Synonyms

Oryzopsis hymenoides

  Basis for Listing

Achnatherum hymenoides (Indian ricegrass) is a characteristic and common grass in arid 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 (Red River Prairie Subsection). 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, A. hymenoides was listed as an endangered species in Minnesota in 1984.

  Description

Achnatherum hymenoides is a native, perennial bunchgrass 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 branches. 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.

  Habitat

Achnatherum 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. Beyond Minnesota, A. hymenoides is known to inhabit some of the most arid regions of western North America.

  Biology / Life History

Achnatherum hymenoides is a long-lived perennial that reproduces only by seed. The flowers are wind-pollinated in early summer, and the seeds ripen and are shed in mid to late summer. The seeds are gathered from the ground by small mammals that cache the seeds for future consumption. The caches are shallowly buried at scattered locations throughout the home range of the forager. This is reported to have a beneficial effect on seed germination and seedling survival when compared to seeds that are not cached (Longland & Dimitri 2016). Ants are also known to cache the seeds of A. hymenoides but, for reasons unknown, ant-cached seeds do very poorly (Longland et al. 2001). However, it has been reported that A. hymenoides is unexpectedly prevalent near ant mounds, possibly an indirect effect of ants defoliating potential competitors while not defoliating A. hymenoides (Nowak et al. 1990).

A distinctive feature of A. hymenoides is its ability to withstand extremely dry conditions. This appears to be facilitated by rhizosheaths on the roots. A rhizosheath consists of a layer of sand grains tightly bound to the root by mucilage. How this might benefit grasses in excessively dry environments is not well understood, but it likely increases water-holding capacity and may harbor nitrogen-fixing bacteria (Bergmann et al. 2009).

  Conservation / Management

It is probable that population levels of A. hymenoides in Minnesota have declined in recent years because of an incremental loss of habitat. This is the result of incompatible 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.

  Best Time to Search

The best time to search for Achnatherum hymenoides is when the species is in flower during June or in fruit from July to mid-August.

  Conservation Efforts in Minnesota

Nearly all potential habitat of A. 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 A. hymenoides populations has not yet been assessed.

  Authors/Revisions

Welby R. Smith (MNDNR), 2020

(Note: all content ©MNDNR)

  References and Additional Information

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.

Bergmann, D., M. Zehfus, L. Zierer, B. Smith, and M. Gabel. 2009. Grass rhizosheaths: associated bacterial communities and potential for nitrogen fixation. Western North American Naturalist 69(1):105-114.

Longland, W. S., and L. A. Dimitri. 2016. Can seed caching enhance seedling survival of Indian ricegrass (Achnatherum hymenoides) through intraspecific facilitation? Plant Ecology 217:1523-1532.

Longland, W. S., S. H. Jenkins, S. B. Vander Wall, J. A. Veech, and S. Pyare. 2001. Seedling recruitment in Oryzopsis hymenoides: are desert granivores mutualists or predators? Ecology 82(11):3131-3148.

Nowak, R. S., C. L. Nowak, T. DeRocher, N. Cole, and M. A. Jones. 1990. Prevalence of Oryzopsis hymenoides near harvester ant mounds: indirect facilitation by ants. Oikos 58(2):190-198.

West, N. E., K. H. Rea, and R. O. Harniss. 1979. Plant demographic studies in sagebrush-grass communities of southeastern Idaho. Ecology 60(2):376-388.