Legume species differ in the responses of their functional traits to plant diversity

Plants can respond to environmental impacts by variation in functional traits, thereby increasing their performance relative to neighbors. We hypothesized that trait adjustment should also occur in response to influences of the biotic environment, in particular different plant diversity of the community. We used 12 legume species as a model and assessed their variation in morphological, physiological, life-history and performance traits in experimental grasslands of different plant species (1, 2, 4, 8, 16 and 60) and functional group (1–4) numbers. Mean trait values and their variation in response to plant diversity varied among legume species and from trait to trait. The tall-growing Onobrychis viciifolia showed little trait variation in response to increasing plant diversity, whereas the species with shorter statures responded in apparently adaptive ways. The formation of longer shoots with elongated internodes, increased biomass allocation to supporting tissue at the cost of leaf mass, reduced branching, higher specific leaf areas and lower foliar δ¹³C values indicated increasing efforts for light acquisition in more diverse communities. Although leaf nitrogen concentrations and shoot biomass:nitrogen ratios were not affected by increasing plant diversity, foliar δ¹⁵N values of most legumes decreased and the application of the ¹⁵N natural abundance method suggested that they became more reliant on symbiotic N₂ fixation. Some species formed fewer inflorescences and delayed flowering with increasing community diversity. The observed variation in functional traits generally indicated strategies of legumes to optimize light and nutrient capturing, but they were largely species-dependent and only partly attributable to increasing canopy height and community biomass with increasing plant diversity. Thus, the analysis of individual plant species and their adjustment to growth conditions in communities of increasing plant diversity is essential to get a deeper insight into the mechanisms behind biodiversity–ecosystem functioning relationships.     

Plant community diversity influences allocation to direct chemical defence in Plantago lanceolata

Background

Forecasting the consequences of accelerating rates of changes in biodiversity for ecosystem functioning requires a mechanistic understanding of the relationships between the structure of biological communities and variation in plant functional characteristics. So far, experimental data of how plant species diversity influences the investment of individual plants in direct chemical defences against herbivores and pathogens is lacking.

Methodology/Principal Findings

We used Plantago lanceolata as a model species in experimental grasslands differing in species richness and composition (Jena Experiment) to investigate foliar concentrations of the iridoid glycosides (IG), catalpol and its biosynthetic precursor aucubin. Total IG and aucubin concentrations decreased, while catalpol concentrations increased with increasing plant diversity in terms of species or functional group richness. Negative plant diversity effects on total IG and aucubin concentrations correlated with increasing specific leaf area of P. lanceolata, suggesting that greater allocation to light acquisition reduced the investment into these carbon-based defence components. In contrast, increasing leaf nitrogen concentrations best explained increasing concentrations of the biosynthetically more advanced IG, catalpol. Observed levels of leaf damage explained a significant proportion of variation in total IG and aucubin concentrations, but did not account for variance in catalpol concentrations.

Conclusions/Significance

Our results clearly show that plants growing in communities of varying species richness and composition differ in their defensive chemistry, which may modulate plant susceptibility to enemy attack and consequently their interactions with higher trophic level organisms.     

Adaptive survival mechanisms and growth limitations of small-stature herb species across a plant diversity gradient

Several biodiversity experiments have shown positive effects of species richness on aboveground biomass production, but highly variable responses of individual species. The well-known fact that the competitive ability of plant species depends on size differences among species, raises the question of effects of community species richness on small-stature subordinate species. We used experimental grasslands differing in species richness (1-60 species) and functional group richness (one to four functional groups) to study biodiversity effects on biomass production and ecophysiological traits of five small-stature herbs (Bellis perennis, Plantago media, Glechoma hederacea, Ranunculus repens and Veronica chamaedrys). We found that ecophysiological adaptations, known as typical shade-tolerance strategies, played an important role with increasing species richness and in relation to a decrease in transmitted light. Specific leaf area and leaf area ratio increased, while area-based leaf nitrogen decreased with increasing community species richness. Community species richness did not affect daily leaf carbohydrate turnover of V. chamaedrys and P. media indicating that these species maintained efficiency of photosynthesis even in low-light environments. This suggests an important possible mechanism of complementarity in such grasslands, whereby smaller species contribute to a better overall efficiency of light use. Nevertheless, these species rarely contributed a large proportion to community biomass production or achieved higher yields in mixtures than expected from monocultures. It seems likely that the allocation to aboveground plant organs to optimise carbon assimilation limited the investment in belowground organs to acquire nutrients and thus hindered these species from increasing their performance in multi-species mixtures.     

