Beyond shading: Litter production by neighbors contributes to overyielding in tropical trees

Abstract

The influence of biodiversity on ecosystem functioning is now well established. However, our ability to predict the ecological consequences of biodiversity changes remains limited by our poor understanding of the mechanisms underlying biodiversity effects. We disentangled the contributions of light competition and residual neighborhood interactions in a 10‐year‐old biodiversity experiment with tropical trees that display overyielding, i.e., higher community‐level yields in mixtures compared with monocultures. We developed models of individual tree growth that partition the effects of neighboring trees into shading and residual effects assumed to reflect primarily belowground interactions. These models reject the hypothesis that reduced light competition in mixtures is the only mechanism driving overyielding. After factoring out the effects of shading, litter production by neighbors was a far better predictor of tree growth than traditional crowding indices; it contributed to overyielding by producing pairwise interactions that ranged from competitive to facilitative, but which, on average, concentrated competition within species. Consistent with litter‐mediated biodiversity effects, the magnitude of overyielding increased over time. Our results provide evidence for diversity effects extending beyond that of light and reveal the neglected role of litter‐mediated interactions among trees.