Evidence for divergence in phenology over morphology in response to limiting similarity in montane communities of Rhododendron

The coexistence of closely related species is key to understanding the nature of biodiversity hotspots where in situ diversification has yielded rich communities of close relatives. Limiting similarity predicts that co-occurring species are differentiated in their niches; identifying the axes of differentiation in sympatric close relatives can thus help reveal the eco-evolutionary dynamics of community assembly. For flowering plants, these axes may be temporal (related to reproductive phenology) or morphological (related to functional traits). We collected fine-scale data on abundance, morphology and phenology over a flowering season for 34 species of Rhododendron (Ericaceae) spanning a 2700 m elevation gradient in a nature reserve in the eastern Hengduan Mountains, China. We used null models to test for patterns of clustering versus overdispersion in species' abundances, phylogenetic relatedness and functional traits across sites, and applied joint distribution models to examine the correlates of pairwise associations. We found that species tended to be spatially aggregated, indicating that communities are not strongly structured by competitive exclusion. At higher elevation sites, species tended to be vegetatively more similar (clustered) and closely related. Environmental variables, including climate and topography, were strong predictors of species' ranges. No evidence of niche differentiation was detected along spatial or morphological (functional) axes, but along the temporal axis, the phenology of co-occurring species showed significant divergence, and was less phylogenetically conserved compared to morphological traits. Synthesis. Local communities of Rhododendron in its centre of diversity are structured by environmental filtering and the effects of limiting similarity. Evidence for the latter is apparent in the pervasive phenological divergence of co-occurring species, likely driven by reproductive interference from shared pollinators. The evolutionary lability of flowering time appears to render it the quickest path to coexistence for recently diverged species that experience secondary contact in this biodiversity hotspot.