Competitive ability underpins the effect of spatial aggregation on plant performance.

Most plant species exhibit spatially clustered distributions. Theory suggests such conspecific aggregation can delay competitive exclusion by sparing weak competitors. However, the extent to which spatial aggregation increases species performance and which species are likely to benefit from it remain largely unknown. In this study, we asked (1) whether spatial aggregation enhances plant performance and (2) whether the effects are biologically predictable. For the second question, we focused on "the competition-relatedness hypothesis" and the "competitive asymmetry hypothesis," which relate the effect of spatial arrangement to niche and competitive ability differences between species, respectively. We performed phylogenetic meta-analyses to investigate whether phylogenetic and ecological differences among competitors explain the effect of spatial arrangement. We found idiosyncratic responses of plant species to spatial aggregation. While some species performed better when conspecific individuals were aggregated, others did so when conspecifics and heterospecifics were randomly distributed. The non-negligible number of species benefiting more from intraspecific aggregation indicates that intraspecific competition is sometimes weaker than interspecific competition. Further, the result contrasts with the assumption of the competition-relatedness hypothesis, which postulates the strongest competition among conspecifics, suggesting that this hypothesis does not hold for at least these species. Although phylogeny did not predict the effect of spatial arrangement, interspecific plant height differences did: Species performed better in an aggregation treatment when they were smaller than competitors. Collectively, our results lend more support for the competitive asymmetry hypothesis that interspecific differences in competitive ability underpin the effect of spatial arrangement on plant performance. Moreover, they suggest that spatial processes, such as dispersal limitation, may play an important role in plant coexistence.