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

Plants growing along steep elevational gradients experience variations in abiotic conditions. The elevational gradient also afects the diversity and abundance of pollinators associated with these plants. As a result, plants may have locally adapted foral traits. However, detailed assessments of multiple foral traits along elevational gradients are often neglected despite the traits being important for plant sexual reproduction. We tested the association of foral traits with pollinators in response to elevation by identifying pollinators and measuring morphological and biochemical foral traits as well as studying the breeding systems of ten aggregated Rhododendronspecies in the Sikkim Himalaya. Corolla length, nectar volume and distance between stamen and stigma signifcantly decreased with elevation. In contrast, nectar concentrations were positively associated with elevation. Birds, bumblebees and fies were the three dominant pollinator groups. Bird visits showed a strong negative association with elevation while visits by bumblebees and fies 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 foral traits and pollinator groups in response to elevation may help generate testable hypotheses on alpine plant–pollinator responses to climate warming.