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Astragalus flexuosus var. flexuosus | Slender Milk-vetch |
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Basis for ListingAstragalus flexuosus var. flexuosus (slender milk-vetch) occurs throughout the western plains and grasslands of North America, but its occurrence in Minnesota is at the far eastern edge of the species' range. In Minnesota, it occurs in nine counties along the westernmost part of the Minnesota River valley and in the northern part of the Prairie Coteau. Historically, it may have occurred in several more adjoining counties, but apparently was never widespread in the state. DescriptionThe overall impression of A. flexuosus var. flexuosus is that of a low, sprawling plant (less than knee-high), that appears almost viny. It is a perennial legume that can form clumps nearly 1 m (3.3 ft.) in diameter, with several to many stems giving rise to the clump from a branched crown. Though they tend to recline along the ground, the tips of the stems turn up. The color of the leaves and stems is greenish or grayish because of the hairs on the plant. The leaves have 11-25 leaflets arranged along 2 sides of the leaf axis, with a leaflet at the tip. Leaflets are linear to narrowly oblong-lanceolate; leaflet tips are blunt, squared-off, or with a small notch at the tip. At the base of the leaf are 2 leafy stipules, which are 1.5-7 mm (0.06-0.28 in.) long and connected at their bases; their tips are separate. HabitatIn Minnesota, A. flexuosus var. flexuosus is found in dry prairies and in mesic prairies, and most commonly in hill prairies, which are frequently dry-mesic in nature. Both types of prairie are dominated by grasses, but in dry prairie the midheight and shortgrass species are prominent, while in mesic prairie, tallgrasses dominate. Astragalus flexuosus var. flexuosus seems to be found across quite a range of moisture and topographic conditions, from dry hill prairie to rolling mesic prairie. It can be found on level terrain, such as on flat uplands at the top of blufflines, or on moderate and steep slopes, and it can be found mid-slope or at the crest of slopes. It grows on all slope aspects (west-, south-, east-, and north-facing). Soils where the species is found are typically loams, including sandy loam, clay loam, and gravelly clay loam. Biology / Life HistoryLike most legumes, A. flexuosus var. flexuosus is insect pollinated. Many species of Astragalus are somewhat toxic to livestock. They can have either nitrotoxins or toxic alkaloids in their tissues, and some species accumulate selenium. All of these substances can cause illness in livestock with prolonged grazing. However, many Astragalus species also have high forage value and cattle will graze on them, especially in the early spring when other forage is scarce. Astragalus flexuosus var. flexuosus does contain nitrotoxins, but does not contain toxic alkaloids or accumulate selenium (Williams and Barneby 1977; Ralphs et al. 2002). Based on available literature, it does not appear that A. flexuosus var. flexuosus is an especially problematic species for grazing livestock. Conservation / ManagementThe remnant habitats for A. flexuosus var. flexuosus are mostly in grazed landscapes (either currently grazed or grazed in the past). Given this setting, the species faces several threats. Overgrazing causes erosion and compaction, which may degrade prairie habitats. The common practice of managing active pastures with herbicide to control Cirsium spp. (thistles) further threatens prairie forbs (Minnesota County Biological Survey 2007). Many pastures are also heavily invaded by non-native cool-season grasses such as Poa pratensis (Kentucky blue grass) and Bromus inermis (smooth brome), which outcompete native species for light, space, nutrients, and/or water. Other non-native herbs such as Euphorbia virgata (leafy spurge) can also be a problem. Lastly, hilly habitats of A. flexuosus var. flexuosus are threatened by habitat conversion. While not as big a concern as it is for Astragalus missouriensis var. missouriensis, aggregate quarrying may be a threat where A. flexuosus var. flexuosus populations are located on gravelly sites. Several programs and resources are available to land managers and landowners who want to help protect and manage remaining prairie parcels, including the Native Prairie Bank Program, the Native Prairie Tax Exemption Program, and a prairie restoration handbook. Best Time to SearchThe best time to search for A. flexuosus var. flexuosus is when the plants are flowering or fruiting, from about June through early September. Conservation Efforts in MinnesotaOver 1/3 of the existing populations of A. flexuosus var. flexuosus are on lands owned by state or federal agencies or private conservation organizations. This affords the species some level of protection from development, and some of these areas are being actively managed to perpetuate the prairie habitats. However, no known attempt has been made to specifically assess the conservation needs of A. flexuosus var. flexuosus within these areas or to determine the effects of current management practices on its status. |