During the past two decades, biodiversity experiments have provided strong evidence for positive relationships between plant diversity and a range of ecosystem functions. However, the mechanisms for biodiversity effects are not yet well understood and the interpretation of experimental results remains a subject of debate. A central issue of the controversy around positive biodiversity effects is the occurrence and significance of transgressive overyielding, in other words the phenomenon that the yield of a mixture exceeds the yield of all component species when grown as monocultures.

Using a large-scale biodiversity experiment in Germany (The Jena Experiment), we analyzed the effects of four different components of plant diversity on aboveground biomass production over a period of five years. These components of plant diversity were the number of species (1 – 16) and the number (1 – 4), presence, and abundance of particular functional groups (legumes, tall herbs, small herbs, grasses). While the issue of transgressive overyielding is commonly addressed at the level of different species, we focused on patterns at the level of different functional groups.

Results/Conclusions

We found strong effects of all four components of biodiversity on aboveground community biomass. Increasing the number of species or functional groups resulted in an increase in aboveground community biomass in all years. However, the strongest influence was exerted by particular functional groups: while legumes and tall herbs had positive effects, small herbs and grasses had on average no effect on community biomass. Using replacement series diagrams, we found that all four functional groups enhanced community biomass when their relative abundance was lower than 1/3 of the total community biomass. The latter analyses provided strong evidence for transgressive overyielding between functional groups, meaning that all four functional groups could enhance the absolute yield of a mixture beyond the yield of communities lacking that particular functional group. We conclude that 1) aboveground community biomass is most strongly affected by the presence of particular functional groups, and positively but less strongly related to species richness and functional group richness, and that 2) transgressive overyielding can occur between several combinations of functional groups.