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Montana Field Guides

Sweetwater Milkvetch - Astragalus aretioides
Other Names:  Astragalus sericoleucus var. aretioides

Species of Concern

Global Rank: G4
State Rank: S2S3
* (see State Rank Reason below)

Agency Status
USFWS:
USFS:
BLM:
MNPS Threat Rank: 3
C-value:

External Links






State Rank Reason (see State Rank above)
Sweetwater milkvetch is a reginal regional endemic from Montana south through Wyoming to Colorado and Utah, known in Montana only from exposed ridges and outcrops in the Pryor Mountains / Bighorn Canyon area. Threats to the species' viability in Montana appear to be minimal. Trend data are unavailable.
  • Details on Status Ranking and Review
    Sweetwater Milkvetch (Astragalus aretioides) Conservation Status Review
    Review Date = 05/06/2013
    View State Conservation Rank Criteria
    Population Size

    Score2 - Small: Generally 2,000-10,000 individuals.

    Range Extent

    Score2 - Regional or State Endemic or Small Montana Range: Generally restricted to an area <100,000 sq. miles (equivalent to 2/3 the size of Montana or less) or Montana contributes 50% or more of the species’ range or populations OR limited to 2-3 Sub-basins in Montana.

    CommentSmall Montana range.

    Area of Occupancy

    Score2 - Low: Generally occurring in 4-10 Subwatersheds (6th Code HUC’s).

    Environmental Specificity

    Score1 - Moderate: Species is restricted to a specific habitat that is more widely distributed or to several restricted habitats and is typically dependent upon relatively unaltered, good-quality habitat (C Values of 5-7).

    Trends

    ScoreNA - Rank factor not assessed.

    Threats

    Score0-1 - Low to Medium.

    Intrinsic Vulnerability

    Score0-1 - Low to Moderate Vulnerability.

    Raw Conservation Status Score

    Score 7 to 9 total points scored out of a possible 16 (Rarity factors and threats only).

 
General Description
Sweetwater Milkvetch is a long-lived perennial with short, profusely branching stems that form mounded cushions which are 1-3 dm in diameter. The pinnately compound leaves are 6-15 mm long and have 3 narrowly lance-shaped leaflets and a prominent papery sheath around the petiole where it meets the stem. Foliage is densely covered with silky hairs. Pea-like, magenta flowers are paired on short stems among the leaves. The upper petal is 6-8 mm long and partly reflexed upwardly. The calyx is 3-4 mm long. Fruit pods are narrowly egg-shaped, 4-5 mm long, densely silky and hairy, and are usually hidden amongst the leaves.

This taxon has also been treated as A. sericoleucus (M.E. Jones) Barneby var. aretioides M.E. Jones.

Phenology
Flowering occurs mid-June to mid-July and extends into August under some conditions.

Diagnostic Characteristics
Sweetwater milkvetch belongs to a distinctive section of Astragalus recognized by leaves with three leaflets and a cushion-like growth form. Of these, Astragalus gilviflorus and A. hyalinus have larger, yellow flowers and a calyx tube longer than 5 mm. Astragalus barrii has a banner petal longer than 8 mm and is found on clay soils east of the Bighorn Mountains.

Species Range
Present
 


Range Comments
MT south to CO; Big Horn and Carbon counties (Lesica 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).

Summary of Observations Submitted for Montana
Number of Observations: 19

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density

Recency

 

(Observations spanning multiple months or years are excluded from time charts)



Habitat
Sweetwater milkvetch grows in the foothills and montane zone (about 4400-7800 feet) on exposed ridges and slopes in thin soil usually derived from limestone or calcareous sandstone. It typically occurs in openings of Douglas fir, growing with Eritrichium howardii, Shoshonea pulvinata, Petrophyton caespitosum, and Hymenoxys acaulis.

Ecology
Cushion plants with a woody caudex, such as Sweetwater milkvetch, are often long-lived and slow-growing (Ehrlen and Lehtila 2002). This plant grows on harsh sites with sparse vegetation and presumably minimal competition. The breeding system and pollinators are unknown, but legumes have a tendency to be outcrossers, requiring pollination to set fruit.

Management
This plant often grows in sites that are quite remote and/or not easily accessible, and most are on public lands. Wild horse trailing has been observed at some locations, but the impacts, if any, are unknown.

References
  • Additional ReferencesLegend:   View Online Publication
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    • Barneby, R. C. 1956. Pugillus Astragalorum XIX: Notes On A. Sericoleucus Gray and Its Immediate Relatives. Amer. Midl. Nat. 55: 504-507.
    • Barneby, R.C. 1964. Atlas of North American Astragalus. 2 Vols. New York Botanical Garden, Bronx, New York. 1188 pp.
    • Dorn, R.D. 1988. Vascular Plants of Wyoming. Mountain West Publ., Cheyenne.
    • Ehrlen, J. and K. Lehtila. 2002. How perennial are perennial plants? Oikos 98: 308-322.
    • Lesica, P., M. T. Lavin, and P. F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
    • Welsh, S.L, N.D. Atwood, S. Goodrich, and L.C. Higgins. 1993. A Utah Flora, second edition, revised. Brigham Young University, Provo, UT.
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Citation for data on this website:
Sweetwater Milkvetch — Astragalus aretioides.  Montana Field Guide.  .  Retrieved on , from