The herbicide imazapic is registered for use on rangelands and provides effective short-term control of certain invasive annual grasses. However, details about optimal application rates for downy brome and susceptibility of simultaneously seeded species are lacking. Thus, we investigated downy brome and seeded species responses to variable rates of imazapic (0, 35, 70, 105, and 140 g ai/ha) in two plant communities (salt desert shrub and Wyoming big sagebrush). In autumn 2003, plots were treated with imazapic and seeded with one of five perennial plant materials (Siberian wheatgrass [‘Vavilov’ and the experimental source Kazak]; prostrate kochia [‘Immigrant’ and the experimental source 6X], and Russian wildrye [‘Bozoisky II’]). Downy brome cover and seeded species establishment were evaluated in spring 2004 and 2006. Downy brome cover in 2004 decreased with increasing imazapic rate at both sites, although more so at the Wyoming big sagebrush site. In 2006, no difference in downy brome cover existed among herbicide rates at the Wyoming big sagebrush site. At the salt desert shrub site, the high rate of imazapic reduced downy brome cover by about 25% compared to untreated plots. ‘Vavilov’ Siberian wheatgrass was the only seeded species with lower downy brome cover in 2006 than 2004. Seeded species establishment increased with imazapic rate in the salt desert shrub community, but in the Wyoming big sagebrush community it peaked at intermediate rates and declined at higher rates. Variation in downy brome control and seeded species establishment might have been associated with differences in precipitation, soil organic matter, and disturbance history between sites. Overall, imazapic was useful for helping establish desirable perennial species, but unless downy brome is reduced below a critical threshold, favorable precipitation can return sites to pretreatment levels within two years.

No abstract

Allelopathic interference between plants has generally been discussed in terms of the production of toxic complex biochemicals; however, complex biochemicals may not be the only substances plants use to interfere with one another. It has also been suggested that inorganic elements may be used in an allelopathic manner. If, through phytoenrichment or root exudates, a plant is able to increase the bioavailable levels of a particular element and tolerate the levels better than its neighbors, it can produce an allelopathic effect. Elemental allelopathy has been implicated as the cause for the success of a number of invasive weeds, including Acroptilon repens, Tamarix spp., Halogeton glomeratus, Salsola iberica, and Mesambryenthemum crystallinum. Phytoenrichment of elements can occur through hyperaccumulation and litter deposition and by altering rhizosphere chemistry. Reported cases of elemental allelopathy have involved three types of elements: heavy metals and soluble salts in terrestrial systems and elemental S in aquatic systems. For the most part, studies that have reported elemental allelopathy have been inconclusive. In order to prevent overreaching conclusions in the study of biochemical allelopathy, criteria were set that can be adapted to the study of elemental allelopathy. Of the studies reviewed, the most common criteria left uninvestigated were whether the plant was actually responsible for changing the concentration of the element and whether the increased levels of an element negatively affected other species. If the study of elemental allelopathy is to avoid the same problems often associated with the study of biochemical allelopathy, these criteria should be included in investigations of elemental allelopathy.

Downy brome has converted millions of hectares of Great Basin rangelands from shrubland to annual grass-dominated systems. Methods for removing downy brome from sites that already have perennial grasses established are especially needed because of the difficulty in re-establishing perennial species. In this study, early spring applications of glyphosate alone and glyphosate plus imazapic were monitored for 2 yr. Cover of downy brome was reduced from 45 to 10% by spring application of glyphosate alone and to approximately 1% with the addition of imazapic. Perennial grass cover was not affected by the spring application of glyphosate. The addition of imazapic reduced perennial grass cover in the first year; however, it recovered by the second year. The selective response to glyphosate may be due to differences in growth stage or dormancy characteristics between annual and perennial species. Our findings suggest spring application of glyphosate may provide an alternative approach for managing annual grasses on Great Basin rangelands.

Although Acroptilon repens (L.) DC. (Russian knapweed) is known to concentrate zinc (Zn) in upper soil layers, the question of whether the elevated Zn has an allelopathic effect on restoration species has not been addressed.