Astragalus sinuatus (Whited’s milkvetch)
Flowers of Astragalus sinuatus (Photo: Julie K. Combs)
Astragalus sinuatus Piper
Critically Endangered (CR) according to IUCN Red List criteria.
South-facing slopes of rocky hillsides; in loess soils with small amounts of volcanic ash.
A small number of Astragalus species are toxic to livestock, but this is not likely to be the case for A. sinuatus, which is occasionally browsed by elk, deer and livestock.
About this species
Whited’s milkvetch is a white-flowered, perennial herb belonging to the pea and bean family (Leguminosae). The common name refers to Kirk Whited (born in 1852 in Michigan), who collected specimens of this species in Washington State and whose name is honoured in the specific epithet of the synonym, Astragalus whitedii. The specific epithet sinuatus refers to the sinuously wavy suture (seam along which the fruit opens) along the upper surface of the pod.
Astragalus whitedii, Phaca sinuata, Homalobus sinuatus, Homalobus whitedii
Geography and distribution
Restricted to Washington State in the USA, Whited’s milkvetch is found only along the western edge of the Columbia Basin in Chelan County. It occurs on rocky hillsides, growing in loess soils with small amounts of volcanic ash, at 250-610 metres above sea level.
Overview: Astragalus sinuatus is a robust, perennial herb with a woody taproot and knotty root-crown. The plant is covered with stiff hairs up to 0.6 mm long. Each plant has multiple stems that grow along the ground, but have the terminal branchlets bending upwards.
Leaves: The widely-spreading leaves are 2-7 cm long and are mostly sessile (borne directly on the stem and lacking a leaf stalk or petiole). Each leaf is composed of 11-17 flat or loosely folded leaflets each measuring 6-16 mm in length.
Flowers: The inflorescence is a raceme of 8-16 flowers. The calyx (comprising a tube and five calyx teeth) is densely hairy, with black, or mixed black and white, hairs. The petals are whitish. The standard petal (banner) is 16.6-20 mm long and deeply notched at the apex; the two wing petals (each comprising a blade and basal claw) are 14.5-17.3 mm long; and the clawed keel petals are 12-13.4 mm long.
Fruits: The mature fruit is a leathery to sub-woody pod, 2-3 cm long and 5-7 mm in diameter with a narrow, wavy suture along the upper surface and a cord-like suture along the lower surface (the upper and lower sutures together form the seam along which the fruit opens). There are 24-30 ovules (immature seeds) per pod.
Seeds: The seeds are smooth, dull brown and about 3.5 mm long.
Threats and conservation
This species is listed by the U.S. Fish and Wildlife Service (USFWS) as a 'Species of Concern', and is Critically Endangered (CR) according to IUCN Red List criteria. An estimated 5,000 individuals of Astragalus sinuatus remained in the wild in 1994, although this number may have decreased since as a result of threats to its habitat. The invasion of non-native species (such as the grass Bromus tectorum), grazing and agricultural development all threaten the dry hillside habitat in which it grows. Herbivorous insects are also a major threat, and although pods have the potential to produce up to 30 seeds, the presence of specialist and generalist seed predators means that in reality each pod often only contains one or two mature seeds.
Periodic fires, on a 30-90 year cycle, probably played a role historically in maintaining the habitat of this species. The more frequent fires typical of recent years may promote weedy species at the expense of A. sinuatus. Suppression of fires and any increase in grazing could lead to invasion by sagebrush (Artemisia tridentata) and to a further decrease in A. sinuatus.
Seeds from one population are held in the Berry Botanic Garden Seed Bank for Rare and Endangered Plants of the Pacific Northwest (Oregon, USA). Seeds are also held at the Miller Seed Vault at the University of Washington Botanic Gardens. The seed pods are so tough that it can take a pair of pliers to crack them open and extract the seed, and given that each pod normally contains only one or two seeds, seed-collecting can be quite a challenge!
Conservation assessments carried out at Kew
Astragalus sinuatus is being monitored as part of the 'IUCN Sampled Red List Index for Plants', which aims to produce conservation assessments for a representative sample of the world’s plant species. This information will then be used to monitor trends in extinction risk and help focus conservation efforts where they are needed most.
Barneby, R.C. (1964). Atlas of North American Astragalus. In: Mem. N. Y. Bot. Gard. 13: 315-316.
Bjork, C.R. & Fishbein, M. (2006). Astragalus asotinensis (Fabaceae), a newly discovered species from Washington and Idaho, United States. Novon 16: 299-303.
Combs, J.K., S.E. Reichard, M.J. Groom, D.L. Wilderman and P.A. Camp. 2011. Invasive competitor and native seed predators contribute to rarity of the narrow endemic Astragalus sinuatus. Ecological Applications 21: 2498–2509
Contu, S. (2010). Astragalus sinuatus. Assessment using IUCN Categories and Criteria 3.1 (IUCN 2001). Royal Botanic Gardens, Kew.
Washington Natural Heritage Program (1997). Field Guide to Selected Rare Plants of Washington. Washington State Department of Natural Resources.
Kew Science Editor: Gwilym Lewis
Kew contributors: Steve Davis
Copyediting: Emma Tredwell
Kew would like to thank the following contributors: Julie K. Combs (University of Washington, School of Forest Resources) for images firstname.lastname@example.org
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