Aristida purpurea var. longiseta (Steud.) Vasey
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Basis for Listing
Aristida purpurea var. longiseta (red three-awn) is a perennial grass that occurs in dry and dry-mesic prairies in western Minnesota. Since over 98% of the prairie and savanna habitat that was present in the state before settlement has been lost (Minnesota's Remaining Native Prairie), there is a limited amount of potential habitat remaining for this species in Minnesota. There are only 20-30 extant locations of A. purpurea var. longiseta in the state, and almost all of them are in the North Central Glaciated Plains Section of southwestern Minnesota. Over 1/3 of the locations are in Traverse County, in the bluffs overlooking the historic Glacial River Warren valley. Prairie habitats in western Minnesota continue to be threatened by overgrazing, invasion by non-native species, gravel mining, biofuel and wind-energy production, and the expansion of residential developments. Because of its rarity, habitat loss, and ongoing habitat threats, A. purpurea var. longiseta was listed as a special concern species in Minnesota in 1984.
Aristida purpurea var. longiseta is a midheight perennial grass that grows in small tufts or bunches. The leaves are 4-16 cm (1.6-6.3 in.) long and 1-1.5 mm (0.04-0.06 in.) wide (Allred 2003). Moreover, the leaves are involute, meaning the edges of the leaves are rolled in toward each other (Van Bruggen 1985; Allred 2003), which gives them a stiff, wiry look. The flowering culms are 10-40 cm (3.9-15.7 in.) high, with the ascendingly branched, narrow panicles 5-15 cm (2.0-5.9 in.) long (Allred 2003). The seeds are borne in single-flowered spikelets at the ends of panicle branches. Each seed is enclosed in a conspicuous pair of translucent glumes, with the second (upper) glume, at 14-25 mm (0.55-0.98 in.) twice as long as the first (lower) glume, at 8-12 mm (0.31-0.47 in.). The lemma, which is fused to the seed, bears 3 long, filamentous awns at its tip. These are nearly equal in length, typically 4-14 cm (1.6-5.5 in.) (Allred 2003). Before ripening, the awns are soft and flexuous and extend from the seed in alignment with its long axis. They have a reddish tinge and glint in the sunlight, imparting to the panicles a resemblance to wisps of fine, coppery hair or feathers. When the seed is ripe, the awns turn pale, stiffen, and bend out sharply from the seed tip at a steep angle to the axis of the seed and 120° to each other (like 3 equally spaced spokes of a wheel). At this stage the panicles suggest tangles of very long-legged, delicate spiders.
There are several varieties of Aristida purpurea, but A. purpurea var. longiseta is the only variety known from Minnesota. It has the most robust lemmas and awns of any of the varieties. Other species in the genus Aristida that occur in Minnesota have much shorter awns (< 4 cm (1.6 in.)), are much smaller-statured, and are all annuals.
Aristida purpurea var. longiseta grows in dry and dry-mesic prairies in western Minnesota, and it is represented in both the northern and southern types of these communities. These habitats have well-drained soils and are dominated by grasses. Aristida purpurea var. longiseta has been found on fine-textured soils over glacial till, and on coarse-textured soil over sand and gravel deposits. Specific soil types include clay loams, sandy loams, and sandy or gravelly soil. The species grows on crests of ridges, mid to upper hill slopes, steep side slopes, and occasionally on gentle slopes. It is most often found on south and west-facing slopes, but can be found on any slope aspect. It may also be found in areas apparently degraded by erosion from past grazing.
Prairie plants growing in association with A. purpurea var. longiseta include Bouteloua curtipendula var. curtipendula (side-oats grama), Schizachyrium scoparium var. scoparium (little bluestem), Hesperostipa spartea (porcupine grass), Symphyotrichum sericeum (silky aster), and Liatris punctata var. punctata (dotted blazing star). It also sometimes co-occurs with Dalea candida var. oligophylla (western white prairie clover), another special concern species.
Biology / Life History
Aristida purpurea var. longiseta is a drought tolerant grass, and it germinates at high temperatures (40°C) (Evans and Tisdale 1972). Its roots elongate rapidly during periods of favorable soil moisture. It needs well-drained soils and is not able to grow well in saturated or moist soils. The seeds of Aristida purpurea var. longiseta have a sharp, hard base called a callus. This enables the seed to burrow into hard soils (Evans and Tisdale 1972). The awns and sharp callus also aid in seed dispersal, by wind to some extent but perhaps primarily by animal transport. The awns allow the wind to catch and move the seeds along, and the awns together with the callus readily entangle the seeds in the fur of animals.
According to Sevidec and Barker's (1997) description of the phenology of A. purpurea var. longiseta in North Dakota and Minnesota, growth begins in early May, flowering occurs in late July, and seed setting continues through September. Studies of the species in Idaho found it flowered in late June (Evans and Tisdale 1972).
Conservation / Management
The remnant habitats for A. purpurea var. longiseta are often in hilly, grazed landscapes (either currently grazed or grazed in the past). Given this setting, the species faces several threats. Overgrazing degrades prairie vegetation, causing the decline or disappearance of many of the less tolerant native species, and it facilitates erosion that degrades the habitat. However, because it is unpalatable to domestic livestock, A. purpurea var. longiseta has been shown to increase under heavy grazing (Evans and Tisdale 1972). Also, the species is able to grow in areas of thin, eroded soils. Nonetheless, there are other threats. Many remnant prairies and prairie pastures are 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. Sand and gravel mining is another threat. Sand and gravel deposits present in the moraines and outwash deposits are in high demand for use in concrete. Mining the gravel substrate destroys the prairie habitat and the plant populations that are present.
Prescribed fire is recommended as a management tool to reduce the dominance of non-native, cool-season grasses. Aristida purpurea var. longiseta is a warm-season grass and is not harmed by early spring or fall burns. Fire can also be used to target certain invasive species, and if woody cover is increasing at the site, it can keep invading shrubs and trees in check. Targeted use of herbicide may be necessary to control some exotic species. Several programs and resources are available to land managers and landowners 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 Search
The best time to search for A. purpurea var. longiseta is July through September when it is flowering and fruiting.
Conservation Efforts in Minnesota
Only four populations of A. purpurea var. longiseta are protected on lands set aside for conservation. 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 conservation efforts have been undertaken to specifically manage for this species within these areas or elsewhere.
Allred, K. W. 2003. Aristida. Pages 315-342 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 25. Oxford University Press, New York.
Evans, G. R., and E. W. Tisdale. 1972. Ecological characteristics of Aristida longiseta and Agropyron spicatum in west-central Idaho. Ecology 53:137-142.
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.
Sevidec, K. K., and W. T. Barker. 1997. Selected North Dakota and Minnesota range plants. North Dakota State University Extension Service, North Dakota State University, Fargo, North Dakota. 270 pp.
Van Bruggen, T. 1985. The vascular plants of South Dakota. Second Edition. Iowa State University Press, Ames, Iowa. 476 pp.