The effects of pollen limitation on population dynamics of snow lotus (Saussurea medusa and S. laniceps, Asteraceae): Threatened Tibetan medicinal plants of the eastern Himalayas

Abstract

Pollen limitation reduces seed production and may reduce plant population growth rate. Plants may be particularly prone to pollen limitation if they require pollinators, occur at high elevation, and have human-mediated reductions in plant density due to harvesting. We found that two rare monocarpic Tibetan plant species, known as the Himalayan Snow Lotus, both require pollinators and that seed production in Saussurea medusa (Asteraceae) but not S. laniceps is limited by pollen receipt. We created deterministic and stochastic stage-structured matrix models for S. medusa, and found that pollen supplementation significantly increases population growth rate. However, even when pollen is not limiting, S. medusa is likely at risk for extinction in the next 50 years. Our results for this monocarpic plant differ from other population studies on pollen limitation in polycarpic plant species since the magnitude of pollen limitation for seed production was relatively low, and yet the sensitivity of population growth to changes in seed production was relatively high.