Specimens intermediate between Astragalus diaphanus var. diurnus and var. diaphanus have been collected along the John Day River from Grant and Wheeler counties, north of the known range of var. diurnus.
South Fork John Day milkvetch is a tap-rooted annual, or possibly short-lived perennial, with several cespitose, spreading, strigose stems 10-40 cm long. Leaves are 3-5 cm long with slender petioles. Leaflets number 9-15, are oblong to obovate, glabrous above and strigose beneath, and 0.5-1.0 cm long. Racemes are short, few-flowered, and are borne on peduncles mostly shorter than the leaves. The calyx is strigose and approximately 0.4 cm long, the subulate teeth equaling or nearly equaling the tube. The corolla is cream or purplish-tinged and 0.5-0.8 cm long. Pods are inflated, ovoid-reniform, finely whitish-strigillose, sessile, 1.5-2.0 cm long and over half as wide. Pods are 1-celled but the lower suture is slightly intruded and forms a thin partial partition.
Astragalus diaphanus var. diaphanus very closely resembles South Fork John Day milkvetch and both occur along the John Day River. However, the two taxa can be distinguished by fruit morphology. South Fork John Day milkvetch pods are distinctly inflated and ovoid-reniform, with a scarcely developed partition, whereas A. d. var. diaphanus pods are linear-oblong, strongly arcuate, and compressed, with a nearly complete partition formed by the intrusion of the lower suture.
When to survey
Surveys for South Fork John Day milkvetch should be completed when the taxon is fruiting and can be distinguished from the closely related Astragalus diaphanus var. diaphanus, typically from mid May to mid June.
South Fork John Day milkvetch usually occurs on dry, barren slopes in gravelly, shallow soils overlying basalt. It is found in openings in juniper woodland at elevations ranging from 760-1100 m (2500-3600 ft).
Associated plant species include Achillea sp., Agropyron sp., Alyssum sp., Artemisia tridentata, A. rigida, Blepharipappus sp., Bromus sp., Cryptantha sp., Ericameria nauseosa, Eriogonum compositum, E. microthecum, E. sphaerocephalum, E. strictum, Epilobium sp., Eriophyllum lanatum, Gutierrezia sarothrae, Juniperus occidentalis, Lewisia rediviva, Lomatium sp., Phacelia sp., Poa sp., Purshia sp., Salvia dorii, and Senecio sp.
This taxon is restricted to the south fork of the John Day River within the Blue Mountains ecoregion.
A primary threat to South Fork John Day milkvetch is habitat destruction due to excavation of cinders for road maintenance. The species is strongly influenced by spring precipitation and may be negatively impacted by prolonged periods of unfavorable climatic conditions. Other potential threats to the species include exotic weed invasions, changes in historic fire regimes, and off-road vehicle use.
Did you know?
The type specimen for this taxon was collected by Thomas Howell in 1885. The taxon was first described by Watson as Astragalus diurnus the following year.
Croft, L. K., W. R. Owen, and J. S. Shelly. 1997. Interior Columbia Basin Ecosystem Management Project Analysis of Vascular Plants. U.S. Forest Service, Pacific Northwest Research Station. Available at http://www.icbemp.gov/science/croft_1.pdf
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Peck, M. E. 1961. A manual of the higher plants of Oregon. Binfords and Mort, Portland, Oregon.