WHAT MAKES GREAT BASIN SAGEBRUSH ECOSYSTEMS INVASIBLE BY BROMUS TECTORUM?

Authors: CHAMBERS, JEANNE - USDA-FOREST SERVICE; ROUNDY, BRUCE - BYU; Blank, Robert - Bob; MEYER, SUSAN - USDA-FOREST SERVICE; WHITTAKER, ALLISON - UTAH DIV OF WILDLIFE RES

Submitted to: Ecological Monographs
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2005
Publication Date: 2/15/2007
Citation: Chambers, J.C., Roundy, B.A., Blank, R.R., Meyer, S.E., Whittaker, A. 2007. What makes Great Basin sagebrush ecosystems invasible by bromus tectorum? Ecological Monographs. 77:117-145.

Interpretive Summary: Factors that influence ecosystem susceptibility to invasion by nonnative species are poorly understood. We examined the effect of herbaceous vegetation removal and prescribed burning on cheatgrass establishment over a two-year period in two sagebrush-dominated ecosystems in the Great Basin. The invasibility of sagebrush ecosystems by cheatgrass varies across elevational gradients and is most closely related to temperature at higher elevations and soil water availability at lower elevations.

Technical Abstract: The factors that influence ecosystem susceptibility to invasion by nonnative species are poorly understood, but there is increasing evidence that spatial and temporal variability in resource availability can have both large-scale and long-term effects on invasion processes. We conducted a study in Artemisia tridentata dominated ecosystems at two locations in the Great Basin that examined differences in resource availability (soil water and nitrate) over elevation gradients and in response to experimental treatments including removal of perennial herbaceous competition and fire. We evaluated differences in invasibility of Bromus tectorum over the elevation gradients and in response to direct and interacting effects of removal and fire. We monitored environmental conditions, soil variables, and B. tectorum establishment and reproduction over a 2-year period. Soil water availability and soil nitrate availability (measured as days when soil matric potential was > -1.5 MPa and µmoles of NO3- sorbed to resin capsules per day-in-the-ground in interspace microsites) decreased over the topographic gradient from high to low elevation sites. Lower elevation sites had greater annual variability in soil water availability than upper elevation sites. Soil nitrate levels were highest at all elevations when soils were the wettest - early in the growing season (Oct-April sample period) and during a wet year. Nitrate availability was not more variable on low than high elevation sites. High variability in available soil water coupled with lower average perennial herbaceous cover at low elevations may result in increased potential for invasion when available soil water reaches a certain threshold. Removal of perennial herbaceous vegetation generally increased days of available water at 13-15 cm and 28-30 cm depths irrespective of burning. Burning without removal had marginally significant effects on soil water and then only in the first year after treatment. The removal and burn treatments had similar effects on soil nitrate. Burning resulted in mortality of target shrubs, but perennial herbaceous vegetation increased in cover on plots without removal. Disturbances that remove perennial herbaceous vegetation appear to have greater and longer-lasting effects on soil water and nitrate in the upper soil profile than disturbances like fire that only remove shrubs if perennial herbaceous species are a substantial component of the system. Bromus tectorum had the lowest overall establishment, biomass and seed production on high elevation, A. tridentata sites in both locations and both years. High elevation sites (2380 and 2274 m) had the highest days of available soil water and among the highest levels of nitrate, but low degree days indicating colder and shorter growing seasons. Establishment and production of B. tectorum on mid elevation sites (2190 and 2085 m) also depended on growing season indicating that ecophysiological limitations due to cold temperatures can restrict its growth and reproduction and that the elevation at which these limitations become apparent differs among years. Establishment, biomass and seed production were variable on low elevation sites and best explained by soil characteristics of the sites and spatial and temporal variation in soil water. Invasion potential of B. tectorum increased significantly with herbaceous species removal and fire. The magnitude of the response varied among sites and between years, but the direction was remarkably consistent. Removal and fire had relatively minor effects on emergence and survival, but effects on biomass and seed production were both highly significant and additive. Biomass and seed production increased 2 to 3 times following removal, 2 to 6 times after burning, and 10 to 30 times following removal and burning. Our data provide evidence that invasibility of A. tri