COS 158-3 - Increased flower lifespans and decreased seed sets in three subalpine wildflowers following decreased pollination services

Background/Question/Methods

Global pollinator declines threaten the important pollination services they provide to plants. The production and maintenance of flowers represent a plant’s reproductive effort, which ideally translates into reproductive success. However, flowers are resource intensive structures and their production and maintenance are costly—if pollination services are reduced, it is unclear how plants may respond regarding reproductive effort and reproductive success. It is therefore important to understand how plants may alter resource and energy investment to flowers in the face of declining pollination services in an effort to maintain reproductive success. To address this issue, we ask the following questions: (1) How does variation in pollination services influence floral longevity (i.e., reproductive effort)? And (2) How does variation in floral longevity influence plant reproductive success (i.e., seed set)? To answer these questions, we manipulated pollination services to three subalpine wildflower species in the Colorado Rocky Mountains (Delphinium nuttallianum, Hydrophyllum fendleri, and Erigeron speciosus): pollination services were reduced via bagging flowers (50-75% of open flowers) and increased via pollen supplementation. We then tagged and monitored individual flowers on each plant to quantify floral longevity. After flower senescence and fruit maturation, we collected all tagged flowers and quantified seed set.

Results/Conclusions

Overall, the three species exhibited similar patterns of floral longevity and seed set in response to experimental variation in pollination services, despite differences in floral morphologies, pollinator diversity, and phenology windows. Plants receiving reduced pollination services experienced longer floral lifespans compared to those in the control treatment that received naturally available pollination services. Flowers of H. fendleri and E. speciosus that received supplemented pollination services did not differ in floral longevity compared to the control treatment; in contrast, D. nuttallianum flowers receiving supplemented pollination services had similar floral lifespans compared to both the control and reduced pollination treatments. Seed sets of D. nuttallianum and H. fendleri flowers tended to have narrower and lower boundaries when receiving reduced pollination services compared to flowers receiving the control or supplemented pollination services, while seed sets of E. speciosus flowers receiving reduced pollination services had wider boundaries, but on average producing lower seed sets than the control and supplemented treatments. Taken together, these results provide insight into how plants may manage trade-offs when investing in floral resources, and subsequently seed production, in response to decreasing pollination services as they produce and maintain necessary floral resources while maintaining reproductive success.