Montana Field Guide

PLANTS: A low-growing herbaceous perennial with deep, cord-like rhizomes. Stems are 10-20 cm tall, and zig-zag. Source: Lesica et al. 2012.

LEAVES: Pinnately-compound leaves that are arranged alternately on lax stems. Each leaf is comprised of 5-15 obovate leaflets, 5-15 mm long, with blunt to rounded tips. Lower leaf stipules are connate and 5-10 mm long. Foliage is thinly to densely covered with stiff, two-forked (dolabriform) hairs which are appressed to the leaf surface. Source: Lesica et al. 2012.

INFLORESCENSE: Yellowish-white (ochroleucus) flowers are arranged in crowded racemes that arise from the axils of the upper leaves, being barely longer than the leaves. Racemes 3-7 cm long, each with 15-40 flowers. The calyx is united (campanulate) and thinly covered with light-colored hairs. The pea-like flowers have a reflexed upper petal (banner) that is notched at the tip. Source: Lesica et al. 2012.

Flowering and fruiting occur in June and July.

Nearly 40 Astragalus species are documented in south-central Montana. Despite this high diversity, few species resemble Wind River Milkvetch.

Wind River Milkvetch – Astragalus oreganus, SOC
*Stems zigzag. 10-20 cm tall.
*Leaves have 5-15 broadly elliptic to nearly round leaflets (obovate) with blunt to rounded tips, and 2-forked hairs (dolabriform).
*Leaf stipules form a papery sheath at the base of each petiole, completely surrounding the stem (connate), 5-10 mm long.
*Flowers are yellowish-white (ochroleucus), 15-40, crowded in racemes, and cylindrical.
*Flowering stems are barely longer than the leaves.

Low Milkvetch - Astragalus lotiflorus
*Leaves with 5–15 linear-elliptic leaflets with acute tips.
*Leaf stipules lanceolate, 2–5 mm long, and distinct.
*Flowers are arranged in almost sessile racemes with 2–8 spreading flowers.
*Flowering stems are shorter than the leaves.

Canadian Milkvetch - Astragalus canadensis
*Stems do not zigzag, 15-60 cm tall.
*Leaves have 11-23 narrowly elliptic leaflets.
*Leaf stipules are lanceolate-attenuate, 5-10 mm long.
*Flowers 30-100 spreading to declining in crowded racemes.

Across its range in Montana and Wyoming, it occurs in barren bluffs, gullied knolls, dunes, and detritus under cliffs or buttes (Barneby 1964). Wind River Milkvetch grows in sandy, calcareous soil of sagebrush steppe in the valleys of the Pryor Mountain desert portion of Montana. The species forms large colonies in sandy soil below 5,000 feet (Lesica and Achuff 1992). It most often grows in the Chugwater Formation (Lesica and Achuff 1992).

ASSOCIATED SPECIES
Wind River Milkvetch is commonly associated with Big Sagebrush (Artemisia tridentata), Needle and Thread (Stipa comata), and Shadscale (Atriplex cofertifolia) in sagebrush steppe communities.

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).

PAPILIONACEOUS FLOWER
*Species belonging to the Pea Subfamily (Papilionoideae) have papilionaceous flowers (Elpel 2000). This applies to almost all species in the northern latitudes (Elpel 2000), including Wind River Milkvetch.
*Papilionaceious refers to the petals of the flowers that appear like a butterfly.
*The 5 petals (collectively called the corolla) have a bilateral symmetry: large upper petal (banner), two lateral petals (wings), and usually two fused lower petals (keel). The keel petals resembling a boat.

FLOWERS
Calyx: The sepals are united (campanulate; calyx), 6-10 mm long, tipped with teeth (1-3 mm long), and thinly covered with light-colored hairs.

Corolla: Cream-colored (ochroleucus). Banner petal is 10-15 mm, reflexed and notched at the tip. Lateral petals are 13.2-17.6 mm. Keel petal is 10-13 mm.

FRUITS
The fruit is a legume (seedpod). Legumes are fleshy, green, linear-oblong, and 10-16 mm long. In cross-section they are round. When mature, the legumes are nearly erect, leathery, and thinly covered with long soft hairs.

RESEARCH NATURAL AREAS and AREA OF CRITICAL ENVIRONMENTAL CONCERN PROGRAM
The Research Natural Area (RNA) program is a nationwide system created to protect a network of federally administered public lands established for the purpose of scientific research, maintaining biological diversity, and education (USFS 2014). The intent is to designate RNAs that represent the full array of North American ecosystems with their biological communities, habitats, natural phenomena, and geological and hydrological formations. These intact protected areas are managed to maintain their natural and primitive character with a minimum of human disturbance. Each RNA also serves as a baseline of ecological information that can be used to compare against other similar, yet managed ecological systems.

