Submitted to: Journal of Plant Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2006
Publication Date: 11/1/2007
Citation: Tepedino, V.J., Toler, T.R., Bradley, B.A., Hawk, J.L., Griswold, T.L. 2007. Pollination biology of a disjunct population of the endangered sandhills endemic Penstemon haydenii S. Wats. (Scrophulariaceae) in Wyoming, USA. Journal Of Plant Ecology. 193:1:59-69
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 and southcentral Wyoming. It possesses large, showy, and unusually (for Penstemon) fragrant flowers which attract large numbers of flower visitors. Presently unknown is whether the disjunct Wyoming population of this early successional, colonizing species actually depends on those flower-visitors for sexual reproduction as do members of the Nebraska population, or if Wyoming individuals have adopted other means of reproducing. We found complete agreement between Wyoming and Nebraska populations in breeding system. 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 Wyoming to Nebraska. 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 the endangered plant Penstemon haydenii, at several south central Wyoming U.S.A. occurrences. In agreement with earlier studies in Nebraska, we found Wyoming plants to be self-incompatible and pollinator-dependent for sexual reproduction. As in Nebraska, fruit set did not differ between our experimental cross-pollination treatment and an open-pollinated control. However, unlike Nebraska, open-pollinated treatments in Wyoming produced significantly fewer seeds per fruit than did the experimental outcrossing treatment. Because flower visitation rates were high in all five Wyoming sites studied, it is unlikely that insufficient pollination reduced seeds per fruit. Instead, we suggest several other hypotheses: 1) a large proportion of pollen deposited on receptive stigmas is self rather than non-self, and therefore incompatible; 2) due to genetic drift and/or the founder effect, the Wyoming metapopulation has fewer S-alleles and few mating types compared to the Nebraska metapopulation, from which it is likely a derivative; or 3) resource competition for developing ovules on open-pollinated inflorescences but not on experimental inflorescences. Pollinators were several species of native bees in the families Apidae (particularly bumblebees), Halictidae (small sweat bees), and Megachilidae (especially in the genus Osmia); and the masarid wasp Pseudomasaris vespoides. Especially important was the megachilid bee Osmia brevis which was very abundant and one of only two species present at all five sites; the sweat bee Lasioglossum (Dialictus) pruinosum was the other. Herbivory on reproductive structures by domestic livestock and native mammals was high. Conservation of this species may require managing grazing schedules so as to keep cattle out of areas with plants until seeds