The consequences of delayed flowering phenology in a sky-island plant, pointleaf manzanita

Background/Question/Methods

Climate change-induced shifts in flowering phenology can alter both the biotic and abiotic environment experienced by plants, potentially leading to decreased temporal overlap with pollinators and exposure to conditions that negatively affect fruit and seed set.  We explored the relationship between flowering phenology and reproductive output in pointleaf manzanita (Arctostaphylos pungens) in a sky island habitat in the Sonoran Desert.  Although pointleaf manzanita is one of the first plants to bloom in our study area, long-term records suggest flowering is shifting later.  To determine the consequences of among- and within-plant variation in flowering time, we documented the flowering schedules of 52 plants in our study population and followed the fates of individual flowers throughout two seasons (2012 and 2013).  We also measured visitation rates by potential pollinators.  We harvested fruits to determine fruit mass and seed set of flowers produced at different times.

Results/Conclusions

The patterns we documented indicate that, if pointleaf manzanita flowering continues to shift later, the flowering season may be shorter, fewer total fruits may be produced per plant, and those fruits may weigh less and contain smaller seeds.  The date of flowering onset, date of flower production, number of co-flowering plants, and visitation rate all significantly affected fruit set.  Individual flowering schedules were consistent between years, suggesting that plants that begin flowering late have lower reproductive output each year.  These results demonstrate that delayed phenology in a long-lived plant affects pollination success and reproduction and highlight the value of using natural variation in flowering time to determine the consequences of phenological shifts.