Nutrient stoichiometry and land use rather than species richness determine plant functional diversity

Plant functional traits reflect individual and community ecological strategies. They allow the detection of directional changes in community dynamics and ecosystemic processes, being an additional tool to assess biodiversity than species richness. Analysis of functional patterns in plant communities provides mechanistic insight into biodiversity alterations due to anthropogenic activity. Although studies have consi‐dered of either anthropogenic management or nutrient availability on functional traits in temperate grasslands, studies combining effects of both drivers are scarce. Here, we assessed the impacts of management intensity (fertilization, mowing, grazing), nutrient stoichiometry (C, N, P, K), and vegetation composition on community‐weighted means (CWMs) and functional diversity (Rao's Q) from seven plant traits in 150 grasslands in three regions in Germany, using data of 6 years. Land use and nutrient stoichiometry accounted for larger proportions of model variance of CWM and Rao's Q than species richness and productivity. Grazing affected all analyzed trait groups; fertilization and mowing only impacted generative traits. Grazing was clearly associated with nutrient retention strategies, that is, investing in durable structures and production of fewer, less variable seed. Phenological variability was increased. Fertilization and mowing decreased seed number/mass variability, indicating competition‐related effects. Impacts of nutrient stoichiometry on trait syndromes varied. Nutrient limitation (large N:P, C:N ratios) promoted species with conservative strategies, that is, investment in durable plant structures rather than fast growth, fewer seed, and delayed flowering onset. In contrast to seed mass, leaf‐economics variability was reduced under P shortage. Species diversity was positively associated with the variability of generative traits. Synthesis. Here, land use, nutrient availability, species richness, and plant functional strategies have been shown to interact complexly, driving community composition, and vegetation responses to management intensity. We suggest that deeper understanding of underlying mechanisms shaping community assembly and biodiversity will require analyzing all these parameters.     

氮肥添加对青藏高原高寒草甸6个群落优势种生态化学计量学特征的影响

以青藏高原高寒草甸为研究对象, 通过人工氮肥添加试验, 研究6个群落优势种在不同施氮(N)水平下叶片碳(C)、N、磷(P)元素含量的变化以及生态化学计量学特征。结果表明: 自然条件下, 6个物种叶片N、P质量浓度存在显著的差异, 表现为: 黄花棘豆(Oxytropis ochrocephala)最高, 为24.5和2.51 g·kg^-1, 其叶片N含量低于而P含量高于我国其他草地的豆科植物; 其余5个物种叶片N、P质量浓度分别为11.5-18.1和1.49-1.72 g·kg^-1, 嵩草(Kobresia myosuroides)叶片N含量最低, 垂穗披碱草(Elymus nutans)叶片P含量最低, 与我国其他区域的研究结果相比, 其叶片N和P含量均低于我国其他草地非豆科植物。随氮素添加量的增大, 6种群落优势种叶片的C和P含量保持不变; 其他5种植物叶片N含量显著增加, 黄花棘豆叶片N含量保持不变。未添加氮肥时, 6种植物叶片N:P为7.3-11.2, 说明该区植物生长更多地受N限制。随N添加量的增加, 除黄花棘豆外, 其他5种植物叶片N:P大于16, 表现为植物生长受P限制。综合研究表明, 青藏草原高寒草甸植物叶片N含量较低, 植物受N影响显著, 但不同物种对N的添加反应不同, 豆科植物黄花棘豆叶片对N添加不敏感, 其他5个物种叶片全N含量随着N添加量的升高而增加, 该研究结果可为高寒草甸科学施肥提供理论依据。     

Simple plant traits explain functional group diversity decline in novel grassland communities of Texas

