Vicky M. Temperton

Bottom-up effects of plant diversity on multitrophic interactions in a biodiversity experiment

Biodiversity is rapidly declining, and this may negatively affect ecosystem processes, including economically important ecosystem services. Previous studies have shown that biodiversity has positive effects on organisms and processes across trophic levels. However, only a few studies have so far incorporated an explicit food-web perspective. In an eight-year biodiversity experiment, we studied an unprecedented range of above- and below-ground organisms and multitrophic interactions. A multitrophic data set originating from a single long-term experiment allows mechanistic insights that would not be gained from meta-analysis of different experiments. Here we show that plant diversity effects dampen with increasing trophic level and degree of omnivory. This was true both for abundance and species richness of organisms. Furthermore, we present comprehensive above-ground/below-ground biodiversity food webs. Both above ground and below ground, herbivores responded more strongly to changes in plant diversity than did carnivores or omnivores. Density and richness of carnivorous taxa was independent of vegetation structure. Below-ground responses to plant diversity were consistently weaker than above-ground responses. Responses to increasing plant diversity were generally positive, but were negative for biological invasion, pathogen infestation and hyperparasitism. Our results suggest that plant diversity has strong bottom-up effects on multitrophic interaction networks, with particularly strong effects on lower trophic levels. Effects on higher trophic levels are indirectly mediated through bottom-up trophic cascades.

Plant species richness and functional composition drive overyielding in a six‐year grassland experiment

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.

Positive interactions between nitrogen-fixing legumes and four different neighbouring species in a biodiversity experiment

The importance of facilitative processes due to the presence of nitrogen-fixing legumes in temperate grasslands is a contentious issue in biodiversity experiments. Despite a multitude of studies of fertilization effects of legumes on associated nonfixers in agricultural systems, we know little about the dynamics in more diverse systems. We hypothesised that the identity of target plant species (phytometers) and the diversity of neighbouring plant species would affect the magnitude of such positive species interactions. We therefore sampled aboveground tissues of phytometers planted into all plots of a grassland biodiversity–ecosystem functioning experiment and analysed their N concentrations, δ15N values and biomasses. The four phytometer species (Festuca pratensis, Plantago lanceolata, Knautia arvensis and Trifolium pratensis) each belonged to one of the four plant functional groups used in the experiment and allowed the effects of diversity on N dynamics in individual species to be assessed. We found significantly lower δ15N values and higher N concentrations and N contents (amount of N per plant) in phytometer species growing with legumes, indicating a facilitative role for legumes in these grassland ecosystems. Our data suggest that the main driving force behind these facilitative interactions in plots containing legumes was reduced competition for soil nitrate (“nitrate sparing”), with apparent N transfer playing a secondary role. Interestingly, species richness (and to a lesser extent functional group number) significantly decreased δ15N values, N concentrations and N content irrespective of any legume effect. Possible mechanisms behind this effect, such as increased N mineralisation and nitrate uptake in more diverse plots, now need further investigation. The magnitude of the positive interactions depended on the identity of the phytometer species. Evidence for increased N uptake in communities containing legumes was found in all three nonlegume phytometer species, with a subsequent strong increase in biomass in the grass F. pratensis across all diversity levels, and a lesser biomass gain in P. lanceolata and K. arvensis. In contrast, the legume phytometer species T. pratense was negatively affected when other legumes were present in their host communities across all diversity levels.

