Astragalus alpinus var. alpinus
Basis for Listing
Astragalus alpinus var. alpinus (alpine milk-vetch) has a wide circumpolar distribution in America and Eurasia, extending south in alpine habitats to New Mexico. In Minnesota, this species is known from only one area in Lake County (Laurentian Uplands Subsection), where plants occur around portions of several shallow, rocky ponds, all within 1.3 km (0.8 mi.) of each other. Although the habitat does not appear to be immediately threatened, it is dependent on a very specific and sensitive hydrological system that could be inadvertently disrupted by a variety of human activities. It has also been compromised by several invasive plant species that require continued control and monitoring. For these reasons, A. alpinus var. alpinus was listed as an endangered species in Minnesota in 1996.
Astragalus alpinus var. alpinus is a perennial that spreads from a below-ground rhizome. The stems are smooth or nearly so. They are up to 30 cm (12 in.) long, though the lower portions will lay or trail on the ground, sometimes forming a loose mat. The leaves are 3-15 cm (1-6 in.) long and composed of 9-26 individual leaflets arranged in a pinnate pattern. Each leaflet is 4-20 mm (0.2-0.8 in.) long and 2-10 mm (0.1-0.4 in.) wide. There are 5-30 flowers arranged in a raceme at the end of the stem. Each flower is 9-12 mm (0.4-0.5 in.) long with a purple keel and white wings. The seeds are borne in pendulous pods 7-17 mm (0.3 – 0.7 in.) long. Several taxonomic varieties of A. alpinus have been described based on variations in pod characteristics. Variety alpinus is found in arctic regions and extends south into South Dakota, Minnesota, Wisconsin, and throughout the Rocky Mountains. It has a densely pubescent pod with dark and light hairs.
In Minnesota, A. alpinus var. alpinus has been found on sandy gravel margins of shallow groundwater-fed ponds (Inland sand/gravel/cobble shore) at a single area in Lake County. The water level in the ponds fluctuates greatly from year to year and often within a single year. The ponds are in a fire-dependent forest/woodland system where the dominant tree species is jack pine (Pinus banksiana). The tree canopy is intermittent, and the amount of sunlight reaching the ground layer is variable throughout the day. Associated species include: Amelanchier spp. (Juneberry), Carex cryptolepis (cryptic sedge), C. viridula (green sedge), Deschampsia cespitosa (tufted hair grass), Lysimachia ciliata (fringed loosestrife), Packera paupercula (balsam ragwort), and Vaccinium spp. (blueberry).
Biology / Life History
The typical habitat for this species across its range is strongly associated with water bodies, particularly streams or interconnected ponds. It is believed that seed dispersal is accomplished by water currents or, specifically, dispersal of the unopened pod rather than the individual seeds (Danvind and Nilsson 1997). The competitive advantage of A. alpinus var. alpinus is likely enhanced by its ability to fix atmospheric nitrogen in the nitrogen-poor habitat where it occurs. This is a characteristic of leguminous plants that make them especially important in maintaining plant community properties (Olsen et al. 2013). Flowering within the inflorescence occurs sequentially from bottom to top. It is pollinated mainly by bumblebees, and the seeds are often preyed upon by dipteran and lepidopteran larvae (Kudo and Molau 1999).
The pond habitat where this species occurs has a very complicated hydrology. The water level varies greatly, sometimes flooding the population. This condition is not a problem for A. alpinus var. alpinus because it is very well adapted to periodic spring flooding. In fact, it may require this natural flood cycle to maintain itself. Population size and even specific location within the site may vary from year to year. Sometimes there may be only a few almost stunted clumps, while in other years the whole area may be covered with robust plants. The densest occurrences of the population are often in the more open and recently flooded beach habitat. Some plants may even be found in the open edge of the Pinus banksiana jack pine forest at the edge of the beach (U.S. Forest Service 2000). Peak flowering occurs primarily in late May through June. The fruiting period typically lasts longer, from August into autumn, but it can be affected by dry conditions or flooding (high water). Above ground portions of plants may be gone or greatly reduced in abundance by mid-summer.
