Can root trait diversity explain complementarity effects in a grassland biodiversity experiment?

Aims The positive relationship between plant biodiversity and com-munity productivity is well established.However,our knowledge about the mechanisms underlying these positive biodiversity effects is still limited.One of the main hypotheses is that com-plementarity in resource uptake is responsible for the positive biodiversity effects:plant species differ in resource uptake strat-egy,which results in a more complete exploitation of the avail-able resources in space and time when plant species are growing together.Recent studies suggest that functional diversity of the community,i.e.the diversity in functional characteristics(‘traits’)among species,rather than species richness per se,is important for positive biodiversity effects.However,experimental evidence for specific trait combinations underlying resource complemen-tarity is scarce.As the root system is responsible for the uptake of nutrients and water,we hypothesize that diversity in root traits may underlie complementary resource use and contribute to the biodiversity effects.Methods In a common garden experiment,16 grassland species were grown in monoculture,4-species mixtures differing in root trait diversity and 16-species mixtures.The 4-species mixtures were designed to cover a gradient in average rooting depth.Above-ground biomass was cut after one growing season and used as a proxy for plant productivity to calculate biodiversity effects.Important Findings Overall,plant mixtures showed a significant increase in biomass and complementarity effects,but this varied greatly between com-munities.However,diversity in root traits(measured in a separate greenhouse experiment and based on literature)could not explain this variation in complementarity effects.Instead,complementa-rity effects were strongly affected by the presence and competitive interactions of two particular species.The large variation in comple-mentarity effects and significant effect of two species emphasizes the importance of community composition for positive biodiversity effects.Future research should focus on identifying the traits asso-ciated with the key role of particular species for complementarity effects.This may increase our understanding of the links between functional trait composition and biodiversity effects as well as the relative importance of resource complementarity and other underly-ing mechanisms for the positive biodiversity effects.