Hummingbird and bee pollination of Penstemon pseudospectabilis

We studied the pollination effectiveness of hummingbirds and bees, and the breeding system and nectar production rate of Penstemon pseudospectabilis M. E. Jones in the Chiricahua Mountains, Arizona. With pink rather than red flowers, a corolla tube as long as a hummingbird bill but wide enough to admit bees, and hummingbird-typical nectar sugar composition, P. pseudospectabilis may be adapted for pollination by both hummingbirds and bees. Manual outcrossing yielded more than twice as many seeds/flower as manual selfing. When all visitors were excluded most plants produced some seeds through autonomous self-pollination (7/flower), but seed set increased more than two-fold when flowers were manually selfed. Flowers visited only by small, mainly halictid bees had seed set similar to the hand-self treatment (significantly better than no visits), whereas those visited mainly by hummingbirds and honeybees had seed set comparable to the hand-outcrossed treatment. The daily nectar production rate of 3.96 mg sugar/flower was relatively high for hummingbird flowers of the western United States (typical range: 2-4 mg sugar/flower), rather than being intermediate between hummingbird and bee flowers. Hummingbirds visited an observation patch at least hourly. Our results support a previous conclusion about a "mixed pollination system" in this species, and provide an example of a Penstemon species which is significantly self-incompatible and attracts high-energy pollinators (hummingbirds), but retains the ability to self-pollinate and profit from small bee visitors.