Mechanisms responsible for the positive diversity–productivity relationship in Minnesota grasslands

Janneke Hille Ris Lambers,* W. Stanley Harpole, David Tilman, Johannes Knops and Peter B. Reich Department of Ecology, Evolution and Behavior, University of Minnesota, Saint Paul, MN 55108, USA School of Natural Resource Sciences, University of Nebraska, Lincoln, Lincoln, NE 68583, USA Department of Forest Resources, University of Minnesota, Saint Paul, MN 55108, USA *Correspondence: E-mail: [email protected] Abstract Species extinctions have spurred debate on whether interactions among few or among many species cause a positive diversity–productivity relationship in experimentally assembled grasslands. We addressed this question by quantifying the productivity of 14 species across an experimental diversity gradient in Minnesota. We found that interspecific interactions leading to coexistence and competitive displacement both determine which species overyield; i.e. are more productive at high diversity. Overyielding species were either superior N competitors (C4 grasses) or N fixers (legumes). Surprisingly, these species were not most productive in monoculture, thus, the selection of productive species in diverse plots did not cause the positive diversity– productivity relationship. Both positive (with legumes) and negative interspecific interactions (with C4 grasses) determined whether individual species overyielded. Foliar pathogens did not cause overyielding, although other natural enemies may be responsible. Overyielding species are not displacing underyielding species over time, implying that other diversity-promoting interactions also operate in this experiment.