Combined MRI–PET dissects dynamic changes in plant structures and functions

Unravelling the factors determining the allocation of carbon to various plant organs is one of the great challenges of modern plant biology. Studying allocation under close to natural conditions requires non-invasive methods, which are now becoming available for measuring plants on a par with those developed for humans. By combining magnetic resonance imaging (MRI) and positron emission tomography (PET), we investigated three contrasting root/shoot systems growing in sand or soil, with respect to their structures, transport routes and the translocation dynamics of recently fixed photoassimilates labelled with the short-lived radioactive carbon isotope (11)C. Storage organs of sugar beet (Beta vulgaris) and radish plants (Raphanus sativus) were assessed using MRI, providing images of the internal structures of the organs with high spatial resolution, and while species-specific transport sectoralities, properties of assimilate allocation and unloading characteristics were measured using PET. Growth and carbon allocation within complex root systems were monitored in maize plants (Zea mays), and the results may be used to identify factors affecting root growth in natural substrates or in competition with roots of other plants. MRI-PET co-registration opens the door for non-invasive analysis of plant structures and transport processes that may change in response to genomic, developmental or environmental challenges. It is our aim to make the methods applicable for quantitative analyses of plant traits in phenotyping as well as in understanding the dynamics of key processes that are essential to plant performance.

Plant diversity surpasses plant functional groups and plant productivity as driver of soil biota in the long term.

Background One of the most significant consequences of contemporary global change is the rapid decline of biodiversity in many ecosystems. Knowledge of the consequences of biodiversity loss in terrestrial ecosystems is largely restricted to single ecosystem functions. Impacts of key plant functional groups on soil biota are considered to be more important than those of plant diversity; however, current knowledge mainly relies on short-term experiments. Methodology/Principal Findings We studied changes in the impacts of plant diversity and presence of key functional groups on soil biota by investigating the performance of soil microorganisms and soil fauna two, four and six years after the establishment of model grasslands. The results indicate that temporal changes of plant community effects depend on the trophic affiliation of soil animals: plant diversity effects on decomposers only occurred after six years, changed little in herbivores, but occurred in predators after two years. The results suggest that plant diversity, in terms of species and functional group richness, is the most important plant community property affecting soil biota, exceeding the relevance of plant above- and belowground productivity and the presence of key plant functional groups, i.e. grasses and legumes, with the relevance of the latter decreasing in time. Conclusions/Significance Plant diversity effects on biota are not only due to the presence of key plant functional groups or plant productivity highlighting the importance of diverse and high-quality plant derived resources, and supporting the validity of the singular hypothesis for soil biota. Our results demonstrate that in the long term plant diversity essentially drives the performance of soil biota questioning the paradigm that belowground communities are not affected by plant diversity and reinforcing the importance of biodiversity for ecosystem functioning.

Effects of plant diversity on invertebrate herbivory in experimental grassland

The rate at which a plant species is attacked by invertebrate herbivores has been hypothesized to depend on plant species richness, yet empirical evidence is scarce. Current theory predicts higher herbivore damage in monocultures than in species-rich mixtures. We quantified herbivore damage by insects and molluscs to plants in experimental plots established in 2002 from a species pool of 60 species of Central European Arrhenatherum grasslands. Plots differed in plant species richness (1, 2, 4, 8, 16, 60 species), number of functional groups (1, 2, 3, 4), functional group and species composition. We estimated herbivore damage by insects and molluscs at the level of transplanted plant individuals (“phytometer” species Plantago lanceolata, Trifoliumpratense, Rumexacetosa) and of the entire plant community during 2003 and 2004. In contrast to previous studies, our design allows specific predictions about the relative contributions of functional diversity, plant functional identity, and species richness in relation to herbivory. Additionally, the phytometer approach is new to biodiversity-herbivory studies, allowing estimates of species-specific herbivory rates within the larger biodiversity-ecosystem functioning context. Herbivory in phytometers and experimental communities tended to increase with plant species richness and the number of plant functional groups, but the effects were rarely significant. Herbivory in phytometers was in some cases positively correlated with community biomass or leaf area index. The most important factor influencing invertebrate herbivory was the presence of particular plant functional groups. Legume (grass) presence strongly increased (decreased) herbivory at the community level. The opposite pattern was found for herbivory in T. pratense phytometers. We conclude that (1) plant species richness is much less important than previously thought and (2) plant functional identity is a much better predictor of invertebrate herbivory in temperate grassland ecosystems.

Aboveground overyielding in grassland mixtures is associated with reduced biomass partitioning to belowground organs.