Previous research has found that plant diversity declines more quickly in exotic than native grassland plots, which offers a model system for testing whether diversity decline is associated with specific plant traits. In a common garden experiment in the Southern Great Plains in central Texas, USA, we studied monocultures and 9-species mixtures of either all exotic or all native grassland species. A total of 36 native and exotic species were paired by phylogeny and functional group. We used community-level measures (relative abundance in mixture) and whole-plant (height, aboveground biomass, and light capture) and leaf-level traits (area, specific leaf area, and C:N ratio) to determine whether trait differences explained native-exotic differences in functional group diversity. Increases in species’ relative abundance in mixture were correlated with high biomass, height, and light capture in both native and exotic communities. However, increasing exotic species were all C₄ grasses, whereas, increasing native species included forb, C₃ grass and C₄ grass species. Exotic C₄ grasses had traits associated with relatively high resource capture: greater leaf area, specific leaf area, height, biomass, and light capture, but similar leaf C:N ratios compared to native C₄ grasses. Leaf C:N was consistently higher for native than exotic C₃ species, implying that resource use efficiency was greater in natives than exotics. Our results suggest that functional diversity will differ between grasslands restored to native assemblages and those dominated by novel collections of exotic species, and that simple plant traits can help to explain diversity decline.     

Metabolomics Unravel Contrasting Effects of Biodiversity on the Performance of Individual Plant Species

In spite of evidence for positive diversity-productivity relationships increasing plant diversity has highly variable effects on the performance of individual plant species, but the mechanisms behind these differential responses are far from being understood. To gain deeper insights into the physiological responses of individual plant species to increasing plant diversity we performed systematic untargeted metabolite profiling on a number of herbs derived from a grassland biodiversity experiment (Jena Experiment). The Jena Experiment comprises plots of varying species number (1, 2, 4, 8, 16 and 60) and number and composition of functional groups (1 to 4; grasses, legumes, tall herbs, small herbs). In this study the metabolomes of two tall-growing herbs (legume: Medicago x varia; non-legume: Knautia arvensis) and three small-growing herbs (legume: Lotus corniculatus; non-legumes: Bellis perennis, Leontodon autumnalis) in plant communities of increasing diversity were analyzed. For metabolite profiling we combined gas chromatography coupled to time-of-flight mass spectrometry (GC-TOF-MS) and UPLC coupled to FT-ICR-MS (LC-FT-MS) analyses from the same sample. This resulted in several thousands of detected m/z-features. ANOVA and multivariate statistical analysis revealed 139 significantly changed metabolites (30 by GC-TOF-MS and 109 by LC-FT-MS). The small-statured plants L. autumnalis, B. perennis and L. corniculatus showed metabolic response signatures to increasing plant diversity and species richness in contrast to tall-statured plants. Key-metabolites indicated C- and N-limitation for the non-leguminous small-statured species B. perennis and L. autumnalis, while the metabolic signature of the small-statured legume L. corniculatus indicated facilitation by other legumes. Thus, metabolomic analysis provided evidence for negative effects of resource competition on the investigated small-statured herbs that might mechanistically explain their decreasing performance with increasing plant diversity. In contrast, taller species often becoming dominant in mixed plant communities did not show modified metabolite profiles in response to altered resource availability with increasing plant diversity. Taken together, our study demonstrates that metabolite profiling is a strong diagnostic tool to assess individual metabolic phenotypes in response to plant diversity and ecophysiological adjustment.     