A type of RNA is the Area of Critical Environment Concern (ACEC). The ACEC program was established in the 1976 Federal Lands Policy and Management Act and is managed by the Bureau of Land Management (BLM) (Wikipedia Contributors 2020). Places designated as ACECs require special management to protect important historical, cultural, and scenic values, fish and wildlife, or other natural resources (BLM 2021). The BLM has extended the purpose of RNAs to that of preserving gene pools of typical and endangered plants and animals (BLM 2015; BLM 2021).

Pryor Foothill RNA/ACEC
This RNA-ACEC was established in the Approved Resource Management Plan by the MT/Dakotas BLM, Billings Field Office, recommended for approval by the MT/Dakotas BLM State Director in 2015, and is awaiting final approval by the U.S Congress (BLM 2015). The RNA-ACEC is approximately 2,606 acres in size. The management goals for the Pryor Foothills RNA/ACEC are to protect a large concentration of BLM's special status plant species and rare plant communities (unique vegetation) and to protect significant historic and cultural values in the Gyp Springs area. Large, intact populations of Wind River Milkvetch live in the Pryor Foothill RNA-ACEC. A scattering of populations occur in vicinity of the Pryor Foothill RNA-ACEC. See Range Comments.

The area encompassing Gypsum Creek and Crooked Creek, which includes the present-day Pryor Foothill RNA/ACEC was recognized in 1991 as having the highest concentration of high-priority plant species' populations (Lesica and Achuff 1992). Surveys from the 1980's to 1990's found populations of eight rare species Astragalus geyeri, Astragalus oreganus, Camissonia andina, Cleome lutea, Grayia spinosa, Leptodactylon caespitosum, Malacothrix torreyi, and Mentzelia pumila, of which remain as either Montana Species of Concern or Potential Species of Concern. The area's concentration of unique and diverse plant species partially results from diverse habitats, terrain that varies from highly dissected foothills with deep canyons to broad alluvial valleys, and soils derived from calcareous standstone, Chugwater sandstone, and limestone. In their 1992 report the authors recommended that this area would serve as the best choice for a single reserve in the Pryor Mountain Desert (Lesica and Achuff 1992).

GRAZING
Livestock grazing occurs in some of the areas were Wind River Milkvetch is found. It is thought that the plant is likely unpalatable to livestock. Information on the interaction between Wind River Milkvetch and grazing is needed.

In Montana, current and potential long-term threats to Wind River Milkvetch include (Lesica and Achuff 1992; Montana Native Plant Society 2009 and 2021):

Gypsum Mining: Gypsum is a common mineral found in the Chugwater Formation. Gypsum claims have been made on public lands in vicinity of Wind River Milkvetch. Potential negative impacts from mining activities include direct removal and trampling of plants. Potential indirect negative impacts from mining activities include disturbance to habitat from soil erosion, soil compaction, and colonization by invasive species. Soils in the Chugwater Formation are known to be very susceptible to erosion.

Invasive Species: The arid landscape in which Wind River Milkvetch occurs is predominately in native, intact habitat. However, Cheatgrass (Bromus tectorum), African mustard (Malcolmia africans), and Halogeton (Halogeton glomeratus) are known to occur along roads near to some Wind River Milkvetch occurrences. Halogeton outcompetes many native species by increasing the salinity of the soil (Sauer 1988) and has become locally common in the Pryor Mountain Desert region (Lesica and Achuff 1992). There is the potential for negative impacts based on the assumption that these exotics would be more competitive for soil water and nutrient resources, thus displacing native plants.

Off-road Vehicle Recreation: Off-road vehicle (ORV) use commonly occurs further north in Pryor Mountains. In the Big Horn Canyon National Recreational Area one species' occurrence grows along a road and may be vulnerable to vehicular disturbance. Where ORV use overlaps with Wind River Milkvetch populations there is the potential to damage plants by trampling, compacting soils, and increasing erosion.

STATE THREAT SCORE REASON
Reported threats to Montana's populations of Oregon Milkvetch include impacts from existing weed populations, livestock trampling and off-highway vehicle (OHV) disturbance (MTNHP Threat Assessment 2021). Habitat is found on Chugwater Formation substrates, a material that is especially prone to erosion and is known for gypsum rich layers. Surface disturbance in this habitat can directly damage populations and subsequently exacerbate co-occurring threats. Potential surface disrubance in the vicinity of populaitons is a concern where demand for gypsum extraction is ongoing.