Inter-perennial gaps may indicate invasibility of sagebrush systems

Background/Question/Methods

Land Managers throughout the Intermountain West are acutely aware of the growing problem of Bromus tectorum invasion into sagebrush steppe ecosystems. Both scientists and managers are searching for ways to combat this problem, and part of the solution is to understand how sites respond to disturbance (whether intended or unintended).  SageSTEP is a regional experiment designed to evaluate different methods to maintain sagebrush habitat while reducing fire fuel loads in the Great Basin.  Fuel treatments being evaluated at the whole-plot level are prescribed fire, mechanical thinning of sagebrush (Artemisia tridentata ssp. wyomingensis) by mowing, and aerial application of the herbicide tebuthiuron (Spike 20P®) to thin sagebrush. Additionally, the pre-emergent herbicide imazapic (Plateau®) was applied to subplots within fuels treatments to reduce cheatgrass.  SageSTEP researchers are examining these treatments and their influence on gap size between perennial plants as an indicator of a sagebrush ecosystem’s resilience, or a site’s ability to recover from disturbance versus converting to cheatgrass.  

Results/Conclusions

Preliminary results include short term (3-years) post-treatment effects.  Prescribed fire reduced perennial tall grass cover immediately post fire (7% to 4%), but cover recovered quickly.  Prescribed fire also increased inter-perennial gaps from a mean of 230 cm to 375 cm.  The application of imazapic also contributed to increasing gap sizes and bare ground by decreasing perennial short grass cover (4% to 3%) and perennial forbs cover (1.6% to 1.3%), while also reducing cheatgrass cover (7% to 1%) and annual forb cover (3% to 1%).  While these reductions from Plateau seem minimal, they resulted in increases in bare ground (4%) and in gap sizes from a mean of 170 cm to 230 cm.  While an increase in bare ground and gap size does not guarantee an increase in cheatgrass cover, it opens an ecological niche that decreases the ecosystem’s resistance to cheatgrass invasion and potentially promotes erosion.  To maintain a system’s resilience and reduce the risk of cheatgrass invasion, managers might consider maintaining the smallest gaps possible among perennial grasses. Measuring gap size between perennial plants may provide an excellent early warning indicator of invasion potential.