Showing papers by "Alexandra Weigelt published in 2009"

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

Abstract: 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 disproportion- ate 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.

Biodiversity for multifunctional grasslands: equal productivity in high-diversity low-input and low-diversity high-input systems

Abstract: . Modern grassland management seeks to provide many ecosystem services and experimental studies in resource-poor grasslands have shown a positive relationship between plant species richness and a variety of ecosystem functions. Thus, increasing species richness might help to enhance multifunctionality in managed grasslands if the relationship between species richness and ecosystem functioning is equally valid in high-input grassland systems. We tested the relative effects of low-input to high-input management intensities and low to high plant species richness. Using a combination of mowing frequencies (1, 2 or 4 cuts per season) and fertilisation levels (0, 100 and 200 kg N ha−1 a−1), we studied the productivity of 78 experimental grassland communities of increasing plant species richness (1, 2, 4, 8 or 16 species with 1 to 4 functional groups) in two successive years. Our results showed that in both years higher diversity was more effective in increasing productivity than higher management intensity: the 16-species mixtures had a surplus of 449 g m−2 y−1 in 2006 and 492 g m−2 y−1 in 2007 over the monoculture yields whereas the high-input management resulted in only 315 g m−2 y−1 higher productivity in 2006 and 440 g m−2 y−1 in 2007 than the low-input management. In addition, high-diversity low-input grassland communities had similar productivity as low-diversity high-input communities. The slopes of the biodiversity – productivity relationships significantly increased with increasing levels of management intensity in both years. We conclude that the biological mechanisms leading to enhanced biomass production in diverse grassland communities are as effective for productivity as a combination of several agricultural measures. Our results demonstrate that high-diversity low-input grassland communities provide not only a high diversity of plants and other organisms, but also ensure high forage yields, thus granting the basis for multifunctional managed grasslands.

Positive biodiversity–productivity relationship due to increased plant density

Abstract: 1. Positive effects of biodiversity on plant productivity may result from diversity-induced changes in the size or density of individual plants, yet these two possibilities have never been tested at the same time in a biodiversity experiment with a large species pool. Here, we distinguish between size effects and density effects on plant productivity, using data from 198 experimental grassland communities that contained 1–16 species. Plant modules such as tillers or rosettes were defined as relevant units, being equivalent to plant individuals in the majority of species. 2. In agreement with previous studies, we found positive effects of species richness on above-ground productivity. We show that this positive biodiversity effect resulted from diversity-induced increases in module density rather than from increases in module size. In contrast, variation in productivity within diversity levels was related to module size rather than module density. 3. The size–density relationships varied among plant functional groups and among species but their average response to increasing species richness paralleled the pattern observed at the level of the entire plant communities: species richness had a positive effect on above-ground species biomass and species module density but not on species module size. Twenty-four out of 26 overyielding species had denser populations and 25 out of 28 underyielding species had smaller modules in mixtures than in monocultures. 4. Synthesis: In grasslands, an increase in community productivity must involve an increase in plant size or density. We found that diversity-induced increases in productivity were related to diversity-induced increases in density, whereas diversity-independent increases in productivity were related to increases in plant size. Our results suggest that increased density of overyielding species in mixtures was the main driver of the positive biodiversity–productivity relationship in our experiment. We conclude that the mechanisms leading to enhanced productivity of species-rich as compared with species-poor communities cannot be derived from mechanisms explaining high productivity within communities that contain a particular number of species.

Plant community impacts on the structure of earthworm communities depend on season and change with time.

Abstract: Declining plant diversity potentially threatens essential ecosystem functions driven by the decomposer community, such as litter decomposition and nutrient cycling. Currently, there is no consensus on the interrelationships between plant diversity and decomposer performance and previous studies highlighted the urgent need for long-term experiments. In the Jena Experiment we investigated the long-term impacts of plant community characteristics on the structure of earthworm communities representing key decomposers in temperate grassland. We repeatedly sampled plots varying in plant species richness (1–16 species), plant functional group richness (1–4 groups), and presence of certain plant functional groups (grasses and legumes) three, four, and six years after establishment of the experiment in spring and autumn. The results show that earthworm performance is essentially driven by the presence of certain plant functional groups via a variety of mechanisms. Plant productivity (root biomass) explained most of the detrimental grass impacts (decrease in earthworm performance), while beneficial legume effects likely were linked to high quality inputs of plant residues (increase in earthworm performance). These impacts depended on the functional group of earthworms with the strongest effects on surface feeding anecic earthworms and minor effects on soil feeding endogeic species. Remarkably, effects of plant community characteristics on the composition and age structure of earthworm communities varied between seasons. Moreover, plant diversity effects reported by a former study decreased and detrimental effects of grasses increased with time. The results indicate that plant community characteristics, such as declining diversity, indeed affect the structure of earthworm communities; however, loss of key plant functional groups is likely to be more important than plant species number per se. However, in frequently disturbed ecosystems plant species richness might be important for the recovery and resilience of belowground functions. Moreover, the results accentuate the importance of long-term repeated measurements to fully appreciate the impacts of plant community composition and diversity on ecosystem properties. Single point observations may be misleading and potentially mask the complexity of above-belowground interrelationships.

The analysis of biodiversity experiments: from pattern toward mechanism

Abstract: Meta-analysis of the first generation of biodiversity experiments has revealed that there is a general positive relationship between diversity and ecosystem processes that is consistent across trophic groups and ecosystem types. However, the mechanisms generating these general patterns are still under debate. While there are unresolved conceptual issues about the nature of diversity and complementarity, the debate is partly due to the difficulty of performing a full-factorial analysis of the functional effects of all species in a diverse community. However, there are now several different analytical approaches that can address mechanisms even when full factorial analysis is not possible. This chapter presents an overview and users' guide to these methods. This chapter concludes that the current toolbox of methods allows investigation of the mechanisms for most, if not all, biodiversity and ecosystem functioning experiments conducted to date that manipulate species within a single trophic level (e.g. plant biodiversity experiments). Methods that can address mechanisms in multitrophic studies are a key need for future research.

Advantages of compound-specific stable isotope measurements over bulk measurements in studies on plant uptake of intact amino acids.

Abstract: Increasing interest in the ability of plants to take up amino acids has given rise to questions on the accuracy of the commonly used bulk method to measure and calculate amino acid uptake. This method uses bulk measurements of 13C and 15N enrichment in plant tissues after application of dual-labelled amino acids but some authors have recommended the use of compound-specific stable isotope (CSI) analysis of the plants' amino acids instead. However, there has never been a direct evaluation of both methods. We conducted a field study applying dual-labelled (13C, 15N) amino acids (glycine, valine, tyrosine and lysine) to soil of a Plantago lanceolata monoculture. Root and shoot samples were collected 24 h after label application and the isotope composition of the plant tissues was investigated using bulk and CSI measurements. Enrichment of 13C in the case of CSI measurements was limited to the applied amino acids, showing that no additional 13C had been incorporated into the plants' amino acid pool via the uptake of tracer-derived C-fragments. Compared with this rather conservative indicator of amino acid uptake, the 13C enrichment of bulk measurements was 8, 5, 1.6 and 6 times higher for fine roots, storage roots, shoot and the whole plant, respectively. These findings show that the additional uptake of tracer-derived C-fragments will result in a considerable overestimation of amino acid uptake in the case of bulk measurements. We therefore highly recommend the use of CSI measurements for future amino acid uptake studies due to their higher accuracy.