieval of deep nitrogen), and facilitation of phosphorus availability by one species, are mechanisms that may account for the inferred complementarity. Extending the generalized findings on biodiversity–productivity relationships from well‐studied grasslands to tropical forests is warranted. Mixtures of fast growing trees can out‐perform the average of their component monocultures, whether the metric is biomass accrual or productivity. The modular growth of long‐lived structure enables arborescent species to retain crown space previously captured and may lead to increased spatial partitioning and facilitation of resources over time.     

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.     

Exotic tree seedlings are much more competitive than natives but show underyielding when growing together

Aims Invasive species continue to be a worldwide threat to ecosystems mainly as a cause for biodiversity loss. Forest ecosystems, for example, are subject to a change in species composition due to the invasion of exotic species. Specifying the attributes that cause the strong competitiveness of several exotic species may improve the ability to understand and effectively manage plant invasions in the future. In this study the following hypotheses were tested: (1) biomass production of below- and aboveground plant components of the exotic tree species is higher than that of the natives, resulting in a higher competitiveness of the exotics; (2) the exclusion of root competition has a positive effect on the biomass production of the inferior native species; and (3) mixtures of native and exotic species yield a higher biomass production than the respective monocultures.Methods A pot experiment, containing about 2000 tree seedlings, was established. We investigated the biomass productivity and growth reactions of two native (Quercus robur L., Carpinus betulus L.) and two exotic tree species (Prunus serotina Ehrh., Robinia pseudoacacia L.) in different intra- and interspecific, competitive situations with and without the influence of root competition.Important findings The biomass production of both exotic species was significantly higher and led to a strong competitive advantage, resulting in a biomass decrease of the less competitive native species. The high belowground biomass of both exotic species had a negative effect on the biomass production. The competitive pressure of exotic tree seedlings on the native ones was largely driven by root competition. Furthermore, mixtures of native and exotic tree species had a higher productivity than their growth in monocultures would have predicted. Competition was lower for exotic species in mixtures with the less productive native species compared to the competition in monocultures or in mixture with the other highly productive exotic species. Accordingly, both highly competitive exotic species produced less biomass in mixture with each other compared to monocultures. Despite the significantly higher biomass of P. serotina in all mixtures and in monoculture, R. pseudoacacia seemed to be the dominating species. Due to its strong root competition, R. pseudoacacia significantly reduced the biomass production of P. serotina .     

Density may alter diversity–productivity relationships in experimental plant communities

Contemporary biodiversity experiments, in which plant species richness is manipulated and aboveground productivity of the system measured, generally demonstrate that lowering plant species richness reduces productivity. However, we propose that community density may in part compensate for this reduction of productivity at low diversity. We conducted a factorial experiment in which plant functional group richness was held constant at three, while plant species richness increased from three to six to 12 species and community density from 440 to 1050 to 2525 seedlings m⁻². Response variables included density, evenness and above- and belowground biomass at harvest. The density gradient converged slightly during the course of the experiment due to about 10% mortality at the highest sowing density. Evenness measured in terms of aboveground biomass at harvest significantly declined with density, but the effect was weak. Overall, aboveground, belowground and total biomass increased significantly with species richness and community density. However, a significant interaction between species richness and community density occurred for both total and aboveground biomass, indicating that the diversity–productivity relationship was flatter at higher than at lower density. Thus, high species richness enabled low-density communities to reach productivity levels otherwise seen only at high density. The relative contributions of the three functional groups C₃, C₄ and nitrogen-fixers to aboveground biomass were less influenced by community density at high than at low species richness. We interpret the interaction effects between community density and species richness on community biomass by expanding findings about constant yield and size variation from monocultures to plant mixtures.     

Light partitioning in experimental grass communities

Through complementary use of canopy space in mixtures, aboveground niche separation has the potential to promote species coexistence and increase productivity of mixtures as compared to monocultures. We set up an experiment with five perennial grass species which differed in height and their ability to compete for light to test whether plants partition light under conditions where it is a limiting resource, and if this resource partitioning leads to increased biomass production in mixtures (using relative yield-based methods). Further, we present the first application of a new model of light competition in plant communities. We show that under conditions where biomass production was high and light a limiting resource, only a minority of mixtures outperformed monocultures and overyielding was slight. The observed overyielding could not be explained by species differences in canopy structure and height in monoculture and was also not related to changes in the canopy traits of species when grown in mixture rather than monoculture. However, where overyielding occurred, it was associated with higher biomass density and light interception. In the new model of competition for light, greater light use complementarity was related to increased total energy absorption. Future work should address whether greater canopy space-filling is a cause or consequence of overyielding.     

Plant biomass production and soil nitrogen in mixtures and monocultures of old field Mediterranean annuals

We examine the relationship between plant diversity and ecosystem properties in a Mediterranean grassland. Five legumes, three grasses and two forb species are grown in monocultures and compared with mixtures that include these ten species. Trifolium angustifolium L. (a legume), Lolium rigidum Gaudin (a grass), and Centaurea solstitialis L. (a forb), are replicated in monocultures. Plant cover, root length and biomass, and concentrations of soil nitrate and ammonium are measured in all plots in March and May. Aboveground biomass is measured at a final harvest in late May to early June. Root biomass is significantly higher in the species mixtures than the average of the monocultures. Plant cover and root length are marginally significantly higher (0.05 < P ≤ 0.1) in the mixtures compared to the average of the monocultures. Soil inorganic nitrogen concentrations and aboveground biomass do not significantly differ between the average of the monocultures and the mixtures. Aboveground biomass in T. angustifolium monocultures is significantly higher than in the mixtures, and on average the legume monocultures do not differ significantly from the mixtures. Root length and biomass in L. rigidum monocultures are higher than in the mixtures in March. Nitrate concentrations (which are negatively correlated with root length and biomass) are the lowest in C. solstitialis in May. Thus, we have evidence that some of the measures of ecosystem performance decline in the average of the monocultures when compared with the mixtures, but mixtures never outperform or do more poorly than the best performing monocultures.     

