| Astragalus osterhoutii Jones (Fabaceae) is a long-lived iteroparous perennial endemic to sagebrush steppe. Osterhout's milkvetch is restricted to a 25 square kilometer area near Kremmling, Colorado. Threatened by habitat fragmentation and the construction of the Wolford Mountain Dam, this species was federally listed as endangered in 1989. It was hypothesized that selective grazing in the area during the early 1900's followed by range improvements has caused an increase in big sagebrush density with a resultant increase in competition for soil moisture. Demographic monitoring was used to detect changes in the population structure of Astragalus osterhoutii in response to site variability and annual climatic variation. Prolonged dormancy was documented for this species, with individuals remaining dormant for a maximum of four years. Overall, Astragalus osterhoutii appears to use its longevity to persist in a competitive but relatively stable environment. Results indicate that in patches with high sagebrush cover the growth of the milkvetch may be slowed. A habitat manipulation experiment was implemented to determine the effects of soil surface disturbance and the removal of sagebrush plants on the survival and growth of transplanted milkvetch plants and on milkvetch seedling establishment. Sagebrush removal and soil surface disturbance were manipulated in a two factor, split plot experiment using a randomized complete block design. Survivorship of planted seeds and greenhouse grown transplants was surprisingly low. Results of the habitat manipulation experiment indicate that the role of disturbance in the population dynamics of Astragalus osterhoutii has not been resolved. Experimental treatments did not significantly increase the seedling recruitment of this species. Sagebrush removal may initially increase milkvetch survival, however, the removal of sagebrush may promote the invasion of an exotic species. Competition from introduced crested wheatgrass slows the growth of milkvetch seedlings and possibly delays the transition from juvenile to adult stages. |
