BREEDING BIOLOGY AND FLOWER VISITORS OF THE RARE WHITE RIVER PENSTEMON, PENSTEMON SCARIOSUS VAR. ALBIFLUVIS (SCROPHULARIACEAE)

Authors: LEWINSOHN, JENNIFER - RED BT. GARDEN,SLC,UT; Tepedino, Vincent

Submitted to: Western North American Naturalist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/7/2006
Publication Date: 4/1/2007
Citation: Lewinsohn, J.S., Tepedino, V. 2007. Breeding biology and flower visitors of the rare white river penstemon, penstemon scariosus var. albifluvis (scrophulariaceae). Western North American Naturalist. 67(2): 232-237

Interpretive Summary: One of the most arresting of the many rare plants of North America is White River penstemon, Penstemon scariosus var. albifluvis, an endemic of oil shale lands in the Uintah Basin of Utah and Colorado. This taxon is being considered for listing under the U. S. Endangered Species Act. White River penstemon possesses large, showy, flowers which attract flower visitors, most of which are native non-social bees. Presently unknown is whether White River penstemon actually depends on those flower-visitors for sexual reproduction as do most species of Penstemon, or if they have adopted other means of reproducing. We found that flowers of this species produce only a few fruits and seeds if they are not visited by native bees carrying pollen from a flower on another plant. Self pollen is inferior in achieving pollination. The native pollinating bees are species that nest both in the ground and in existing holes in twigs and dead wood. The pollinator species differ somewhat between the two sites studied. Conservation of this Penstemon necessitates managing habitat so as to encourage bee nesting sites.

Technical Abstract: We studied the breeding system and flower visitors of White River penstemon, a rare endemic from the Uintah Basin of eastern Utah and western Colorado. Bagging treatments coupled with hand-pollinations showed that Penstemon scariosus var. albifluvis has a mixed mating system: while some seeds and fruits are produced via self-pollination (both autogamy and geitonogamy), significantly more are produced when the flowers are cross-pollinated. The primary flower visitors, and likely pollinators, were several species of native twig- and ground-nesting bees in the families Apidae, Halictidae and Megachilidae. We found no differences in fruit or seed production between open-pollinated controls and hand-outcrossed flowers, suggesting that pollinator visits were sufficient to maximize sexual reproduction. Management plans to conserve White River penstemon must recognize that full reproductive success of this rare plant taxon relies upon a suite of pollinating bees and that the species richness of bee visitors should be maintained.