PS 24-63
Imazapic increases resistance to annual grass invasions after fuels reduction treatments in arid Wyoming big sage sites
Imazapic, a pre- and post-emergent herbicide that especially targets annual species, is potentially useful in restoration and fuels reduction treatments in shrublands of the arid western U.S. to control invasive annual grasses, but little is known about the long-term efficacy and potential side effects on native vegetation. Restoration and fuels reduction treatments are employed to encourage native perennial grass reestablishment and growth, and to reduce the intensity and duration of fires, but may have the unintended side-effect of fostering annual invaders. State and transition models predict that sites with codominant native populations should be more resilient to disturbance and more resistant to invasions, and therefore should respond more favorably to management practices. We applied 3 fuels reduction treatments (fire, mowing, and herbicide thinning) and imazapic to 6 Wyoming big sagebrush sites to 1) investigate the ability of imazapic application and pre-treatment perennial grass cover to increase post-treatment resistance to cheatgrass, and 2) to investigate potential negative effects of imazapic on native perennial and annual vegetation, including uncommon native annual forbs (defined as occurring in less than 10% of subplots). 24 to 96 30 x 33m subplots were monitored pre-treatment and 4 years post-treatment at each site.
Results/Conclusions
Imazapic provided ongoing control of cheatgrass across all fuels reduction treatments. By year 4 post-treatment cheatgrass cover was on average 44% greater than pre-treatment levels in plots not treated with imazapic. Over the same period cheatgrass recovered to pre-treatment levels in plots treated with imazapic, but still remained 95% lower than in plots not treated with imazapic. Fuels reduction treatments did not significantly affect cheatgrass cover or influence the effectiveness of imazapic. Imazapic application suppressed perennial tall grass cover by 16%; however, there is evidence that fuels treatments provided a competitive release for perennial tall grasses as cover levels in fire and mowing plots were nearly double those of control plots by four years post-treatment. Annual native forbs were suppressed by imazapic, but were more influenced by interannual variation, and uncommon native annual cover did not differ between imazapic and non-imazapic plots by four years post-treatment. High levels of perennial grass cover increased post-treatment resistance to cheatgrass invasions, while imazapic application provided increased resistance when perennial grass cover was less than roughly 25%. These results suggest that imazapic can increase resistance to annual grass invasions after fuels reduction treatments in Wyoming big sagebrush sites.