Abstract: Congeneric species may coexist at fine spatial scales through niche differentiation, however, the magnitude to which the effects of functional traits and phylogenetic relatedness contribute to their distribution along elevational gradients remains understudied. To test the hypothesis that trait and elevational range overlap can affect local species coexistence, we first compared phylogenetic relatedness and trait (including morphological traits and leaf elements) divergence among closely related species of Rhododendron on Yulong Mountain, China. We then assessed relationships between the overlap of multiple functional traits and the degree of elevational range overlap among species pairs in a phylogenetic context. We found that phylogeny was a good predictor for most functional traits, where closely related species exhibited higher trait similarity and occupied different elevational niches at our study site. Species pairs of the subgenus Hymenanthes exhibited low elevational range overlap and some species pairs of the subgenus Rhododendron showed obvious niche differentiation. Trait divergence is greater for species in the subgenus Rhododendron, and it plays an important role between species pairs with low elevational range overlap. Trait convergent selection takes place between co-occurring closely related species that have high elevational range overlap, which share more functional trait space due to environmental filtering or ecological adaptation in more extreme habitats. Our results highlight the importance of evolutionary history and trait selection for species coexistence at fine ecological scales along environmental gradients.

Key words: comparative phylogenetic analysis, elevational range overlap, Hengduan Mountains, niche conservatism, species coexistence, trait overlap