Plant Species Loss Affects Life-History Traits of Aphids and Their Parasitoids

The consequences of plant species loss are rarely assessed in a multi-trophic context and especially effects on life-history traits of organisms at higher trophic levels have remained largely unstudied. We used a grassland biodiversity experiment and measured the effects of two components of plant diversity, plant species richness and the presence of nitrogen-fixing legumes, on several life-history traits of naturally colonizing aphids and their primary and secondary parasitoids in the field. We found that, irrespective of aphid species identity, the proportion of winged aphid morphs decreased with increasing plant species richness, which was correlated with decreasing host plant biomass. Similarly, emergence proportions of parasitoids decreased with increasing plant species richness. Both, emergence proportions and proportions of female parasitoids were lower in plots with legumes, where host plants had increased nitrogen concentrations. This effect of legume presence could indicate that aphids were better defended against parasitoids in high-nitrogen environments. Body mass of emerged individuals of the two most abundant primary parasitoid species was, however, higher in plots with legumes, suggesting that once parasitoids could overcome aphid defenses, they could profit from larger or more nutritious hosts. Our study demonstrates that cascading effects of plant species loss on higher trophic levels such as aphids, parasitoids and secondary parasitoids begin with changed life-history traits of these insects. Thus, life-history traits of organisms at higher trophic levels may be useful indicators of bottom-up effects of plant diversity on the biodiversity of consumers.     

Effects of N addition on ecological stoichiometric characteristics in six dominant plant species of alpine meadow on the Qinghai-Xizang Plateau,China

以青藏高原高寒草甸为研究对象, 通过人工氮肥添加试验, 研究6个群落优势种在不同施氮(N)水平下叶片碳(C)、N、磷(P)元素含量的变化以及生态化学计量学特征。结果表明: 自然条件下, 6个物种叶片N、P质量浓度存在显著的差异, 表现为: 黄花棘豆(Oxytropis ochrocephala)最高, 为24.5和2.51 g·kg^-1, 其叶片N含量低于而P含量高于我国其他草地的豆科植物; 其余5个物种叶片N、P质量浓度分别为11.5-18.1和1.49-1.72 g·kg^-1, 嵩草(Kobresia myosuroides)叶片N含量最低, 垂穗披碱草(Elymus nutans)叶片P含量最低, 与我国其他区域的研究结果相比, 其叶片N和P含量均低于我国其他草地非豆科植物。随氮素添加量的增大, 6种群落优势种叶片的C和P含量保持不变; 其他5种植物叶片N含量显著增加, 黄花棘豆叶片N含量保持不变。未添加氮肥时, 6种植物叶片N:P为7.3-11.2, 说明该区植物生长更多地受N限制。随N添加量的增加, 除黄花棘豆外, 其他5种植物叶片N:P大于16, 表现为植物生长受P限制。综合研究表明, 青藏草原高寒草甸植物叶片N含量较低, 植物受N影响显著, 但不同物种对N的添加反应不同, 豆科植物黄花棘豆叶片对N添加不敏感, 其他5个物种叶片全N含量随着N添加量的升高而增加, 该研究结果可为高寒草甸科学施肥提供理论依据。     

How plant diversity and legumes affect nitrogen dynamics in experimental grassland communities

Positive relationships between species richness and ecosystem processes such as productivity or nitrogen cycling can be the result of a number of mechanisms. We examined how species richness, biomass, and legume presence, diversity, and abundance explained nitrogen dynamics in experimental grassland plots in northern Sweden. Nitrogen concentrations and '¹⁵N values were measured in plants grown in 28 mixtures (58 plots) including 1, 2, 4, 8 or 12 local grassland species over four years. Values for '¹⁵N declined over time for all three functional groups (grasses, legumes, and non-leguminous forbs), suggesting greater reliance on N fixed by legumes over time by all species. Above ground percent nitrogen (%N) also declined over time but root %N and total N did not. Path analysis of above ground data suggested that two main factors affected %N and the size of the N pool. First, higher plant diversity (species richness) increased total N through increased biomass in the plot. Although in the first two years of the experiment this was the result of a greater probability of inclusion of at least one legume, in the last two years diversity had a significant effect on biomass beyond this effect. Second, percent legumes planted in the plots had a strong effect on above ground %N and '¹⁵N, but a much smaller effect on above ground biomass. In contrast, greater plant diversity affected N in roots both by increasing biomass and by decreasing %N (after controlling for effects mediated by root biomass and legume biomass). Increased legume biomass resulted in higher %N and lower '¹⁵N for both non-legume forbs and grasses in the first year, but only for grasses in the third year. We conclude that a sampling effect (greater probability of including a legume) contributed towards greater biomass and total N in high-diversity communities early on in the experiment, but that over time this effect weakened and other positive effects of diversity became more important.     

