Achnatherum hymenoides (Roemer & J.A. Schultes) Barkworth
Achnatherum hymenoides, Oryzopsis hymenoides
Basis for Listing
Oryzopsis hymenoides is a characteristic and common grass in the plains and semiarid regions of the West. However, the eastward extent of its range ends abruptly near the 100th meridian, before it reaches the tallgrass prairie region. The only exceptions are the disjunct populations in western Minnesota. A single population was discovered in 1947 in a sand dune complex in Polk County and it was relocated in 1985. A second population was located in adjacent Norman County in 1987 and is part of the same habitat complex often referred to as the Agassiz Dunes. Given its extreme rarity, O. hymenoides was listed as an endangered species in Minnesota in 1984.
A native, perennial bunchgrass, O. hymenoides is a distinctive species that can be easily identified in the field with a hand lens. The inflorescence is an open, diffuse panicle of one-flowered spikelets borne at the tips of spreading stalks. The lower bracts, which enclose the flower, have a stiff bristle at their tip and are covered with long hairs. The stem is densely tufted.
Oryzopsis hymenoides occurs in a xeric sand dune habitat in the prairie region of the state. The plants seem to prefer the margins of active blowouts where there is exposed sand and little competing vegetation.
Biology / Life History
Oryzopsis hymenoides is a long-lived perennial that reproduces only by seed. Like most grasses it is wind-pollinated, and the seeds are gravity-dispersed. Because its preferred habitat is active sand dunes, it is possible that wind aids the dispersal of seeds to some extent. It is also possible that abrasion of the seeds by the blowing sand may aid germination.
Conservation / Management
It is probable that population levels of O. hymenoides in Minnesota have declined in recent years because of an incremental loss of habitat. This is the result of land use activity and vegetation succession that proceeds from open sand dunes to closed forest. The rate of encroachment of woody cover has been accelerated by wildfire suppression, which previously kept the dunes free of trees. Population levels could probably recover if appropriate habitat was restored. Other threats to the species' habitat are gravel and sand mining, agricultural production, overgrazing, and off-highway vehicle traffic (West et al. 1979). It is important that a careful census be conducted to determine the full extent of the Minnesota population and to establish management goals.
Conservation Efforts in Minnesota
Nearly all potential habitat of O. hymenoides in Minnesota has been surveyed, at least cursorily, and it is unlikely that many, if any, additional populations will be found. A significant portion of this species' habitat is in public ownership and being managed for conservation purposes. However, the effect this management is having on the O. hymenoides populations has not yet been assessed.
Barkworth, M. E. 2007. Achnatherum. Pages 114-142 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 24. Oxford University Press, New York, New York.
Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence, Kansas. 1,402 pp.
West, N. E., K. H. Rea, and R. O. Harness. 1979. Plant demographic studies in sagebrush grass communities of southeastern Idaho. Ecology 60(2):376-378.