To assess the potential role of biological soil crusts and native perennial grasses in the establishment and resource acquisition of three invasive species of Centaurea in the Columbia Basin Ecoregion, I conducted two different multi-year, multi-site, field-based investigations. In one study, C. maculosa, C. diffusa, and C. solstitialis were experimentally invaded into nine habitat types across three disturbance regimes. In another study, plot-level species removals allowed inference of the competitive effects of native bunchgrasses on Centaurea species and of Centaurea species on the native bunchgrasses. Recruitment of Centaurea species was approximately 1-2% in non-disturbed (control) plots compared to 5-10% in the disturbance treatments. Regionally, within the disturbance treatments, there were no overall differences in establishment between plots where the soil crust was removed and plots where the plants and the soil crusts were removed although species and treatment differences did occur at some sites. Rates of establishment for all three Centaurea species were positively correlated with precipitation across years and sites. Experimental removal of native bunchgrasses did not result in elevated leaf water potential of Centaurea species relative to control plots (grasses and Centaurea individuals present) at any sites for any of the Centaurea species tested. Midday leaf water potentials for Centaurea species decreased from -1.0 MPa in May to -2.5 MPa in July regardless of native bunchgrass presence. Midday leaf water potentials for native bunchgrasses decreased from -1.5 MPa in May to approximately -4.0 MPa in July. Native bunchgrasses maintained greater leaf water potentials in plots where Centaurea individuals had been removed compared to bunchgrasses in control plots early in the season (0.16 MPa mean difference, p = 0.007) and late in the growing season (0.70 MPa mean difference, p < 0.0001). In a greenhouse study, Festuca idahoensis individuals grown with Centaurea maculosa produced 0.54 (+/- 0.1) grams of root mass in the 10-20 cm depth range compared to 1.02 (+/- 0.2) grams of root mass at the same depths, for Festuca idahoensis individuals grown with a conspecific. These data suggest that Centaurea maculosa achieves its competitive effect through negatively affecting the rooting depth of Festuca idahoensis individuals