Increasing Soil Nutrient Loads of European Semi-natural Grasslands Strongly Alter Plant Functional Diversity Independently of Species Loss

Anthropogenically increased input of nitrogen (N) and phosphorous (P) have led to a severe reduction of plant species richness in European semi-natural grasslands. Although it is well established that this species loss is not trait neutral, a thorough analysis of the effects of nutrient addition on trait based functional diversity and functional composition, independently of species loss, is lacking so far. We compiled data on the plant species abundance (relevé’s) of 279 Nardus grasslands from nine European countries, across a gradient of soil N and P content. Functional diversity (Petchy and Gaston’s FDc, weighted FDc and quadratic entropy) and mean trait composition were calculated for each relevé, based on 21 functional traits. Differences in functional diversity and functional composition were related to differences in soil N, atmospheric N deposition, soil P and pH, while controlling for geographic location and species richness. All functional diversity measures decreased with increasing soil N, with wFDc also decreased by soil P, independent of species loss. This was accompanied by clear shifts in functional trait composition, associated with shifts from below-ground competition for nutrients to above-ground competition for light. This resulted in a decrease in insect-pollinated therophytes and chamaephytes and an increase in long-lived, clonal graminoids and hemicryptophytes under increasing soil N and P. These functional community changes can be expected to alter both ecosystem functioning and service provisioning of the studied grasslands. Our research emphasizes the importance of a reduction of both N and P emission throughout Europe for sustainable conservation of these communities.     

Plant species diversity and composition of experimental grasslands affect genetic differentiation of Lolium perenne populations

Contrasting hypotheses exist about the relationship between plant species diversity and genetic diversity. However, experimental data of species diversity effects on genetic differentiation among populations are lacking. To address this, Lolium perenne was sown with an equal number of seeds in 78 experimental grasslands (Jena Experiment) varying in species richness (1, 2, 4, 8 to 16) and functional group richness and composition (1-4; grasses, legumes, small herbs, tall herbs). Population sizes were determined 4years after sowing, and single-nucleotide polymorphism (SNP) DNA markers based on bulk samples of up to 100 individuals per population were applied. Genetic distances between the field populations and the initially sown seed population increased with sown species richness. The degree of genetic differentiation from the original seed population was largely explained by actual population sizes, which suggests that genetic drift was the main driver of differentiation. Weak relationships among relative allele frequencies and species diversity or actual population sizes, and a positive correlation between actual population sizes and expected heterozygosity also supported the role of genetic drift. Functional composition had additional effects on genetic differentiation of L. perenne populations, indicating a selection because of genotype-specific interactions with other species. Our study supports that genetic diversity is likely to be lower in plant communities with a higher number of interspecific competitors. Negative effects of species richness on population sizes may increase the probability of genetic drift, and selection because of genotype-specific interactions depending on species and genotypic community composition may modulate this relationship.     

Long‐term effects of plant diversity and composition on plant stoichiometry

Plant elemental composition can indicate resource limitation, and changes in key elemental ratios (e.g. plant C:N ratios) can influence rates including herbivory, nutrient recycling, and pathogen infection. Although plant stoichiometry can influence ecosystem‐level processes, very few studies have addressed whether and how plant C:N stoichiometry changes with plant diversity and composition. Here, using two long‐term experimental manipulations of plant diversity (Jena and Cedar Creek), we test whether plant richness (species and functional groups) or composition (functional group proportions) affects temporal trends and variability of community‐wide C:N stoichiometry. Site fertility determined the initial community‐scale C:N ratio. Communities growing on N‐poor soil (Cedar Creek) began with higher C:N ratios than communities growing on N‐rich soil (Jena). However, site‐level plant C:N ratios converged through time, most rapidly in high diversity plots. In Jena, plant community C:N ratios increased. This temporal trend was stronger with increasing richness. However, temporal variability of C:N decreased as plant richness increased. In contrast, C:N decreased over time at Cedar Creek, most strongly at high species and functional richness, whereas the temporal variability of C:N increased with both measures of diversity at this site. Thus, temporal trends in the mean and variability of C:N were underlain by concordant changes among sites in functional group proportions. In particular, the convergence of community‐scale C:N over time at these very different sites was mainly due to increasing proportions of forbs at both sites, replacing high mean C:N (C4 grasses, Cedar Creek) or low C:N (legumes, Jena) species. Diversity amplified this convergence; although temporal trends differed in sign between the sites, these trends increased in magnitude with increasing species richness. Our results suggest a predictive mechanistic link between trends in plant diversity and functional group composition and trends in the many ecosystem rates that depend on aboveground community C:N. Synthesis We compared the effect of plant diversity on the temporal dynamics of community stoichiometry in two long‐term grassland diversity experiments: the Cedar Creek and Jena Experiments. Changes in community C:N ratios were accelerated by increasing diversity at both sites, but in opposite directions depending on soil fertility. Stoichiometry changes were driven by shifts of functional group composition differing in their elemental compositions, the identity of the functional groups depending on the site. Thus, we highlighted that community turnover constrained the effect of diversity on plant stoichiometry at both sites     