Conservation / Management
Astragalus alpinus var. alpinus is considered critically imperiled in both Minnesota and Wisconsin, so any reported threats must be taken very seriously. Habitat alteration and the spread of non-native invasive species such as Phalaris arundinacea (reed canary grass), Cirsium arvense (Canada thistle), and C. vulgare (bull thistle) pose a real threat to A. alpinus var. alpinus at the only known site where it occurs in Minnesota. Activities such as road building, ATV and other off-road vehicle use (which may cause rutting), recreational pursuits, chemical alteration of ponds (e.g., rotenone application), permanent changes in hydrology, or changes in forest cover constitute potential threats. Astragalus alpinus var. alpinus colonizes different areas of the habitat in different years. It apparently needs to be able to move around in a habitat larger than is actually occupied at any one time. Water level and unique hydrological processes may be crucial elements for this species' survival, but at this point, more information is needed (U.S. Forest Service 1999, 2000).
Best Time to Search
The best time to search for Astragalus alpinus var. alpinus is when it is in flower or fruit, from late May through autumn.
Conservation Efforts in Minnesota
The Minnesota population of A. alpinus var. alpinus is apparently visited every year or so by U.S. Forest Service personnel to estimate the size of the population and to note presence or absence of colonies (Brzeskiewicz 2003). The general area supporting A. alpinus var. alpinus has been identified as a Unique Biological Area (UBA) by the Superior National Forest, which means management emphasis will be on conserving or enhancing the area (U.S. Forest Service 2004). A personal communication from Forest Service staff in 2019 indicated that an ongoing attempt to control populations of Canada and bull thistle using spraying and hand pulling have been successful in reducing the populations of those species, though they are still present. Reed canary grass also occurs in the area but apparently does not pose an immediate threat to the population of A. alpinus var. alpinus, though that could change quickly.
Welby R. Smith (MNDNR), 2020
(Note: all content ©MNDNR)
References and Additional Information
Barneby, R.C. 1964. Atlas of North American Astragalus, Memoirs, New York Botanical Garden. Volume 13, Parts I and II. New York Botanical Garden, Bronx, New York. 1188 pp.
Brzeskiewicz, M. 2003. Conservation assessment for alpine milkvetch (Astragalus alpinus). United States Forest Service, Eastern Region, Chequamegon-Nicolet National Forest, Park Falls, Wisconsin. 35 pp.
Danvind, M., and C. Nilsson. 1997. Seed floating ability and distribution of alpine plants along a northern Swedish river. Journal of Vegetation Science 8(2):271-276.
Kudo, G., and U. Molau. 1999. Variations in reproductive traits at inflorescence and flower levels of an arctic legume, Astragalus alpinus L.: comparisons between a subalpine and an alpine population. Plant Species Biology 14(3):181-191.
Olsen, S. L., S. M. Sandvid, and O. Totland. 2013. Influence of two N-fixing legumes on plant community properties and soil nutrient levels in an alpine ecosystem. Arctic, Antarctic, and Alpine Research 45(3):363-371.
United States Forest Service. 1999. Population viability assessment in forest plan revision. Statement of purpose and reason. Draft species data records: Astragalus alpinus. United States Forest Service, Region 9, Duluth, Minnesota.
United States Forest Service. 2000. Population viability assessment in forest plan revision. Questions for plant population viability assessment panel: Astragalus alpinus. United States Forest Service, Region 9, Duluth, Minnesota.
United States Forest Service. 2004. Superior National Forest Land and Resource Management Plan. Superior National Forest, Eastern Region, Milwaukee, Wisconsin.
United States Forest Service. Not dated. Condensed sensitive plant species abstracts. United States Forest Service, Eastern Region, Chequamegon-Nicollet National Forests, Park Falls, Wisconsin.
Wisconsin Department of Natural Resources, Bureau of Endangered Resources. 1993. Guide to Wisconsin's Endangered and Threatened Plants. Wisconsin Department of Natural Resources PUBL-ER-067, Madison, Wisconsin. 128 pp.