1. Diversity effects on canopy structure and the use of space and light were investigated in experimental grassland communities of one, two, four, eight or 32 species and of one, two or three functional groups (grasses, legumes and non-leguminous forbs) in north-western Switzerland.
2. Plant species and functional group richness improved the establishment of grassland assemblages and increased mean vegetation cover from 64% in monocultures to 100% in 32-species mixtures.
3. Above-ground biomass and leaf area index increased linearly with the logarithm of plant species number in all 3 years of the experiment. The most species-rich communities produced 143% more biomass than the mean of all monocultures and 25% more biomass than the most productive monoculture in the third year of the experiment.
4. Plant light absorbance per unit ground area increased by 44% with diversity (32-species mixtures compared with monocultures) for two reasons. (i) The lack of species complementarity in terms of success of seedling establishment and plant size in vegetation with low species diversity increased the gap area in the canopy, and consequently more light reached bare ground. (ii) Diverse ecosystems absorbed more light within their closed canopies (+20%) than did less diverse ecosystems, because of better three-dimensional space filling and greater biomass density due to complementarity in plant architecture, particularly between grasses and dicots.
5. The overall canopy height was increased by 46% and the centre of gravity of the canopy was raised by up to 10 cm over the range of experimental diversity treatments from one to 32 species.
6. These results cannot be explained solely by a higher likelihood of the inclusion of more productive dominant species in the more diverse communities (sampling effect), which suggests that biomass production may be determined by the most productive species present in a community. For all important canopy characteristics of multispecies communities, we show that with increasing diversity, mixtures perform better than the best monoculture. This is a strong indication that part of the positive effect of diversity on biomass production is driven by complementary resource use.