Floral traits and community phylogenetic structure shape plant-pollinator interactions in co-occurring Rhododendrons in the Himalaya

Elevation gradients of mountains serve as ideal settings to test the impact of pollinators on plant community assemblages. A shift from bird to insect-mediated pollination is expected with an increase in elevation, and such biotic shifts become more prominent in transition zones or ecotones. This shift in pollinator communities may influence plant communities if pollinators select plants with a specific set of floral traits and flowering phenology to maximize their floral resource (nectar) acquisition. However, empirical studies that examine the relative role of pollinator interactions (such as facilitation and competition) in the assembly of plant communities along elevation gradients are limited. Here we use an integrative framework combining floral traits, pollinators, flowering phenology and community phylogenetic data to reveal the relative role of facilitative and competitive interactions in the assembly of Rhododendron communities along elevation gradients in the Sikkim Himalaya. We find evidence that at lower elevations the Rhododendron community is structured by both facilitation and competition. In contrast, communities in the transition zone and at higher elevations are structured by competition alone. In lower elevations, corolla length, followed by the start of flowering phenology and nectar volume, showed significant phenotypic clustering and supported the facilitation while all other traits were overdispersed, suggesting the role of competition. Furthermore, a clustered phylogenetic structure was predominately observed at lower-elevation Rhododendron communities. In comparison, a dispersed and random phylogenetic structure was observed in the transition zone and at higher elevations. We observed greater overlap in flowering phenology and less pollinator similarity in lower elevation communities, compared to those in higher reaches.

We suggest that in the Himalaya, even at small spatial scales, indirect biotic interactions may significantly contribute to the assembly of interdependent alpine plant communities. Overall, our findings highlight the importance of considering plant-pollinator interactions as important drivers when evaluating plant community assembly processes.