Functional and mycorrhizal trait differences and interspecific competition in a subtropical forest

Tree neighbourhood competitions are fundamental to forest dynamics, yet whether it is driven by niche similarity (approximated by functional trait dissimilarity) or competitive hierarchy remains unresolved, as empirical studies report conflicting support for both hypotheses. Using 52,840 growth records from a 24-ha subtropical forest, we found that heterospecific neighbours consistently reduced growth, but this was independent of dissimilarity in nine key leaf and stem traits. Instead, mycorrhizal type was a powerful predictor: arbuscular mycorrhizal (AM) trees were suppressed by conspecific and conmycorrhizal neighbours but slightly facilitated by ectomycorrhizal (EcM) neighbours, while non-dominant EcM trees were unaffected. These interactions were modulated by species-specific hierarchies, as dominant AM and EcM species asymmetrically suppressed each other. Our results reconcile conflicting support for niche similarity and competitive hierarchy hypotheses by demonstrating that their relative importance depends critically on the choice of functional traits and specific competitive hierarchy between species groups examined.