Aristida longespica var. geniculata (Raf.) Fern.
Basis for Listing
Aristida longespica var. geniculata (slimspike three-awn) is rare or unusual in states adjacent to Minnesota, though it is not uncommon in the south-central and southeastern United States (Allred 2003). Aristida longespica var. geniculata was found for the first time in Minnesota in 1999. While it has probably been here since well before Minnesota became a state, it has been overlooked due to its extreme rarity. Despite targeted surveys of wet meadow habitats since the species’ initial discovery, to date it has been found only in Anoka County (Anoka Sand Plain Subsection) and only in high-quality remnant habitats.
Four distinct populations of this species are known, all within 3-4 miles of each other. It is likely they were all part of a single population at some time in the past, when the habitat was continuous. However, the habitat was fragmented in the 19th and 20th centuries and now exists as isolated remnants. Since 1999, the habitats have experienced even further fragmentation. In addition, A. longespica var. geniculata is an early successional species that appears to be adapted to fire and the seasonal fluctuations of groundwater. In the absence of wildfire, the species will likely require active management such as prescribed burns or brush control in order to maintain its open habitat conditions.
Given the limited amount of potential habitat for the species in the state, the documentation of only four populations despite intensive surveys, the vulnerability of its habitat to degradation or destruction, and the need for active management, Aristida longespica var. geniculata was given the status of endangered in 2013.
Aristida longespica var. geniculata is an annual grass and reaches a height of 15-65 cm (6-26 in.). The stems can be erect, spreading, or nearly prostrate and are usually much-branched. The leaves originate on the stem, rather than from the base of the plant. The leaf blades are flat, 5-14 cm (2-6 in.) long, and 1-2 mm (0.04-0.08 in.) wide. The inflorescence is slender, 6-22 cm (2.5-8.5 in.) long, and 1-4 cm (0.4-1.5 in.) wide. It usually has short erect or ascending side branches, 1-4 cm (0.4-1.5 in.) long; each side branch has 2-5 spikelets, and each spikelet consists of 1 flower. The spikelets can be widely spaced or crowded. Each spikelet has 2 glumes, 2-11 mm (0.08-0.43 in.) long, each with 1 vein and sometimes a short awn no more than 1 mm (0.04 in.) long. The lemma of the spikelet is 2.5-10 mm (0.1-0.4 in.) long and has 3 terminal awns. The awns, which give this species its common name, are usually unequal in length and may be as long as 27 mm (1.0 in.) (Allred 2003).
Aristida longespica var. geniculata is distinguished from A. longespica var. longespica by having longer awns. They are in the range of 6-27 mm vs. 1-14 mm (0.24-1.0 in. vs. 0.04-0.55 in.). It is reported that both varieties have roughly the same geographic range and often occur together (Allred 2003). So far, all the specimens seen from Minnesota have been identified as A. longespica var. geniculata.
There are several other species of Aristida occurring in Minnesota, some are common and some are rare. Telling them apart generally requires the use of a technical key or consultation with an expert.
Habitats of A. longespica var. geniculata in Minnesota have been described as wet meadow and wet prairie, occurring on saturated sandy soil. The landscape setting for these habitats is the Anoka Sandplain. This is a large flat sandy area covering portions of several counties, just north of the Twin Cities. It was formed from sand deposited by glacial meltwater near the end of the last glaciation. The vegetation on the Anoka Sandplain at the time of American settlement (ca 1850) was largely prairie and savanna, with areas of shallow wetlands (MNDNR 2005). These wetlands are sustained by a seasonally fluctuating water table, with cyclical flooding and periodic drawdowns. It is these wetlands that support the last occurrences of A. longespica var. geniculata in Minnesota.
During the last half of the 19th century and the first half of the20th century, nearly all the prairie and savanna habitats on the Sandplain were cleared for agriculture (Minnesota’s Remaining Native Prairie), and most of the wetlands were drained or filled. Most surviving fragments of native habitats have experienced degradation and loss of species due to their small size and their isolation from the ecosystem functions that sustained them.
Biology / Life History
Aristida longespica var. geniculata is an annual warm season grass. In Minnesota, it occurs in biologically diverse, graminoid-dominated communities that are considered fire-maintained. In other words, without periodic wildfire the community would succeed into a forested community of some sort and become unsuitable for A. longespica var. geniculata.
Some plant species that occur in this type of community are believed to maintain stable populations by creating a viable bank of seeds in the soil. This is especially important for annual species such as A. longespica var. geniculata, which would otherwise be vulnerable to random events, which could result in the loss of an entire cohort of seeds. It is unknown if A. longespica var. geniculata maintains a seed bank.
Conservation / Management
The greatest concern for the survival of A. longespica var. geniculata in Minnesota is maintaining the integrity of the native plant community of which it is a member. The greatest threat, other than outright destruction, is increased competition from non-native invasive species. Controlling invasive species is an ongoing task that will be made easier if soil disturbances are kept to a minimum and natural hydrological functions are maintained.
Best Time to Search
Aristida longespica var. geniculata is a late season species and does not reach peak development until August. For this reason, searches need to be conducted in August or September.
Welby R. Smith (MNDNR), 2018
(Note: all content ©MNDNR)
References and Additional Information
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.
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.
Penskar, M. R. 2004. Special plant abstract for Aristida longespica (three-awned grass). Michigan Natural Features Inventory, Lansing.
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.