Floral attractants in an alpine environment: linking floral volatiles, flower size and pollinators

In high mountain environments, the availability of pollinators may decrease as elevation increases, affecting plant reproductive success. Floral volatile organic compounds (VOCs) are relevant to pollinator attraction; however, few studies have explored the variation of floral VOCs at different elevational sites. We analyzed the floral VOCs, flower size and nectar volume in Bidens triplinervia (a species with a generalized pollination system) and Penstemon roseus (with bee and bird pollination system) at three elevations (2800, 3300, and 3700 m) in the Nevado de Colima Volcano in Mexico. We recorded visitation rate and the identity of pollinators and explored the relationships between floral traits and pollinator visitation. The floral scent profile (composition and relative amounts of VOCs) of B. triplinervia and P. roseus differed between the three elevations. The highest number of VOCs and the largest flowers were found at high elevation, which had the lower visitation rate. Nectar volume was higher at low elevation and nectar concentration was lower at middle elevation. In B. triplinervia, Bombus ephippiatus was the main pollinator at low and middle elevation, while Musca domestica predominated at the highest elevation. Penstemon roseus was visited by hummingbirds and bumblebees, which were the most frequent pollinators at all three elevations. The floral traits (i.e., flower size, floral VOCs and nectar) were significantly associated with pollinator visitation. This study provides evidence of floral VOCs variation at different elevations, which was correlated with the visitation rate of local pollinators. The larger flowers at higher elevations may increase the attraction of pollinators where the environment is more extreme and erratic.