Physiognomy and Structure: This alliance is characterized by sparse to dense annual grasslands with low cover of shrub or perennial graminoids or forbs (<5% absolute cover).

Floristics: This interior western U.S. ruderal annual grassland alliance is strongly dominated (>90% relative canopy cover) by invasive, exotic annual grass species such as Bromus tectorum, and less commonly Bromus arvensis (= Bromus japonicus), Bromus hordeaceus, Bromus madritensis, Taeniatherum caput-medusae, or a mixture of other exotic annual grasses. Cover of perennials is low (<5% absolute cover).

Dynamics:  Bromus tectorum is an annual grass and is able to complete its lifecycle in the spring before drying out mid-summer. Its fine structure makes it extremely flammable when dry, and it will increase the fire frequency of a site (FEIS 2001). Frequent fires favor Bromus tectorum because they eliminate competing perennial vegetation and increase soil nitrogen, but do not kill all the Bromus tectorum seeds, which survive in the unburned organic material (FEIS 2001). This altered ecological process has promoted the spread of Bromus tectorum and other exotic annual bromes at the expense of sagebrush shrublands in large parts of the western U.S. (Young and Evans 1973, 1978, Daubenmire 1975).

T. Naumann (pers. comm. 2005) reported successful restoration of cheatgrass-invaded systems by the use of prescribed fire, timed and controlled so as to destroy the seeds of Bromus tectorum while stimulating growth in remnant native warm-season grasses. She also reported that prescribed fire was least successful in areas of shallow soils, presumably because native grasses cannot develop sufficient root mass to compete with cheatgrass. Work by Redente and others (e.g., Redente et al. 1992) indicates that, under some circumstances, native grass and shrub species can regain competitive advantage over annuals such as Bromus tectorum if a source of carbon, such as sugar or sawdust, is added to the system. Amending the soil with carbon increases the activity of soil microbes and results in the reduction of plant-available nitrogen.

This type is most common where disturbances have eliminated or largely set back the native vegetation. Where the brome grasses are invading native vegetation, the types may still be tracked as native types, since the native species may still persist. A recent study (Karl et al. 1999) found that, despite strong seed and seedling production by the exotic brome grasses (Bromus arvensis, Bromus tectorum), the large amount of herbaceous biomass produced by the two vegetatively propagating native grasses Bouteloua gracilis and Pascopyrum smithii suggests that these native grasses may well maintain their ecological importance in the stands.

Evans et al. (2001) studied the invasion by cheatgrass of an undisturbed native grassland in Canyonlands National Park (Virginia Park). Their study showed that Bromus may cause a short-term decrease in nitrogen loss by decreasing substrate availability and denitrification enzyme activity, but in the long term, nitrogen loss is likely to be greater in invaded sites because of increased fire frequency and greater nitrogen volatilization during fire. A study by Englund (2004) at the same site showed decreasing levels of soil organic carbon as Bromus tectorum, with its shallow root systems, replaced perennial grasses with their more massive root systems.

In Nevada, Beatley (1976) found dense stands of the introduced winter annual grass Bromus tectorum growing in disturbed Artemisia shrublands. Bromus rubens is more common in lower elevation sites and Bromus tectorum is most common in higher elevation sagebrush and pinyon-juniper communities.