Using plant traits to understand the contribution of biodiversity effects to community productivity in an agricultural system

Summary Increasing biodiversity generally enhances productivity through selection and complementarity effects not only in natural but also in agricultural systems. However, explaining why diversity enhances productivity remains a central goal in agricultural science. In a field experiment, we constructed monocultures, 2- and 4-species mixtures from eight crop species with and without fertilizer and both in temperate Switzerland and semi-arid Spain. We measured environmental factors and plant traits and related these in structural equation models to selection and complementarity effects to explain yield differences between monocultures and mixtures. Increased crop diversity increased yield in Switzerland. This positive biodiversity effect was driven to almost same extents by selection and complementarity effects, which increased with plant height and C:N ratio, respectively. Also, ecological processes driving yield increases from monocultures to mixtures differed from those responsible for yield increases through the diversification of mixtures. While selection effects were mainly driven by one species, complementarity effects were linked to higher productivity per unit N. Yield increases due to mixture diversification were driven only by complementarity and were not mediated through the measured traits, suggesting that ecological processes beyond those measured in this study were responsible for positive diversity effects on yield beyond 2-species mixtures.