Astragalus microcymbus
Author: Barneby

Skiff milkvetch

Fabaceae (Pea Family)

Close up of Astragalus microcymbus by Bernadette Kuhn
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Close up of Astragalus microcymbus by Michelle DePrenger-Levin
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Close up of Astragalus microcymbus by Michelle DePrenger-Levin
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Ranks and Status

Global rank: G1
State rank: S1
Federal protection status: BLM Sensitive, USFWS Candidate
State protection status: None

[+] Description and Phenology

Astragalus microcymbus by Karen Cleaver.  Please also see 1997 profile.
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General description: Herbaceous, perennial plants with purplish stems 2.5-6 dm long. Leaves are 2-4 cm long, leaflets are 3-9 mm long. Flowers are white, tinged with purple, and arranged in loose racemes of 7-14 flowers. Pods are terete and up to 8 mm long (Spackman et al. 1997).

Look Alikes: Similar to Astragalus flexuosus which differs in having pods over 1 cm in length (Spackman et al. 1997). 

Phenology: Flowers in mid to late May through June, and occassionally into early July. Fruit may be observed as early as late May, but it is always observable by mid-June. Peak fruiting period is mid-July. By late August plants are dry and fruits have dehisced (Colorado Natural Heritage Program).

[+] Habitat

Habitat of Astragalus microcymbus by Michelle DePrenger-Levin
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Open sagebrush or juniper-sagebrush communities on moderately steep to steep slopes. Often found in rocky areas with a variety of soil conditions from clay to cobbles, gray to reddish in color (Spackman et al. 1997).  Associated plant taxa include Purshia tridentata,Yucca harrimaniae, Artemisia frigida, Chaenactis, Penstemon caespitosa, Symphoricarpos, Sedum lanceolatum, Phlox, Hesperia comata, Castilleja, and Poa.

Elevation Range: 7,592 - 8,573 feet (2,314 - 2,613 meters)

[+] Distribution

Distribution of Astragalus microcymbus in Colorado.
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Colorado endemic: Yes
Global range: Colorado endemic known from Gunnison County, and extending into the edge of Saguache County. Estimated range is 168 square kilometers, calculated in GIS in 2008 by the Colorado Natural Heritage Program by drawing a minimum convex polygon around the known occurrences.

[+] Threats and Management Issues

Summary results of an analysis of the status of Astragalus microcymbus based on several ranking factors. This species was concluded to be "moderately conserved”. From Rondeau et al. 2011.
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Rabbit herbivory and recreational use by ORVs are considered to be the primary threats to the species (Rondeau et al. 2011). Denver Botanic Gardens monitoring data, along with Lyon (1990) indicate that rabbit herbivory is heavy and that rabbits may be selecting Astragalus microcymbus over other local plant species. About 20 miles of single track trails have been constructed within A. microcymbus habitat (pers. comm. Tom Grant 2006), with the potential to become popular with motorcyclists. Other recreational activities and grazing also pose possible threats. Astragalus microcymbus habitat is further fragmented by utility corridors, trails, roads, development, and cheat grass invasion.

[+] References

    • Ackerfield, J. 2012. The Flora of Colorado. Colorado State University Herbarium. 433 pp.
    • Barneby, R.C. 1964. Atlas of North American Astragalus. 2 Vols. New York Botanical Garden, Bronx, New York. 1188 pp.
    • Colorado Native Plant Society. 1989. Rare plants of Colorado. Rocky Mountain Nature Association, Colorado Native Plant Society, Estes Park, Colorado. 73 pp.
    • Colorado Natural Heritage Program. 2005. The Second Annual Colorado Rare Plant Symposium: G1 Plants of Colorado. Symposium Minutes. Available on-line
    • Colorado Natural Heritage Program. 2006. Biological Conservation Datasystem. Colorado State University, Ft. Collins, CO.
    • Denver Botanic Gardens. 2008. Demographic Analysis of Astragalus microcymbus (Fabaceae), an Endemic Species of Gunnison County, Colorado, USA.
    • Heil, K.D. and J.M. Porter. 1990. Status report for Astragalus microcymbus Barneby. January 2, 1990.
    • Johnston, B.C. 1979. USFS Summary Sheet. Unpublished report.
    • Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.
    • Lawrence, S. 1993 Endangered species petition. Unpublished report prepared for the US Fish and Wildlife Service, Washington, D.C..
    • Lyon, P. 1990. Field survey for Astragalus microcymbus.
    • Neely, B., S. Panjabi, E. Lane, P. Lewis, C. Dawson, A. Kratz, B. Kurzel, T. Hogan, J. Handwerk, S. Krishnan, J. Neale, and N. Ripley. 2009. Colorado Rare Plant Conservation Strategy, Developed by the Colorado Rare Plant conservation Initiative. The Nature Conservancy, Boulder, Colorado, 117 pp.
    • O'Kane, S. L. 1988. Colorado's Rare Flora. Great Basin Naturalist. 48(4):434-484.
    • Peterson, J. S., B.C. Johnston, and W. Harmon. 1981. Astragalus microcymbus status report of 3 March 1981.
    • Peterson, J.S. 1982 Plant Species of Special Concern, Astragalus microcymbus. Unpublished manuscript.
    • Rondeau, R., K. Decker, J. Handwerk, J. Siemers, L. Grunau, and C. Pague. 2011. The state of Colorado's biodiversity 2011. Prepared for The Nature Conservancy. Colorado Natural Heritage Program, Colorado State University, Fort Collins, Colorado.
    • Spackman, S., B. Jennings, J. Coles, C. Dawson, M. Minton, A. Kratz, and C. Spurrier. 1997. Colorado rare plant field guide. Prepared for Bureau of Land Management, U.S. Forest Service and U.S. Fish and Wildlife Service by Colorado Natural Heritage Program.
    • US Fish and Wildlife Service. 1993 c. Memorandum to report on trip to Gunnison to visit Astragalus microcymbus and Astragalus anisus. USDOI, FWS, Ecological Services, Western Colorado Office, Grand Junction, CO. Unpublished report.
    • USDA, NRCS. 2013. The PLANTS Database. National Plant Data Team, Greensboro, NC 27401-4901 USA.
    • Weber, W. A. and R. C. Wittmann. 2012. Colorado Flora, Western Slope, A Field Guide to the Vascular Plants, Fourth Edition. Boulder, Colorado. 532 pp.

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