1 We tested the hypothesis that the competitive effect of multi-species mixtures on a single target plant can be described as a simple additive function of the effects of single species. 2 In a controlled field competition experiment on sand, we used four common species of dry acidic grasslands (Hieracium pilosella, Jasione montana, Corynephorus canescens and Festuca ovina) to study the effects of both, single- and multi-species neighbourhoods on the target species H. pilosella. 3 Target plant biomass points at a competitive ranking of neighbour species with F. ovina as the strongest competitor followed by C. canescens and H. pilosella, while J. montana has a lower effect on the target. The analysis of the competitive response gives no indication of competitive release or amplification with increasing number of neighbouring species. 4 Further analysis with yield-density models reveals that the competition intensity of most multi-species mixtures is well predicted by the effect of pairwise interactions. Hence, for the majority of species combinations, our results indicate additive effects of species competition. 5 Specific species combinations, however, significantly deviate from this result. For combinations of C. canescens with either one of the herbs, the models show a significantly better fit if a non-additivity parameter is included. The important missing factor to describe target plant growth is species-specific biomass of neighbouring plants and here root biomass is the most crucial determinant. The addition of each of these factors as non-additive parameters significantly increases the predictive power of the model. 6 Our work adds to the increasing evidence that interactions in diverse plant communities are not simply a sum of their component species interactions. For a better understanding of the effect of competition on the structure and development of diverse plant communities, future studies need to consider both additive and non-additive effects of competing species.
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