Ecological restoration of shrub–steppe communities in the western United States is often hampered by invasion of exotic annual grasses during the process. An important question is how to create restored communities that can better resist reinvasion by these weeds. One hypothesis is that communities comprised of species that are functionally similar to the invader will best resist invasion, while an alternative hypothesis is that structurally more complex and diverse communities will result in more effective competitive exclusion. In this field experiment, we examined the effects of restored community structure on the invasion success of three annual grass weeds (downy brome, jointed goatgrass, and cereal rye). We created replicated community plots that varied in species composition, structural complexity and density, then seeded in annual grass weeds and measured their biomass and seed production the following year, and their cover after 1 and 3 yr. Annual grass weeds were not strongly suppressed by any of the restored communities, indicating that it was difficult for native species to completely capture available resources and exclude annual grass weeds in the first years after planting. Perennial grass monocultures, particularly of the early seral grass bottlebrush squirreltail, were the most highly invaded communities, while structurally complex and diverse mixtures of shrubs (big sagebrush, rubber rabbitbrush), perennial grasses (bluebunch wheatgrass and bottlebrush squirreltail) and forbs (Lewis flax, Utah sweetvetch, hairy golden aster, gooseberryleaf globemallow) were more resistant to invasion. These results suggest that restoration of sagebrush steppe communities resistant to annual grass invasion benefits from higher species diversity; significant reduction of weed propagule pressure prior to restoration may be required.

Nomenclature: Cereal rye, Secale cereale L.; downy brome, Bromus tectorum L.; jointed goatgrass, Aegilops cylindrica Host; big sagebrush, Artemisia tridentata Nutt.; bluebunch wheatgrass, Pseudoroegneria spicata (Pursh) Á. Löve; bottlebrush squirreltail, Elymus elymoides (Raf.) Swezey; gooseberryleaf globemallow, Sphaeralcea grossulariifolia (Hook. and Arn.) Rydb.; hairy golden aster, Heterotheca villosa (Pursh) Shinners; Lewis flax, Linum lewisii Pursh; rubber rabbitbrush, Ericameria nauseosa (Pall. ex Pursh) G.L. Nesom & Baird; Utah sweetvetch, Hedysarum boreale Nutt.

Management Implications: Our study of annual grass weed invasion dynamics in a sagebrush steppe restoration experiment emphasizes the importance of combining the basic principles of ecological restoration and weed science to understand how the plant community at a particular site is likely to respond to restoration activities, and how the restoration species will interact with invasive exotic species in the area through time. Fundamental properties of a restoration site include its intrinsic productivity, as defined by climate and soils, its history of disturbance and use, and the pool of potential colonizing species, both native and exotic, that already occupy the area. Ecological theory indicates that sites with a history of disturbance are most invasible, as the absence of an intact plant community means resources are likely to be incompletely utilized, thus providing more resources for an invader. Sites subject to high propagule pressure from invasive species in the area are also at higher risk. The least invasible sites are generally those with low levels of disturbance and an absence of potential invaders. Restoration strategies will be different for sites at different points along this continuum of invasibility. Our study site was historically highly disturbed, with a history of fertilizer additions. Under this scenario the best strategy was to establish a native commu