• Rocky Mountain Research Station
• 240 West Prospect
• Fort Collins, CO 80526
• (970) 498-1100

Understanding Invasion of the Exotic Annual, Cheatgrass

Both the environmental and ecological factors that influence community susceptibility to invasive species are largely unknown. To date, the focus of both research and management has been on specific methods for controlling these species following invasion. Consequently, we still know very little about which communities are most susceptible to invasion or about how fire influences the invasion process. Increasing our understanding of the environmental and ecological factors that determine invasibility will allow us to develop management techniques, including prescribed fire, aimed at preventing initial invasion or expansion. From a regional perspective, this approach may be as or more effective than management schemes designed to reduce or eliminate established populations of invasive species.

To increase our understanding of the factors that increase the susceptibility of sagebrush (Artemisia tridentata) ecosystems to invasion by cheatgrass (Bromus tectorum), we conducted a study in sagebrush ecosystems at two locations in the Great Basin. Because cheatgrass often increases in response to elevated resources following disturbance, we examined differences in resource availability and cheatgrass invasibility over elevation gradients and in response to direct and interacting effects of removal of perennial herbaceous vegetation and fire. We monitored environmental conditions, soil variables, and cheatgrass establishment and reproduction over 2-yrs.

We found that soil water and nitrate availability decreased with decreasing elevation. Lower elevation sites had lower and more variable soil water availability than upper elevation sites. Soil nitrate levels were highest at all elevations when soils were wettest, but nitrate availability was not more variable at lower elevations. Removal of herbaceous perennials increased both soil water and nitrate availability, but burning without removal had only minor effects on these resources. Cheatgrass had low establishment, biomass and seed production on high elevation sites and on a mid elevation site during a cold, short growing season. Although our data show that cheatgrass has the potential to germinate at these cold temperatures, it appears that there are ecophysiological limitations on cheatgrass growth resulting from cold temperatures. Cheatgrass establishment, biomass and seed production were variable at low elevations and were best explained by soil characteristics and spatial and temporal variation in soil water. Plant removal and fire alone had only minor effects on emergence and survival. However, biomass and seed production increased 2 to 3 times following removal, 2 to 6 times after burning, and as much as 10 to 30 times following removal and burning.

Our study indicates that invasibility of cheatgrass varies across elevation gradients and appears closely related to temperature at higher elevations and soil water availability at lower elevations. High variability in soil water and lower average perennial herbaceous cover may increase invasion potential at lower elevations. Soil water and nitrate availability increase following either fire or removal, but on intact sites native perennials typically increase following fire limiting cheatgrass growth and reproduction. Following resource fluctuations, invasibility is lowest on sites with relatively high cover of perennial herbaceous species, i.e., sites in high ecological condition.

References

"Field germination potential of cheatgrass in relation to disturbance and elevation." Rangeland Ecology and Management; and

"What makes Great Basin sagebrush ecosystems invasible by Bromus tectorum?" Ecological Monographs.