Hummingbird-syndrome traits, breeding system, and pollinator effectivenessin two syntopic Penstemon species

Red-flowered, tubular Penstemon barbatus and Penstemon pinifolius, which ha ve probably adapted independently to hummingbird pollinators, coflower on r ecently burned high-elevation slopes in the Chiricahua Mountains in Arizona . Hummingbirds visit both species but visit P. barbatus at a higher rate. W e compared several traits and asked whether the species differed in special ization for hummingbird pollination. The corollas of P. pinifolius were 30% narrower than those of P. barbatus, but they were similar in tube length. Although a narrower corolla appeared to decrease access by large insects, s mall halictid bees were the only common insect visitors, and they gathered nectar from both species. Species differences in nectar production rate and concentration were minor, being smaller than within-species differences be tween dry and rainy periods. Taller inflorescences and a wider corolla tube may contribute to greater hummingbird use of P, barbatus. Penstemon pinifo lius was fully self-compatible and weakly autogamous, whereas P. barbatus w as strongly self-incompatible and had a longer flower life. Exclusion of hu mmingbirds reduced seed set fourfold in both species, supporting the infere nce from morphology and rewards that hummingbirds are the primary pollinato rs. In the absence of hummingbirds, halictids increased the seed set of P. barbatus relative to exclusion of all visitors, but they did not increase t he seed set of P. pinifolius. Despite sharing the basic set of hummingbird- syndrome traits, the two species differed in degree of investment in indivi dual flowers and in adaptations for outcrossing.