Leaf decomposition depends on nutritional trait values but increasing trait variability does not always increase decomposition efficiency

The mass ratio (MRH) and niche complementarity (NCH) hypotheses can explain how leaf trait composition drives decomposition, an ecosystem process linked to nutrient cycling and carbon sequestration. However, few studies have used an experiment designed to disentangle the role of the mechanisms proposed by these hypotheses. This is especially true regarding the role of leaf functional traits for decomposition rates in tropical ecosystems. Here, we quantified the biomass loss of 120 leaf mixtures assembled according to four quasi-orthogonal combinations of different mean trait values (community-weighted mean; CWM) and trait variability (functional diversity; FD) of three leaf functional traits (leaf nitrogen and leaf magnesium concentrations and specific leaf area). We found that CWM values of leaf nutritional traits were positively related to greater biomass loss. This supports the hypothesis that the mean trait values of leaf mixtures can drive biomass loss (MRH). However, contrary to NCH expectations, in some circumstances, increasing trait variability of leaf nutritional traits decreased biomass loss. Our results reinforce some previous evidence that, together, CWM and FD can explain leaf decomposition and highlight that the mean resource quality of leaf mixtures is a driver of biomass loss. Also, as previously reported for temperate ecosystems, trait variability does not always increase leaf decomposition in tropical ecosystems. Therefore, there is a need to consider simultaneously both MRH and NCH in future studies, using an appropriate design, keeping in mind that both mechanisms will always be present in any species mixture or combination.     

Predicting individual plant performance in grasslands

Plant functional traits are widely used to predict community productivity. However, they are rarely used to predict individual plant performance in grasslands. To assess the relative importance of traits compared to environment, we planted seedlings of 20 common grassland species as phytometers into existing grassland communities varying in land‐use intensity. After 1 year, we dug out the plants and assessed root, leaf, and aboveground biomass, to measure plant performance. Furthermore, we determined the functional traits of the phytometers and of all plants growing in their local neighborhood. Neighborhood impacts were analyzed by calculating community‐weighted means (CWM) and functional diversity (FD) of every measured trait. We used model selection to identify the most important predictors of individual plant performance, which included phytometer traits, environmental conditions (climate, soil conditions, and land‐use intensity), as well as CWM and FD of the local neighborhood. Using variance partitioning, we found that most variation in individual plant performance was explained by the traits of the individual phytometer plant, ranging between 19.30% and 44.73% for leaf and aboveground dry mass, respectively. Similarly, in a linear mixed effects model across all species, performance was best predicted by phytometer traits. Among all environmental variables, only including land‐use intensity improved model quality. The models were also improved by functional characteristics of the local neighborhood, such as CWM of leaf dry matter content, root calcium concentration, and root mass per volume as well as FD of leaf potassium and root magnesium concentration and shoot dry matter content. However, their relative effect sizes were much lower than those of the phytometer traits. Our study clearly showed that under realistic field conditions, the performance of an individual plant can be predicted satisfyingly by its functional traits, presumably because traits also capture most of environmental and neighborhood conditions.     

Species richness and identity affect the use of aboveground space in experimental grasslands

Complementary resource use is regarded as a mechanism that contributes to positive relationships between biodiversity and ecosystem functioning. Here, we used a biodiversity experiment composed of nine potentially dominant species (grasses: Alopecurus pratensis, Arrhenatherum elatius, Dactylis glomerata, Phleum pratense, Poa trivialis; legumes: Trifolium pratense, T. repens; non-legume herbs: Anthriscus sylvestris, Geranium pratense) to test for differences among monocultures and mixtures and for effects of species richness and the presence of particular species on the use of aboveground space. The number of rooting shoots determined in a line transect increased from monocultures to mixtures. Particularly, the presence of A. elatius in mixtures caused a higher shoot density at the community level. The number of pin contacts per sampling point (cumulative cover) at the community level, analysed with the point intercept method, was higher in mixtures than monocultures, and higher in mixtures with than without A. elatius. The effect was attributable to increased densities across the strata of the vertical stand profile as well as to an increase in community height. The impact of species richness on the use of aboveground space differed considerably between individual species. A. elatius achieved increased densities across all strata of the stand profile, while D. glomerata reached higher densities with a more pronounced use of space in the upper strata with increasing species richness of mixtures. Cumulative cover of P. pratense and A. pratensis was not affected by species richness, while the remaining species decreased space use mostly in the upper strata with increasing species richness or in mixtures with the competitively superior A. elatius. Our study shows that potentially dominant species are limited in their ability for adaptive responses to canopy shading. Nevertheless, the differential responses to species richness of individual species with regard to vertical niche occupation resulted in positive diversity effects on aboveground space use at the community level.