Whited's Milkvetch (Astragalus sinuatus)

The region that Astragalus sinuatus occupies is subject to occasional fire, localized erosion, slope movement, and frost heaving. Astragalus sinuatus does not seem to take full advantage of, or require these localized disturbances (Gamon 1995). Although plants may show a short-term positive response from fire and other mild disturbances, it seems unlikely that they are required for long term survival of the population. It is found in early, mid and late successional stages. Population numbers become stabilized as the climax ecosystem is reached (Gamon 1995).Occasional fire (30-90 year interval) probably played a role historically in maintaining open habitat for this species. Now, fire may lead to increased success of weedy annuals (WNHP 1999). Other disturbances can also have a negative impact as they may increase the number of weedy species which may eventually out-compete A. sinuatus (Gamon 1995).The plant actively grows for only a short period every year. Leaf emergence is most likely initiated in late March or early April. The plant is fully leafed-out by mid April, just when floral buds appear. Most flowers bloom between late April and early May, and the majority of fruits develop by mid-May. Seed is fully mature by the middle to end of July. Most leaves begin to senesce by June, and in summer, the only identifying features remaining may be the empty seedpods (Gamon 1995). Astragalus sinuatus is sensitive to fluctuating precipitation levels. During drought years, many plants remain dormant, never emerging from the ground. Those that do emerge may not flower (Gamon 1995).Knowledge of the reproductive biology of Astragalus sinuatus is scant. It is believed that they are a predominantly out-crossing species, although some members of the genus Astragalus can self-pollinate as well. Bumblebees have been observed in and around the plant (Gamon 1995). The incredibly tough pods do not naturally split along the """"seam"""" as a pea or a bean would. They open only at the end, and seeds may fall out by shaking in the wind or as the pod rolls downhill. Pods may open more as they are weathered and rolled along the ground. Seeds must be scarified (have their surface scratched slightly), before they will germinate. Most seeds are dispersed close to the parent plant or slightly downhill from it. A. sinuatus is more common lower on slopes (Gamon 1995).Like other members of the Pea Family (Fabaceae), Astragalus sinuatus can develop a symbiotic relationship with nitrogen fixing bacteria that live in their roots. This gives them a competitive advantage over other species in nitrogen poor soils and increases the available nitrogen in the soil (Gamon 1995).Cattle do not seem to eat Astragalus sinuatus plants. They do, however, compact the soil, accelerate soil erosion, and contribute to an increase in weedy, annual, non-native species, such as cheatgrass (Bromus tectorum) (Gamon 1995). Areas most heavily used by cattle have lower population densities and lower seedling recruitment than areas less heavily used by livestock.