Interspecific competition alters leaf stoichiometry in 20 grassland species

The extensive use of traits in ecological studies over the last few decades to predict community functions has revealed that plant traits are plastic and respond to various environmental factors. These plant traits are assumed to predict how plants compete and capture resources. Variation in stoichiometric ratios both within and across species reflects resource capture dynamics under competition. However, the impact of local plant diversity on species‐specific stoichiometry remains poorly studied. Here, we analyze how spatial and temporal diversity in resource‐acquisition traits affects leaf elemental stoichiometry of plants (i.e. the result of resource capture) and how flexible this stoichiometry is depending on the functional composition of the surrounding community. Therefore, we assessed inter‐ and intraspecific variations of leaf carbon (C), nitrogen (N), and phosphorus (P) (and their ratios) of 20 grassland species in a large trait‐based plant diversity experiment located in Jena (Germany) by measuring leaf elemental concentrations at the species‐level along a gradient in plant trait dissimilarity. Our results show that plants showed large intra‐ and interspecific variation in leaf stoichiometry, which was only partly explained by the functional group identity (grass or herb) of the species. Elemental concentrations (N, P, but not C) decreased with plant species richness, and species tended to become more deviant from their monoculture stoichiometry with increasing trait dissimilarity in the community. These responses differed among species, some consistently increased or decreased in P and N concentrations; for other species, the negative or positive change in P and N concentrations increased with increasing trait difference between the target species and the remaining community. The strength of this relationship was significantly associated to the relative position of the species along trait gradients related to resource acquisition. Trait‐difference and trait‐diversity thus were important predictors of how species’ resource capture changed in competitive neighbourhoods.     

Inorganic soil nitrogen under grassland plant communities of different species composition and diversity

We measured aboveground plant biomass and soil inorganic nitrogen pools in a biodiversity experiment in northern Sweden, with plant species richness ranging from 1 to 12 species. In general, biomass increased and nitrate pools decreased with increasing species richness. Transgressive overyielding of mixed plant communities compared to the most productive of the corresponding monocultures occurred in communities with and without legumes. N₂-fixing legumes had a fertilizing function, while non-legumes had a N retaining function. Plant communities with only legumes had a positive correlation between biomass and soil nitrate content, whereas in plant communities without legumes they were negatively correlated. Both nitrate and ammonium soil pools in mixed non-legume communities were approximately equal to the lowest observed in the corresponding monocultures. In mixed legume/non-legume communities, no correlation was found for soil nitrate with either biomass or legume biomass as percentage of total biomass. The idea of complementarity among species in nitrogen acquisition was supported in both pure non-legume and mixed non-legume/legume communities. In the latter, however, facilitation through increased nitrogen availability and retention, was probably dominating. Our results suggest that diversity effects on biomass and soil N pools through resource use complementarity depend on the functional traits of species, especially N₂ fixation or high productivity.     

