Avenula hookeri (Scribn.) Holub
Click to enlarge
Avena hookeri, Helictotrichon hookeri
Basis for Listing
In the northern Great Plains Avenula hookeri typically occurs in prairie habitats, but in the northern Rocky Mountains it also occurs in subalpine meadows and montane forest openings (Tucker 2007). The species just barely reaches Minnesota at the eastern edge of its geographic range, where it occurs only in dry, sandy or gravelly prairies. Suitable habitats for A. hookeri were probably in short supply even before settlers plowed the prairies, and the situation has only gotten worse. At this point in history, very little true prairie habitat remains in Minnesota, and much of what does remain is in a biologically impoverished condition or threatened by activities such as gravel mining and overgrazing of livestock. Years of abuse and neglect have cost us much of the species diversity and ecosystem services that prairies originally had to offer. One of the many species to have suffered a decline is A. hookeri, which was listed as a special concern species in Minnesota in 1996.
Avenula hookeri is a perennial, mid-height grass with smooth, erect stems that may reach a height of 75 cm (2.5 ft.), though it is typically much shorter, around 30 cm (1 ft.) tall. It tends to form clumps rather than a sod. The leaves are flat or longitudinally folded, 4-20 cm (1.6-7.9 in.) long, 1-4.5 mm (0.04-0.18 in.) wide, and have an acute, membranous ligule 3-7 mm (0.12-0.28 in.) long. The leaf blade tips are "boat-shaped" and the margins of the blades are whitish. The inflorescence is an erect or ascending panicle 6-11 cm (2.4-4.3 in.) long and 0.8-2.5 cm (0.31-0.98 in.) wide. Each branch of the panicle has 1 or 2 spikelets that are 12-16 mm (0.47-0.63 in.) long, and each spikelet has 3-6 flowers. The glumes are 9-14 mm (0.35-0.55 in.) long with 3-5 veins, and they have an acute tip. The callus at the base of the glume is a cluster of hairs less than 1 mm (0.04 in.) long. The lemmas are 10-12 mm (0.39-0.47 in.) long, shorter than the adjacent glumes, and have a twisted awn 10-17 mm (0.39-0.67 in.) long that projects out beyond the spikelet. The paleas are 6-8.75 mm (0.24-0.34 in.) long and have a shallowly toothed tip.
Avenula hookeri occurs in dry prairies on morainic hills, bluffs, and beach ridges in the northwestern part of the state, specifically in communities classified as northern dry prairies (Minnesota Department of Natural Resources 2005). Soils are well-drained, sandy, sandy loam, or gravelly. Dominant species are mid-height and short grasses such as Schizachyrium scoparium var. scoparium (little bluestem) and Hesperostipa spartea (porcupine grass), and a variety of forbs such as Liatris punctata var. punctata (dotted blazing star) and Symphyotrichum sericeum (silky aster).
Biology / Life History
Avenula hookeri is a cool-season species that blooms late spring/early summer and becomes relatively inconspicuous during the hot, dry part of summer (mid-July through August), but puts up a conspicuous tuft of bright green new blades in late summer.
Conservation / Management
As with most prairie species, the survival and perpetuation of A. hookeri is dependent on protection and effective management of its habitat. Habitat loss and degradation comes from many sources, but perhaps the most common sources are gravel mining and overgrazing by domestic livestock. The latter often leads to an increase in non-native weed species. The cause of this problem is easily remedied by reducing or eliminating grazing, but the non-native species can be a problem that persists for years or decades. The tenacious persistence of weeds highlights the need to keep intact, diverse prairie communities free of invasive species from the beginning. Encroaching woody plant species, even native woody species, can also become a problem if they become overabundant, and are best controlled by dormant season spring burning. If burning is consistently done during the dormant season, it should not harm A. hookeri.
Conservation Efforts in Minnesota
As of Spring 2010, there were approximately 22 documented records of A. hookeri in Minnesota, though eight populations have not been seen since the 1960s or earlier. Seven of the 22 records occur (or occurred) on public lands including one State Park, three Scientific and Natural Areas, and two Wildlife Management Areas. However, the effects of management, or lack of management, on these populations is not currently being monitored.
Bailey, A. W. 1978. Use of fire to manage grasslands of the Great Plains: Northern Great Plains and adjacent forests. Pages 691-693 in D. N. Hyder, editor. Proceedings of the first international rangeland congress, Denver, Colorado.
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.
Tucker, G. C. 2007. Avenula. Pages 698-699 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 24. Oxford University Press, New York, New York.