• Energy Research

Plant diversity has been shown to increase community biomass in experimental communities, but the mechanisms resulting in such positive biodiversity effects have remained largely unknown. We used a large‐scale six‐year biodiversity experiment near Jena, Germany, to examine how aboveground community biomass in grasslands is affected by different components of plant diversity and thereby infer the mechanisms that may underlie positive biodiversity effects. As components of diversity we defined the number of species (1–16), number of functional groups (1–4), presence of functional groups (legumes, tall herbs, small herbs, and grasses) and proportional abundance of functional groups. Using linear models, replacement series on the level of functional groups, and additive partitioning on the level of species, we explored whether the observed biodiversity effects originated from disproportionate effects of single functional groups or species or from positive interactions between them.Aboveground community biomass was positively related to the number of species measured across functional groups as well as to the number of functional groups measured across different levels of species richness. Furthermore, increasing the number of species within functional groups increased aboveground community biomass, indicating that species within functional groups were not redundant with respect to biomass production. A positive relationship between the number of functional groups and aboveground community biomass within a particular level of species richness suggested that complementarity was larger between species belonging to different rather than to the same functional groups. The presence of legumes or tall herbs had a strong positive impact on aboveground community biomass whereas the presence of small herbs or grasses had on average no significant effect. Two‐ and three‐way interactions between functional group presences were weak, suggesting that their main effects were largely additive. Replacement series analyses on the level of functional groups revealed strong transgressive overyielding and relative yields >1, indicating facilitation. On the species level, we found strong complementarity effects that increased over time while selection effects due to disproportionate contributions of particular species decreased over time. We conclude that transgressive overyielding between functional groups and species richness effects within functional groups caused the positive biodiversity effects on aboveground community biomass in our experiment.

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

AbstractThe strength of biodiversity–ecosystem functioning (BEF) relationships varies within and across studies, depending on the investigated ecosystem function and diversity facet (e.g., species richness or functional composition), limiting our ability to translate BEF results into recommendations for management and conservation. The variability in BEF relationships is particularly high when considering complex multitrophic communities and can be explained by food web contexts. Here we examine how different plant diversity facets affect biomass stocks and energy flows of each trophic group depending on their position in the trophic network. We used coupled aboveground–belowground multitrophic networks of energy dynamics, assembled across the experimental gradients of grassland plant species richness, functional diversity, and presence of plant functional groups. We compared the strengths of these diversity effects between trophic groups, trophic levels, aboveground versus belowground subnetworks, and types of ecosystem functions. Plant species richness, functional trait diversity, and the presence of legumes and grasses were influential drivers of ecosystem energetics. The effects of plant species richness across the food web often operated through mechanisms of plant functional‐trait diversity. The effects of plant species richness attenuated across trophic levels. Legume presence strengthened the top‐down control (predation) of primary consumers. We found an overall mismatch in the strength of diversity effects on flows versus stocks. Some trophic groups showed even contrasting direction in responses of their stocks and flows to plant diversity. This indicates that plant diversity constrains consumer functioning by means other than only altered consumer biomass. Responses of flows and stocks to plant diversity differed between trophic groups, and aboveground versus belowground parts. Individual stocks and energy flows were responsive to different biodiversity facets, highlighting the importance of the explicit consideration of individual functions and diversity facets for a comprehensive multitrophic understanding. For example, legume presence increased aboveground processes but reduced plant carbon uptake and belowground plant production. Plant communities containing legumes lost more biomass to herbivores, had faster decomposition, and channeled less energy to soil detritus. An important implication of these results is that targeted grassland management would profit from focusing on specific plant diversity facets depending on the ecosystem function or service of interest.

AbstractAimThe aim of this study was to investigate the role of traits in beetle community assembly and test for consistency in these effects among several bioclimatic regions. We asked (1) whether traits predicted species’ responses to environmental gradients (i.e. their niches), (2) whether these same traits could predict co‐occurrence patterns and (3) how consistent were niches and the role of traits among study regions.LocationBoreal forests in Norway and Finland, temperate forests in Germany.TaxonWood‐living (saproxylic) beetles.MethodsWe compiled capture records of 468 wood‐living beetle species from the three regions, along with nine morphological and ecological species traits. Eight climatic and forest covariates were also collected. We used Bayesian hierarchical joint species distribution models to estimate the influence of traits and phylogeny on species’ niches. We also tested for correlations between species associations and trait similarity. Finally, we compared species niches and the effects of traits among study regions.ResultsTraits explained some of the variability in species’ niches, but their effects differed among study regions. However, substantial phylogenetic signal in species niches implies that unmeasured but phylogenetically structured traits have a stronger effect. Degree of trait similarity was correlated with species associations but depended idiosyncratically on the trait and region. Species niches were much more consistent—widespread taxa often responded similarly to an environmental gradient in each region.Main conclusionsThe inconsistent effects of traits among regions limit their current use in understanding beetle community assembly. Phylogenetic signal in niches, however, implies that better predictive traits can eventually be identified. Consistency of species niches among regions means niches may remain relatively stable under future climate and land use changes; this lends credibility to predictive distribution models based on future climate projections but may imply that species’ scope for short‐term adaptation is limited.

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.

Numerous studies have reported positive effects of species richness on plant community productivity. Such biodiversity effects are usually quantified by comparing the performance of plant mixtures with reference monocultures. However, several mechanisms, such as the lack of resource complementarity and facilitation or the accumulation of detrimental agents, suggest that monocultures are more likely than mixtures to deteriorate over time. Increasing biodiversity effects over time could therefore result from declining monocultures instead of reflecting increases in the functioning of mixtures. Commonly, the latter is assumed when positive trends in biodiversity effects occur. Here, we analysed the performance of 60 grassland species growing in monocultures and mixtures over 9 years in a biodiversity experiment to clarify whether their temporal biomass dynamics differed and whether a potential decline of monocultures contributed significantly to the positive net biodiversity effect observed. Surprisingly, individual species' populations produced, on average, significantly more biomass per unit area when growing in monoculture than when growing in mixture. Over time, productivity of species decreased at a rate that was, on average, slightly more negative in monocultures than in mixtures. The mean net biodiversity effect across all mixtures was continuously positive and ranged between 64-217 g per m(2). Short-term increases in the mean net biodiversity effect were only partly due to deteriorating monocultures and were strongly affected by particular species gaining dominance in mixtures in the respective years. We conclude that our species performed, on average, comparably in monocultures and mixtures; monoculture populations being slightly more productive than mixture populations but this trend decreased over time. This suggested that negative feedbacks had not yet affected monocultures strongly but could potentially become more evident in the future. Positive biodiversity effects on aboveground productivity were heavily driven by a small, but changing, set of species that behaved differently from the average species.