Pollinators limit seed production in an early blooming rare plant: evidence of a mismatch between plant phenology and pollinator emergence

Abstract

The reproductive ecology of rare plants is seldom studied, yet the persistence of plant populations depends on successful mutualisms with pollinators. As atmospheric temperatures rise, phenology of plants and pollinators may become mismatched. We investigated the reproduction of Trifolium barnebyi (Barneby's clover), a mat‐forming perennial endemic to central Wyoming, USA, that grows in the crevices of sandstone bedrock. Our objectives were to evaluate a method for monitoring changes in cover as well as assess the pollination and seed‐set of T. barnebyi. We established five monitoring transects using a divided frame to estimate percent cover. We conducted seed‐set experiments at three locations to measure self‐pollination and the degree to which pollinating insects limited seed production. We used vane traps and bee bowls to capture pollinators, and examined pollen carried on bees. Percent cover along transects declined over the four year period and was associated with spring precipitation. Trifolium barnebyi did not self‐pollinate and relied on pollinators to produce seeds. The number and mass of viable seeds per flower, and the number of bees captured increased as the season progressed, indicating that more and larger seeds were made when more pollinators were present. Blooming of T. barnebyi ranged between April and June, depending on the microhabitat the plant lived in, and we observed much higher seed production in later‐blooming plants. Pollen from T. barnebyi was primarily carried by Andrena bees, although we found smaller amounts of pollen on seven other bee genera. A mismatch in timing between blooming and pollinating insect emergence could limit seed production in T. barnebyi if plants bloom earlier over time and bee emergence does not follow the same phenology. Rare plants can be pollinated by rare pollinators showing that conserving these pollinators is crucial for rare plants and early blooming species may be higher at risk.