Astragalus microcymbus Barneby (Fabaceae) is a rare endemic to Gunnison and Saguache counties in western Colorado, ranked as critically imperiled by the Colorado Natural Heritage Program. The purpose of this study was to further investigate this plant’s reproductive ecology including reproductive success, pollination ecology and post-primary dispersal seed ecology. To investigate these components of A. microcymbus reproduction, this study was guided by the following questions: 1.) What is the reproductive success of A. microcymbus as measured by fruit to flower and seed to ovule ratios? Does reproduction vary across sites and what factors influence its reproductive success? 2.) Does A. microcymbus require pollinators for successful seed set? If so, what are its primary pollinators? 3.) Does A. microcymbus have a viable soil seed bank and how does it vary across the landscape? Can insight into the secondary dispersal mechanisms and occurrence patterns of A. microcymbus be gained by the spatial distribution of the seed bank and individuals? Our methods included flower, fruit, seed, and ovule counts, monitoring herbivory interactions, conducting pollinator exclusion experiments, pollinator visitation observations and identification, collecting soil seed bank samples, and describing microhabitat characteristics of individuals. In 2019, we found mean fruit set per plant ranging from 0.25 to 0.30, and seed set per fruit averaging 0.29. In 2020, low reproduction was observed due to the impacts of small mammal herbivory and drought, which limited the reproductive success study and the pollinator exclusion experiment. The genera Ashmeadiella (Megachilidae) and Anthidium (Megachilidae) were observed visiting and eliciting the pollination mechanism of flowers. Astragalus microcymbus exhibited a soil seed bank that decreased in density with increasing distance from parent plant. At the intermediate distance from the parent plant (0.5 m), fewer seeds were found in microsites with higher bare ground exposure, suggesting the process of overland flow or wind influence seed dispersal. Finally, established plants occurred in areas closer to vegetation and with higher vegetation cover, suggesting vegetation patches may serve as seed catches and/or nurse plants facilitating seedling establishment and survival. This information broadens our understanding of the life-history and ecology of this rare endemic and informs conservation efforts.