We investigated effects of plant species richness in experimental grassland plots on annual above- and belowground biomass production estimated from repeated harvests and ingrowth cores, respectively. Aboveground and total biomass production increased with increasing plant species richness while belowground production remained constant. Root to shoot biomass production ratios (R/S) in mixtures were lower than expected from monoculture performance of the species present in the mixtures, showing that interactions among species led to reduced biomass partitioning to belowground organs. This change in partitioning to belowground organs was not confined to mixtures with legumes, but also measured in mixtures without legumes, and correlated with aboveground overyielding in mixtures. It is suggested that species-rich communities invest less in belowground biomass than do monocultures to extract soil resources, thus leading to increased investment into aboveground organs and overyielding.

Diversity Promotes Temporal Stability across Levels of Ecosystem Organization in Experimental Grasslands

The diversity–stability hypothesis states that current losses of biodiversity can impair the ability of an ecosystem to dampen the effect of environmental perturbations on its functioning. Using data from a long-term and comprehensive biodiversity experiment, we quantified the temporal stability of 42 variables characterizing twelve ecological functions in managed grassland plots varying in plant species richness. We demonstrate that diversity increases stability i) across trophic levels (producer, consumer), ii) at both the system (community, ecosystem) and the component levels (population, functional group, phylogenetic clade), and iii) primarily for aboveground rather than belowground processes. Temporal synchronization across studied variables was mostly unaffected with increasing species richness. This study provides the strongest empirical support so far that diversity promotes stability across different ecological functions and levels of ecosystem organization in grasslands.

The Jena Experiment: six years of data from a grassland biodiversity experiment

This data set contains species-specific biomass and cover data as well as community leaf area index (LAI) and height from a large grassland biodiversity experiment (Jena Experiment). In this experiment, 82 grassland plots of 20 × 20 m were established from a pool of 60 species belonging to four functional groups (grasses, legumes, and tall and small herbs). In May 2002, varying numbers of plant species were sown into the plots to create a gradient of plant species richness (1, 2, 4, 8, 16, and 60 species) and functional richness (1, 2, 3, and 4 functional groups). Plots were maintained by biannual weeding and mowing. The data set encompasses the 2002–2008 May and August biomass harvests from 3–4 subplots of 0.2 × 0.5 m per experimental plot sorted to species. Moreover, plant species and community cover estimated in an approximately 9-m2 subplot per plot are included in the data set. Each biomass harvest was accompanied by measurements of vegetation height and LAI per plot. Analyses of the community biomass data have identified species richness as well as functional group composition as important drivers of a positive biodiversity–productivity relationship. The data set can be used to study a variety of questions about how plant community composition and structure respond to changes in species richness and functional diversity over time. Sampling is ongoing, and new data will be added. The complete data sets corresponding to abstracts published in the Data Papers section of the journal are published electronically in Ecological Archives at 〈http://esapubs.org/archive〉. (The accession number for each Data Paper is given directly beneath the title.)

N2 fixation and performance of 12 legume species in a 6-year grassland biodiversity experiment

Highly variable effects of legumes have been observed in biodiversity experiments, but little is known about plant diversity effects on N2 fixation of legume species. We used the 15N natural abundance method in a non-fertilized regularly mown 6-year biodiversity experiment (Jena Experiment) to quantify N2 fixation of 12 legume species. The proportion of legume N derived from the atmosphere (%Ndfa) differed significantly among legume species. %Ndfa values were lower in 2004 after setting-up the experiment (73 ± 20) than in the later years (2006: 80 ± 16; 2008: 78 ± 12). Increasing species richness had positive effects on %Ndfa in 2004 and 2006, but not in 2008. High biomass production of legumes in 2004 and 2006 declined to lower levels in 2008. In 2006, legume positioning within the canopy best explained variation in %Ndfa values indicating a lower reliance of tall legumes on N2 fixation. In 2008, larger %Ndfa values of legumes were related to lower leaf P concentrations suggesting that the availability of phosphorus limited growth of legumes. In summary, diversity effects on N2 fixation depend on legume species identity, their ability to compete for soil nutrients and light and may vary temporally in response to changing resource availability.