Invasion and dominance of weedy species is facilitated or constrained by environmental and ecological factors that affect resource availability during critical life stages. We compared the relative effects of season, annual weather, site, and disturbance on potential cheatgrass (Bromus tectorum L.) germination in big sagebrush (Artemisia tridentata Nutt.) communities. Soil water status and temperature in the seedbed were measured continuously for 4 years on 9 big sagebrush sites in Nevada and Utah. Field plots at lower-, middle-, and upper-elevation sites were either undisturbed, or were burned, sprayed with herbicide, or both sprayed and burned. Spraying removed perennial herbaceous vegetation, whereas burning removed sagebrush. We used thermal-germination data from laboratory incubation studies of 18 cheatgrass seedlots and field soil moisture and temperature measurements to model and predict potential germination in the field plots for periods when seedbeds were continuously wet (above −0.5, −1, or −1.5 MPa) and across intermittent wet and dry periods. Season had the greatest effect on potential cheatgrass germination, followed by annual weather, and site variables (elevation and location); the effects of disturbance were minimal. Potential germination was predicted for most sites and years in spring, a majority of sites and years in fall, and few sites or years in winter. Even though disturbance has limited effects on potential germination, it can increase cheatgrass invasion and dominance by reducing perennial herbaceous species resource use and allowing increased cheatgrass growth and reproduction.
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Vol. 60 • No. 6