Xanthisma spinulosum var. spinulosum
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Basis for Listing
Xanthisma spinulosum var. spinulosum occurs throughout the Great Plains, but is rare all along the eastern edge its range including Minnesota. Its distribution in the state is limited to just four counties - Lac qui Parle, Big Stone, Traverse, and Swift. In these four counties, X. spinulosum var. spinulosum is found at fewer than 20 locations. Population sizes at each location range from small to very small, and the habitat patches where the species are found are often also small.
Xanthisma spinulosum var. spinulosum is an exceedingly complex species with 7 described varieties (Hartman 2006). Only one variety occurs in Minnesota and it differs from the other varieties by being a perennial rather than a subshrub, with spreading to sprawling stems that grow to a maximum height of only about 40 cm (15.7 in.). The lower half or two-thirds of each stem will typically have several branches. The leaves are small, usually less than 3 cm (1.2 in.) long, and are divided in a pinnatifid or twice-pinnatifid pattern giving them a lacey appearance. The leaves are numerous, evenly spaced on the stem, and usually covered with hairs, which may be gland-tipped. The leaves give the whole plant a gray-green appearance. Each branch will produce a flower head that is about 1 cm (0.4 in.) across. Each head is rimmed by short yellow ray flowers.
Xanthisma spinulosum var. spinulosum grows in dry prairies, often in the highest and driest part of the habitat. In Minnesota, it is usually found at the tops of south- or west-facing slopes or knolls, on upper sideslopes, or on crests of ridges. Dry prairies are dominated by grasses, especially mid-height and shortgrass species, and forb cover is sparse to patchy. Soils are droughty (excessively well-drained) because of their coarse texture (sands and gravels) or their position on upper slopes or very steep slopes. Associated grasses include Hesperostipa comata (needle-and-thread grass), Bouteloua gracilis (blue grama), Bouteloua curtipendula var. curtipendula (side-oats grama), and Pascopyrum smithii (western wheatgrass). Associated forbs may include Artemisia frigida (sage wormwood), Solidago mollis (soft goldenrod), and several Astragalus spp. (milk vetches).
Biology / Life History
Xanthisma spinulosum var. spinulosum is insect pollinated. It has a pappus of bristles (ring of bristles at the base of the seed) that help to give the seed some loft in the wind, enabling at least short distance seed dispersion by the wind. Because it occurs in prairies, which are fire-maintained habitats, it is reasonable to assume that the species is fire-adapted. Given the dry conditions of it's habitat, it is also likely that X. spinulosum var. spinulosum is drought-tolerant.
Conservation / Management
The remnant habitats for this rare plant are mostly in hilly, grazed landscapes (either currently or in the past). Given this setting, X. spinulosum var. spinulosum faces several threats. The first is simply overgrazing. While X. spinulosum var. spinulosum may not be a preferred forage plant, it is susceptible to the erosion caused in some pastures by heavy grazing. Second, the common practice of managing pastures with herbicide to control Cirsium spp. (thistles) threatens prairie forbs (Minnesota County Biological Survey 2007). Third, many pastures are heavily invaded by non-native cool-season grasses such as Poa pratensis (Kentucky bluegrass) and Bromus inermis (smooth brome). The forth and largest concern for X. spinulosum var. spinulosum, and other species that share its habitat, is sand and gravel mining. The sand and gravel deposits often present in these habitats are in high demand for use in road construction. Mining the gravel substrate obviously destroys the prairie habitat and any populations of X. spinulosum var. spinulosum that might be present. Habitats supporting this species must be protected from aggregate mining if this species is to survive in Minnesota. Apart from habitat protection, prescribed fire is the primary management activity that is necessary. Fire keeps encroachment of shrubs in check and helps reduce the dominance of non-native cool-season grasses.
Conservation Efforts in Minnesota
Only four populations of X. spinulosum var. spinulosum are protected on publicly owned lands including Big Stone National Wildlife Refuge, Farrell Waterfowl Production Area, and Yellow Bank Hills Scientific and Natural Area. One additional population on private land is protected through a conservation easement. Although no known efforts have been undertaken to specifically manage for X. spinulosum var. spinulosum within these areas, brush management in the form of prescribed burns has occurred on the easement parcel and several old fields have been restored to native vegetation at the SNA site.
Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence, Kansas. 1,402 pp.
Hartman, R. L. 2006. Xanthisma. Pages 383-393 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 20, part 2. Oxford University Press, New York, New York.
Minnesota County Biological Survey. 2007. Native plant communities and rare species of the Minnesota River Valley counties. Division of Ecological Resources, Minnesota Department of Natural Resources, St. Paul. 153 pp.
Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the prairie parkland and tallgrass aspen parklands provinces. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 362 pp.