LETTER Overyielding in experimental grassland communities – irrespective of species pool or spatial scale Christiane Roscher, 1 * Vicky M. Temperton, 2 Michael Scherer- Lorenzen, 3 Martin Schmitz, 1,4 Jens Schumacher, 1 Bernhard Schmid, 4 Nina Buchmann, 3 Wolfgang W. Weisser 1 and Ernst-Detlef Schulze 2 1 Institute of Ecology, University of Jena, Dornburger Str. 159, D-07743 Jena, Germany 2 Max Planck Institute for Biogeochemistry, PO Box 100164, D-07749 Jena, Germany 3 Institute of Plant Sciences, ETH-Zurich, Universitaetsstr. 2, CH-8092 Zurich, Switzerland 4 Institute of Environmental Sciences, University of Zurich, Winterthurerstr. 190, CH-8057 Zurich, Switzerland *Correspondence: E-mail: christiane.roscher@uni-jena.de Abstract In a large integrated biodiversity project (ÔThe Jena ExperimentÕ in Germany) we established two experiments, one with a pool of 60 plant species that ranged broadly from dominant to subordinate competitors on large 20 · 20 m and small 3.5 · 3.5 m plots (¼ main experiment), and one with a pool of nine potentially dominant species on small 3.5 · 3.5 m plots (¼ dominance experiment). We found identical positive species richness–aboveground productivity relationships in the main experiment at both scales. This result suggests that scaling up, at least over the short term, is appropriate in interpreting the implications of such experiments for larger-scale patterns. The species richness–productivity relationship was more pronounced in the experiment with dominant species (46.7 and 82.6% yield increase compared to mean monoculture, respectively). Additionally, transgressive overyielding occurred more frequently in the dominance experiment (67.7% of cases) than in the main experiment (23.4% of cases). Additive partitioning and relative yield total analyses showed that both complementarity and selection effects contributed to the positive net biodiversity effect. Keywords Biodiversity, complementarity effect, dominant species, plant species richness, plot size, productivity, selection effect, The Jena Experiment. Ecology Letters (2005) 8: 419–429 INTRODUCTION A central issue in current ecological research is the potential influence of biodiversity on ecosystem functioning (Chapin et al. 1998; Loreau et al. 2001). Environmental conditions, species interactions and the available species pool all influence species diversity and ecosystem properties (Leps ˇ 2004). As such, observational studies cannot provide the same insights as biodiversity experiments, where either diversity or ecosystem properties are experimentally mani- pulated (Pfisterer et al. 2004; Schmid & Hector 2004). For example, clear causal relationships between species richness and ecosystem productivity can be examined only with experimental approaches, keeping environmental conditions constant among treatments (within-site comparisons). Such experiments can best be done in ecosystems that allow easy manipulation of species richness and rapid measurement of productivity, such as perennial grassland communities (Loreau et al. 2002). Experiments with this system have often, but not always found a positive, asymptotic relation- ship between plant species richness on the x-axis and aboveground plant biomass production on the y-axis (e.g. Tilman et al. 1997; Hector et al. 1999; further references in Schmid et al. 2002b). Initial disputes about the possible explanations for the positive relationship have been addressed with the newly developed additive partitioning analysis method that allows separating a net biodiversity effect into contributions of complementarity and selection effects (Loreau & Hector 2001; Hector et al. 2002a). Complementarity occurs if performance of species in mixture is on average higher than expected from their monoculture yields, while the selection effect explains higher productivity of mixtures by the dominance of individual, highly productive species. Despite the advances in experimental design and statis- tical analysis (Schmid et al. 2002a), major questions remain about the ability to extrapolate the relationship between plant species richness and biomass production across time, space, environments, species pools and other factors. Analyses of data from the Cedar Creek experiment in Ecology Letters, (2005) 8: 419–429 doi: 10.1111/j.1461-0248.2005.00736.x Ó2005 Blackwell Publishing Ltd/CNRS