A functional trait-based approach to understand community assembly and diversity–productivity relationships over 7 years in experimental grasslands

Several multi-year biodiversity experiments have shown positive species richness–productivity relationships which strengthen over time, but the mechanisms which control productivity are not well understood. We used experimental grasslands (Jena Experiment) with mixtures containing different numbers of species (4, 8, 16 and 60) and plant functional groups (1–4; grasses, legumes, small herbs, tall herbs) to explore patterns of variation in functional trait composition as well as climatic variables as predictors for community biomass production across several years (from 2003 to 2009). Over this time span, high community mean trait values shifted from the dominance of trait values associated with fast growth to trait values suggesting a conservation of growth-related resources and successful reproduction. Increasing between-community convergence in means of several productivity-related traits indicated that environmental filtering and exclusion of competitively weaker species played a role during community assembly. A general trend for increasing functional trait diversity within and convergence among communities suggested niche differentiation through limiting similarity in the longer term and that similar mechanisms operated in communities sown with different diversity. Community biomass production was primarily explained by a few key mean traits (tall growth, large seed mass and leaf nitrogen concentration) and to a smaller extent by functional diversity in nitrogen acquisition strategies, functional richness in multiple traits and functional evenness in light-acquisition traits. Increasing species richness, presence of an exceptionally productive legume species (Onobrychis viciifolia) and climatic variables explained an additional proportion of variation in community biomass. In general, community biomass production decreased through time, but communities with higher functional richness in multiple traits had high productivities over several years. Our results suggest that assembly processes within communities with an artificially maintained species composition maximize functional diversity through niche differentiation and exclusion of weaker competitors, thereby maintaining their potential for high productivity.     

Nitrogen economy of alpine plants on the north Tibetan Plateau: Nitrogen conservation by resorption rather than open sources through biological symbiotic fixation

Nitrogen (N) is one of the most important factors limiting plant productivity, and N fixation by legume species is an important source of N input into ecosystems. Meanwhile, N resorption from senescent plant tissues conserves nutrients taken up in the current season, which may alleviate ecosystem N limitation. N fixation was assessed by the ¹⁵N dilution technique in four types of alpine grasslands along the precipitation and soil nutrient gradients. The N resorption efficiency (NRE) was also measured in these alpine grasslands. The aboveground biomass in the alpine meadow was 4–6 times higher than in the alpine meadow steppe, alpine steppe, and alpine desert steppe. However, the proportion of legume species to community biomass in the alpine steppe and the alpine desert steppe was significantly higher than the proportion in the alpine meadow. N fixation by the legume plants in the alpine meadow was 0.236 g N/m², which was significantly higher than N fixation in other alpine grasslands (0.041 to 0.089 g N/m²). The NRE in the alpine meadows was lower than in the other three alpine grasslands. Both the aboveground biomass and N fixation of the legume plants showed decreasing trends with the decline of precipitation and soil N gradients from east to west, while the NRE of alpine plants showed increasing trends along the gradients, which indicates that alpine plants enhance the NRE to adapt to the increasing droughts and nutrient‐poor environments. The opposite trends of N fixation and NRE along the precipitation and soil nutrient gradients indicate that alpine plants adapt to precipitation and soil nutrient limitation by promoting NRE (conservative nutrient use by alpine plants) rather than biological N fixation (open sources by legume plants) on the north Tibetan Plateau.     

Impact of Land-Use Intensity and Productivity on Bryophyte Diversity in Agricultural Grasslands

While bryophytes greatly contribute to plant diversity of semi-natural grasslands, little is known about the relationships between land-use intensity, productivity, and bryophyte diversity in these habitats. We recorded vascular plant and bryophyte vegetation in 85 agricultural used grasslands in two regions in northern and central Germany and gathered information on land-use intensity. To assess grassland productivity, we harvested aboveground vascular plant biomass and analyzed nutrient concentrations of N, P, K, Ca and Mg. Further we calculated mean Ellenberg indicator values of vascular plant vegetation. We tested for effects of land-use intensity and productivity on total bryophyte species richness and on the species richness of acrocarpous (small & erect) and pleurocarpous (creeping, including liverworts) growth forms separately. Bryophyte species were found in almost all studied grasslands, but species richness differed considerably between study regions in northern Germany (2.8 species per 16 m²) and central Germany (6.4 species per 16 m²) due environmental differences as well as land-use history. Increased fertilizer application, coinciding with high mowing frequency, reduced bryophyte species richness significantly. Accordingly, productivity estimates such as plant biomass and nitrogen concentration were strongly negatively related to bryophyte species richness, although productivity decreased only pleurocarpous species. Ellenberg indicator values for nutrients proved to be useful indicators of species richness and productivity. In conclusion, bryophyte composition was strongly dependent on productivity, with smaller bryophytes that were likely negatively affected by greater competition for light. Intensive land-use, however, can also indirectly decrease bryophyte species richness by promoting grassland productivity. Thus, increasing productivity is likely to cause a loss of bryophyte species and a decrease in species diversity.     

