Railhead Milkvetch - Astragalus terminalis
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Astragalus terminalus is a regional endemic known from southwest Montana, east-central Idaho and northwest Wyoming. In Montana it is documented from Beaverhead County and the Upper Madison River Valley. The species appears to be vulnerable to intensive grazing and competition from noxious weeds, at least in low-elevation areas.
Railhead milkvetch is a tufted, taprooted perennial herb with several erect stems that reach 5-30 cm in height. The pinnately compound leaves are 5-20 cm long and have 13-21 oblong leaflets with blunt tips. Foliage is sparsely covered with forked gray hairs that branch at the base and spread in opposite directions and are appressed to the surface of leaves or stem. Dense clusters of 10-30 flowers are borne in the axils of upper leaves and become less crowded as the plant matures. The white, pea-like corollas are 12-16 mm long, with the upper petal reflexed and the lower petal purple-spotted. The calyx reaches 4-5 mm and is covered with white or black hairs. The cigar-shaped fruits are glabrous, unstalked, 3-sided in cross-section, and 15-20 mm long. The alpine ecotype is much smaller than plants from the valleys.
Flowering takes place from June to mid-July.
Astragalus atropubescens, A. canadensis and A. scaphoides all have stalked fruits, and the latter two species usually have yellowish flowers.
Railhead milkvetch is a regional endemic of central Idaho, southwestern Montana, and northwestern Wyoming.
Observations in Montana Natural Heritage Program Database
Number of Observations:
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(Observations spanning multiple months or years are excluded from time charts)
For the Intermountain Region, Barneby (1989) describes this species' habitat as "open, stony hillsides and benches along rivers, commonly associated with low sagebrush and calcareous bedrock." In Montana, its habitat varies widely from valley grasslands (Idaho fescue/bluebunch wheatgrass) and open eroding slopes, to ridgetops, barren clay buttes and dry subalpine meadow. Thus it is difficult to predict where this species will occur.
Southeast of Bannack, it is found on sagebrush and grassland slopes between more gentle slopes. At another site in Beaverhead County, it grows in grasslands dominated by bluebunch wheatgrass and needle-and-thread grass, sometimes with sagebrush (Artemisia tridentata and A. tripartita). In the Centennial Valley, it occurs in an Artemisia tridentata-Festuca idahoensis community. In the Upper Madison Valley, it grows in fine-textured soils at the base of eroding, sparsely vegetated slopes. This species appears to favor relatively barren sites that are often alkaline (Heidel and Vanderhorst 1996), and frequently occurs with other "barren site indicators," such as Astragalus vexilliflexus and Haplopappus acaulis (Lesica and Vanderhorst 1995).
Ecological Systems Associated with this Species
Astragalus terminalis may have the capability for prolonged dormancy, since it is morphologically similar and closely related to A. scaphoides which has been shown to remain dormant in some years (Lesica 1995). This can result in the number of above-ground stems varying significantly among years, making accurate population counts difficult (Lesica and Steele 1994).
Observations suggest that railhead milkvetch is palatable and may decrease under some livestock grazing regimes. In one exclosure in the Upper Madison Valley, plants were much denser inside than outside, and many of the plants in a nearby population had inflorescences removed, probably by game. No plants were found across the cattle guard in a grazed pasture (Heidel and Vanderhorst 1996).
Railhead milkvetch faces competition from invasive weeds, especially spotted knapweed, yellow sweet clover and cheatgrass. Centaurea maculosa (spotted knapweed) is invading this species' habitat from roadsides in the Upper Madison Valley (Heidel and Vanderhorst 1996), and has the potential to establish and compete aggressively there and in other areas. Melilotus officinalis (yellow sweet clover) and Bromus tectorum (cheatgrass) were also noted invading a site in Beaverhead County.
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- Barneby, R.C. 1964. Atlas of North American Astragalus. 2 Vols. New York Botanical Garden, Bronx, New York. 1188 pp.
- Barneby, R.C., A. Cronquist, A.H. Holmgren, N.H. Holmgren, J.L. Reveal, P.K. Holmgren. 1989. Fabales. Intermountain Flora: Vascular Plants of the Intermountain West, U.S.A. Volume 3, Part B. Bronx, NY: New York Botanical Garden. 279 pp.
- Heidel, B.L. and J. Vanderhorst. 1996. Sensitive plant species surveys in the Butte District, Beaverhead and Madison Counties. Unpublished report to the Bureau of Land Management. Montana Natural Heritage Program, Helena, Montana.
- Lesica, P. 1995. Demography of Astragalus scaphoides and effects of herbivory on population growth. Great Basin Naturalist 55: 142-150.
- Lesica, P. and B. M. Steele. 1994. Prolonged dormancy in vascular plants and implications for monitoring studies. Natural Areas Journal 14:209-212.
- Lesica, P. and J. Vanderhorst. 1995. Sensitive plant survey of the Sage Creek area, Beaverhead County, Montana. Unpublished report to the Bureau of Land Management. Montana Natural Heritage Program. 36 pp. plus appendices.
- Lesica, P., M. T. Lavin, and P. F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
- Vanderhorst, J.P. 1995. Sensitive plant survey in the Horse Prairie Creek drainage, Beaverhead County, Montana. Unpublished report to the Bureau of Land Management, Butte District. Montana Natural Heritage Program, Helena, Montana. 42 pp. plus appendices.
- Vanderhorst, J.P. and P. Lesica. 1994. Sensitive plant survey in the Tendoy Mountains, Beaverhead County, Montana. Unpublished report to the Bureau of Land Management, Butte District. Montana Natural Heritage Program, Helena, MT. 59 pp. plus appendices.