1 In the Great Basin of the western United States of America, the invasive annual grass Bromus tectorum has extensively replaced native shrub and bunchgrass communities, but the native bunchgrass Elymus elymoides has been reported to suppress Bromus . Curlew Valley, a site in Northern Utah, provides a model community to test the effects of particular species on invasion by examining competitive relationships among Elymus , Bromus and the native shrub Artemisia tridentata . 2 The site contains Bromus /Elymus , Elymus /Artemisia and monodominant Elymus stands. Transect data indicate that Elymus suppresses Bromus disproportionately relative to its above-ground cover. Artemisia seedlings recruit in Elymus stands but rarely in the presence of Bromus . This relationship might be explained by competition between the two grasses involving a different resource or occurring in a different season to that between each grass and Artemisia . 3 Time reflectometry data collected in monodominant patches indicated that in spring, soil moisture use by Bromus is rapid, whereas depletion under Elymus and Artemisia is more moderate. Artemisia seedlings may therefore encounter a similar moisture environment in monodominant or mixed perennial stands. However, efficient autumn soil moisture use by Elymus may help suppress Bromus . 4 In competition plots, target Artemisia grown with Bromus were stunted relative to those grown with Elymus , despite equivalent above-ground biomass of the two grasses. Competition for nitrogen in spring and autumn, assessed with (15) N tracer, appears to be secondary to moisture availability in determining competitive outcomes. 5 Elymus physiology and function appear to play an important role in determining the composition of communities in Curlew Valley, by maintaining zones free of Bromus where Artemisia can recruit.
