Submitted to: The Journal of the Torrey Botanical Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/24/2006
Publication Date: 12/31/2006
Citation: Tepedino, V., Bowlin, W., Griswold, T.L. 2006. Pollination biology of the endangered Blowout Penstemon (Penstemon haydenii S. Wats.: Scrophulariaceae) in Nebraska. The Journal of the Torrey Botanical Society. 133(4): 548-559
Interpretive Summary: One of the most arresting of the many rare plants of North America is Blowout penstemon, Penstemon haydenii an endemic of sandhills listed as endangered under the U. S. Endangered Species Act in 1987. Blowout penstemon inhabits the body and leeward side of recently exposed "blowouts" in sand dunes in western Nebraska. It possesses large, showy, and unusually (for Penstemon) fragrant flowers which attract large numbers of flower visitors. Presently unknown is whether this early successional, colonizing species actually depends on those flower-visitors for sexual reproduction or if it has adopted other ways of reproducing. We found that flowers of this species cannot produce fruits and seeds unless they are visited by native bees carrying pollen from a flower on another plant. Self pollen is inadequate to effect pollination. These native bees are species that nest both in the ground (some may be sand dune specialists) as well as others that nest in common reeds. The pollinator species differ from one sand dune site to another depending on the surrounding habitat and vegetation. Conservation of this species may require providing nesting habitat for native bees as well as modifying grazing schedules to keep cattle out of areas with plants until seeds are mature and/or dispersed, in addition to relaxation of dune stabilization programs.
Technical Abstract: We studied the breeding system and pollinators of Nebraska populations of the endangered plant Blowout penstemon, Penstemon haydenii. In contrast to previous reports, this species is one of the few self-incompatible species of Penstemon. Plants cannot reproduce sexually unless flowers are visited by pollinators bearing pollen from another plant; self-pollen is unable to effect fertilization. We found no indication that fruit or seed production was limited by inadequate pollination. Pollinators were several species of native bees which varied from population to population and from year to year; there did not appear to be a reliable Blowout penstemon pollinator fauna. Differences in the pollinator fauna between sites appeared to be due to the dominant vegetation, reeds at one site and upland shrubs at the other. At one site flowers were pollinated mainly by large apid bees in the genera Habropoda and Bombus, and by (mostly) smaller halictid bees. At the second site, pollinators were primarily intermediate sized megachilid bees, particularly in the genus Osmia, and halictid bees. One presumed megachilid Penstemon specialist, Hoplitis pilosifrons, carried few pollen grains and may be more of a parasite than a pollinator. Conservation of this species may require providing nesting habitat for native bees as well as modifying grazing schedules to keep cattle out of areas with plants until seeds are mature and/or dispersed, in addition to relaxation of dune stabilization programs.