Floral traits determine pollinator visitation in Rhododendron species across an elevation gradient in the Sikkim Himalaya

Plants growing along steep elevational gradients experience variations in abiotic conditions. The elevational gradient also affects the diversity and abundance of pollinators associated with these plants. As a result, plants may have locally adapted floral traits. However, detailed assessments of multiple floral traits along elevational gradients are often neglected despite the traits being important for plant sexual reproduction. We tested the association of floral traits with pollinators in response to elevation by identifying pollinators and measuring morphological and biochemical floral traits as well as studying the breeding systems of ten aggregated Rhododendron species in the Sikkim Himalaya. Corolla length, nectar volume and distance between stamen and stigma significantly decreased with elevation. In contrast, nectar concentrations were positively associated with elevation. Birds, bumblebees and flies were the three dominant pollinator groups. Bird visits showed a strong negative association with elevation while visits by bumblebees and flies increased with elevation. Species with longer corollas and higher nectar volumes showed higher rates of bird visits, while bumblebees were associated with species that had higher nectar concentrations. Fruit set following cross-pollination was high compared to self-pollination, and higher pollen limitation and auto-fertility were observed among species in higher elevations. These observed patterns in the association between floral traits and pollinator groups in response to elevation may help generate testable hypotheses on alpine plant–pollinator responses to climate warming.