Above-ground resource use increases with plant species richness in experimental grassland ecosystems

1. Diversity effects on canopy structure and the use of space and light wer e investigated in experimental grassland communities of one, two, four, eig ht or 32 species and of one, two or three functional groups (grasses, legum es and non-leguminous forbs) in north-western Switzerland. 2. Plant species and functional group richness improved the establishment o f grassland assemblages and increased mean vegetation cover from 64% in mon ocultures to 100% in 32-species mixtures. 3. Above-ground biomass and leaf area index increased linearly with the log arithm of plant species number in all 3 years of the experiment. The most s pecies-rich communities produced 143% more biomass than the mean of all mon ocultures 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 divers ity (32-species mixtures compared with monocultures) for two reasons. (i) T he lack of species complementarity in terms of success of seedling establis hment 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 (+2 0%) than did less diverse ecosystems, because of better three-dimensional s pace filling and greater biomass density due to complementarity in plant ar chitecture, 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 div ersity treatments from one to 32 species. 6, These results cannot be explained solely by a higher likelihood of the i nclusion of more productive dominant species in the more diverse communitie s (sampling effect), which suggests that biomass production may be determin ed by the most productive species present in a community. For all important canopy characteristics of multispecies communities, we show that with incr easing diversity mixtures perform better than the best monoculture. This is a strong indication that part of the positive effect of diversity on bioma ss production is driven by complementary resource use.