Aristida tuberculosa Nutt.
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Basis for Listing
Aristida tuberculosa (seaside three-awn) is a species of rather limited distribution. A portion of the distribution is along the Atlantic coastal plain, where it acquired its common name; another portion is in the upper Mississippi River Valley, primarily in Minnesota, Wisconsin, and Illinois. In Minnesota, A. tuberculosa occurs in a relatively small number of very small and isolated prairie and savanna habitats in the southeastern part of the state (Eastern Broadleaf Forest Province). These habitats are fragile and easily converted to agricultural, commercial, recreational, or residential uses. More than 99% of the prairie and savanna habitat that was present in the state before settlement has already been destroyed or degraded (Minnesota's Remaining Native Prairie), so any further habitat loss or degradation seriously jeopardizes the viability of A. tuberculosa in Minnesota. Habitat restoration is very difficult and expensive and has not been undertaken on a significant scale. For these reasons, Aristida tuberculosa was listed as special concern in Minnesota in 1984. Subsequently, trends were perceived as worsening, and it was listed as threatened in 2013.
Aristida tuberculosa is a small tufted grass, averaging about 30 cm (12 in.) tall, with lateral branches arising from the lower portion of the stem. The leaf-sheaths are smooth on the surfaces, though the margins may be hairy. The ligule is a fringe of hairs. The leaf-blades are inrolled (involute), 10-20 cm (4-8 in.) long, 1-3 mm (0.04-0.12 in.) wide, and rough (scabrous) to the touch. The inflorescence is an open panicle, 10-20 cm (4-8 in.) long, with ascending branches. The spikelets are solitary and contain a single floret, 6-12 mm (0.24-0.47 in.) long.
There are six species of Aristida in Minnesota. All are characterized by an awn with three forks, or barbs, which project from the end of the flower; hence their common name, "triple-awned grasses". Aristida tuberculosa is easily distinguished from the five other species because the base of the three forks is twisted into a single column.
In Minnesota, A. tuberculosa occurs exclusively in dry and loose sand in sand savannas, sand prairies, and dunes, where vegetation is sparse. The plants typically grow in full sunlight, though there may be scattered oak trees or oak groves in the vicinity, especially Quercus macrocarpa (bur oak), Q. ellipsoidalis (northern pin oak), or Q. velutina (black oak).
Biology / Life History
Aristida tuberculosa is a wind-pollinated annual, with a need for open and sparsely vegetated habitats, where there is dry and shifting sand. The bent awns seem to be an adaptation for catching in the fur of mammals, thereby dispersing the seeds. However, it also seems likely that wind is involved in the process of dispersion. It has been reported that the awns have hygroscopic properties that allow the seeds to bury themselves; seeds that do not get buried usually do not germinate or, if they do germinate, the resulting seedlings fail to become established (Collins and Wein 1997).
Conservation / Management
Native sand savanna habitats typically have a few scattered Quercus spp. (oaks) or Populus tremuloides (trembling aspen) trees and scattered shrubs, such as Corylus americana (American hazel). Sometimes these trees and shrubs are in small groves, or they may occur as isolated individuals. Either way, most of the habitat will be open and free of woody vegetation, thereby allowing the wind to create dunes. Over time, these dunes become bowl shaped, with crests and hollows. The hollows, or "blowouts", are the specific habitat where A. tuberculosa occurs. It is essential that these dunes be maintained in this natural and "active" condition if A. tuberculosa is to survive. Under normal conditions, the vegetation in this habitat type is maintained by wildfire and perhaps periodic drought; grazing by bison and elk were probably important at some time in the past. These processes keep the dunes from becoming overgrown by woody vegetation or a dense thatch of herbaceous vegetation, which would eliminate habitat for this rare species. Well-intentioned individuals will sometimes plant non-native pine trees in this sort of habitat, thinking they are promoting "conservation". Unfortunately, this has the opposite effect and should be avoided at all costs. Where trees have been planted in the past, they should be carefully removed. The dune habitats should also be protected from off-highway vehicles (OHVs), which easily damage the fragile soils, giving invasive species an opportunity to get a foothold.
Best Time to Search
The best time to search for A. tuberculosa is when mature reproductive structures are present, from early July through October.
Conservation Efforts in Minnesota
Several known populations of A. tuberculosa occur on public lands that are managed for conservation or recreation purposes. In most cases (not all), the management activities are compatible or even complimentary with the goals of preserving this rare species.
Welby Smith, MN DNR, 1988, 2008, and 2018
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.
Coffin, B., and L. Pfannmuller, editors. 1988. Minnesota's endangered flora and fauna. University of Minnesota Press, Minneapolis. 473 pp.
Collins, B. S., and G. R. Wein. 1997. Mass allocation and self-burial of Aristida tuberculosa florets. Journal of the Torrey Botanical Society 124:306-311.
Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the eastern broadleaf forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 394 pp.
Minnesota Department of Natural Resources. 2006. Tomorrow's habitat for the wild and rare: An action plan for Minnesota wildlife, comprehensive wildlife conservation strategy. Division of Ecological Services, Minnesota Department of Natural Resources. 297 pp. + appendices.
Minnesota Department of Natural Resources. 2009. Map of Minnesota's remaining native prairie 100 years after the public land survey.
Minnesota Department of Natural Resources, Division of Ecological Resources. 2008. Rare species guide: an online encyclopedia of Minnesota's rare native plants and animals [Web Application]. Minnesota Department of Natural Resources, St. Paul, Minnesota. Accessed 1 July 2009.
Wovcha, D. S., B. C. Delaney, and G. E. Nordquist. 1995. Minnesota's St. Croix River Valley and Anoka Sandplain:a guide to native habitats. University of Minnesota Press, Minneapolis. 248 pp.