Bumblebees (Bombus rufofasciatus Smith) pollinate the enclosed inflorescences of the endangered Brahma’s lotus (Saussurea obvallata: Asteraceae) of the Indian Himalaya

Brahma’s lotus (Saussurea obvallata (DC.) Edgew.: Asteraceae) that grows near the permanent snow line in the Himalaya, is one of several so-called “glasshouse” plants, which enclose their inflorescences in large translucent bracts. Around the temple of Kedarnath (Uttarakhand: India), where this study was conducted, the inflorescences are harvested for offerings and for sale to pilgrims. In addition, the plant has many medicinal uses. We report on the reproductive biology of S. obvallata with the aim of understanding its distinctive morphology and also to gain insight into how it might respond to harvesting. The main questions were: (1) How does population density vary in relation to altitude? (2) What is the flowering and leafing phenology of S. obvallata? (3) Do populations differ in morphology? (4) Does the pollen to ovule ratio suggest outcrossing? (5) What pollinates S. obvallata, and how does visitation rate vary across sites? (6) Is the species self-compatible? (7) Does it require pollinators for seed production? (8) Is seed set pollen-limited? (9) Do the bracts enhance seed set, and (10) Is the species semelparous? To answer these questions, we recorded population density and floral visitors across an elevational gradient, conducted pollinator exclusion and hand-pollination experiments, modified floral morphology by folding back the bracts of experimental plants, and observed the life cycle. We found that the flowers were pollinated by rufous-striped bumblebees, Bombus rufofasciatus Smith, which are strong enough to separate and clamber between the whorled bracts. Paradoxically, visitation rates were higher at lower elevation, whereas the plants survived better at higher elevation. The bracts enhanced seed production, probably by keeping the inflorescences dry and warm during the wet monsoon (August). Despite the reduction of density due to harvesting, plants at one site were not pollen-limited and were capable of autonomous self-pollination. Nevertheless, seed set was somewhat higher in plants left open to receive pollinator visits. Self-compatibility and iteroparity are predicted to render the species less sensitive to the harvesting of inflorescences that some other Saussurea species, which are semelparous and self-incompatible. However, the currently high rate of harvesting highlights the need for monitoring and for a resource management strategy.