Montana Field Guide

Barr's Milkvetch is a perennial that grows in low, dense mats. Leaves have 3 narrowly lance-shaped leaflets and are 1-4 cm long, densely covered with short white hairs. Stipules at the leaf bases are membranous. Purple or pinkish-purple flowers that are shaped like pea flowers are borne in a narrow, open, few-flowered inflorescence. The petals are 7-17 mm long. The calyx is 3-5 mm long and densely covered with long, white hairs. The sparsely white, hairy pod is narrowly elliptical, 4-8 mm long, and 1-2 mm in diameter.

In Montana, Barr's milkvetch blooms from late April to mid-June and forms pods in late May and June.

Barr's milkvetch is distinguished by its small, iridescent, bluish-purple to bluish-pink, early-blooming flowers. Petal color may fade, sometimes to yellowish-white, as flowers dry (Taylor and Caners 2002). Flowers are necessary to distinguish A. barrii from four other matt-forming, 3-leafleted Astragalus species that overlap geographically with it in Montana: A. aretioides (Sweetwater milkvetch), A. sericoleucus (silky milkvetch), A. gilviflorus (plains milkvetch) and A. hyalinus (summer milkvetch).

Flowers or their remnants are necessary to distinguish Barr's milkvetch from co-occurring species of Astragalus that have compound leaves with three leaflets and a cushion-forming habit. The flowers of A. aretioides (Sweetwater milkvetch) and A. sericoleucus (silky milkvetch) are typically one-half the size of A. barrii, and the plants tend to grow in denser mats. A. gilviflorus (plains milkvetch) and A. hyalinus (summer milkvetch) typically have larger, cream to whitish flowers, a longer calyx tube (6-16 mm), and shorter peduncles (0-3.5 mm).

In Montana, Barr's milkvetch occurs on sparsely vegetated knobs and buttes, usually with dry, fine-textured, often calcareous soils (Vanderhorst et al. 1998). These habitats are characterized by a scattered ponderosa pine and Rocky Mountain juniper, or often just a sparse shrub cover of big sagebrush (Artemisia tridentata) and/or shadscale (Atriplex confertifolia). In southeastern Montana, common dominants or associates are western wheatgrass (Pascopyrum smithii or Agropyron smithii), thickspike wheatgrass (Elymus lanceolatus or Agropyron dasystachyum), green needlegrass (Nassella viridula or Stipa viridula), little bluestem (Schizachyrium scoparium), bluebunch wheatgrass (Pseudoroegneria spicata or Agropyron spicatum), prairie junegrass (Koeleria macrantha or Koeleria cristata), native legumes, Nuttall's saltbush (Atriplex gardneri) and winterfat (Krascheninnikovia lanata or Ceratoides lanata) (Heidel & Marriott 1996).

Barr's milkvetch is associated with the harsh edaphic and environmental conditions of Badlands areas (Heidel and Marriott 1996). These habitats receive limited rainfall and are subject to high light intensities, potentially limiting plant establishment and survival as a result of high soil water evaporation. Barr's milkvetch often occurs on barren, eroded microsites, and may depend on the climate that maintains these sites to limit competition from other species. At some locations, particularly on steep slopes, soils erode during intense rainstorms. However, because of its cushion habit and dense foliage, the soil beneath Barr's milkvetch plants is protected from this erosion, resulting in plants that are frequently perched atop small pedestals of soil (Heidel and Marriott 1996). This species may remain in a vegetative condition under stressful conditions.

Drought conditions in 1987 and 1988 resulted in large decreases in the species' abundance, especially at lower elevations, in lower precipitation zones and on more exposed sites (Heidel and Marriott 1996). This may be a common cyclical trend. It does, however, indicate that populations may respond negatively to a long-term warmer, dryer climatic shift.

Populations in the Custer National Forest occur in habitats that receive more moisture than in surrounding areas and are thus more successionally advanced (Schassberger 1990). In these areas, mats of Barr's milkvetch have been noted as larger and often vegetative. This may reflect a different age-class distribution, with these populations consisting of larger, older mats that have lower rates of flowering and fruiting, and reduced seedling establishment.

POLLINATORS
The following animal species have been reported as pollinators of this plant species or its genus where their geographic ranges overlap: Bombus vagans, Bombus appositus, Bombus auricomus, Bombus bifarius, Bombus borealis, Bombus centralis, Bombus fervidus, Bombus flavifrons, Bombus huntii, Bombus mixtus, Bombus nevadensis, Bombus rufocinctus, Bombus ternarius, Bombus terricola, Bombus occidentalis, Bombus pensylvanicus, Bombus griseocollis, and Bombus insularis (Macior 1974, Thorp et al. 1983, Mayer et al. 2000, Colla and Dumesh 2010, Wilson et al. 2010, Koch et al. 2012, Miller-Struttmann and Galen 2014, Williams et al. 2014).

Based on its low, tufted growth form, early-season flowering, and frequent occurrence on secondary range, Barr's milkvetch is probably not affected very much by grazing, unless livestock developments are placed near populations. This species occupies high knobs and outcrops that are seldom selected as development sites. However these habitats could be vulnerable to large-scale disruption of native vegetation and competition from invasives plants, such as yellow sweetclover (Melilotus officinalis).

STATE THREAT SCORE REASON
Reported threats to Montana's populations of Barr's Milkvetch include minor impacts from existing weed populations and livestock trampling and grazing. Most of the existing populations occur in the vicinity of coal and methane gas development, where the potential for land surface disturbance to directly impact and subsequently exacerbate threats from invasive plants and livestock is an ongoing concern (MTNHP Threat Assessment 2021).