Earthworm and belowground competition effects on plant productivity in a plant diversity gradient

Diversity is one major factor driving plant productivity in temperate grasslands. Although decomposers like earthworms are known to affect plant productivity, interacting effects of plant diversity and earthworms on plant productivity have been neglected in field studies. We investigated in the field the effects of earthworms on plant productivity, their interaction with plant species and functional group richness, and their effects on belowground plant competition. In the framework of the Jena Experiment we determined plant community productivity (in 2004 and 2007) and performance of two phytometer plant species [Centaurea jacea (herb) and Lolium perenne (grass); in 2007 and 2008] in a plant species (from one to 16) and functional group richness gradient (from one to four). We sampled earthworm subplots and subplots with decreased earthworm density and reduced aboveground competition of phytometer plants by removing the shoot biomass of the resident plant community. Earthworms increased total plant community productivity (+11%), legume shoot biomass (+35%) and shoot biomass of the phytometer C. jacea (+21%). Further, phytometer performance decreased, i.e. belowground competition increased, with increasing plant species and functional group richness. Although single plant functional groups benefited from higher earthworm numbers, the effects did not vary with plant species and functional group richness. The present study indicates that earthworms indeed affect the productivity of semi-natural grasslands irrespective of the diversity of the plant community. Belowground competition increased with increasing plant species diversity. However, belowground competition was modified by earthworms as reflected by increased productivity of the phytometer C. jacea. Moreover, particularly legumes benefited from earthworm presence. Considering also previous studies, we suggest that earthworms and legumes form a loose mutualistic relationship affecting essential ecosystem functions in temperate grasslands, in particular decomposition and plant productivity. Further, earthworms likely alter competitive interactions among plants and the structure of plant communities by beneficially affecting certain plant functional groups.     

Increase of fast nutrient cycling in grassland microcosms through insect herbivory depends on plant functional composition and species diversity

Nutrient cycling in terrestrial ecosystems is affected by various factors such as plant diversity and insect herbivory. While several studies suggest insect herbivory to depend on plant diversity, their interacting effect on nutrient cycling is unclear. In a greenhouse experiment with grassland microcosms of one to six plant species of two functional groups (grasses and legumes), we tested the influence of plant species richness (diversity) and functional composition on plant community biomass production, insect foliar herbivory, soil microbial biomass, and nutrient concentrations in throughfall. To manipulate herbivory, zero, three or six generalist grasshoppers (Chorthippus parallelus) were added to the plant communities. Increasing plant species richness increased shoot biomass and grasshopper performance, without significantly affecting root biomass or insect herbivory. Plant functional composition affected all of these parameters, e.g. legume communities showed the highest shoot biomass, the lowest grasshopper performance and suffered the least herbivory. Nutrient concentrations (dissolved mineral N, PO₄‐P, SO₄‐S) and pH in throughfall increased with herbivory. PO₄‐P and pH increases were positively affected by plant diversity, especially under high herbivore pressure. Plant functional composition affected several throughfall variables, sometimes fully explaining diversity effects. Increasing plant diversity tended to increase soil microbial biomass, but only under high herbivore pressure. Faeces quantities strongly correlated with changes in pH and PO₄‐P; frass may therefore be an important driver of throughfall pH and a main source of PO₄‐P released from living plants. Our results indicate that insect herbivory may significantly influence fast nutrient cycling processes in natural communities, particularly so in managed grasslands.