Landscape position influences resistance to Bromus tectorum invasion and short-term resilience is limited

Background/Question/Methods

Bromus tectorum L. (hereafter Bromus) is a widespread, invasive annual grass that has greatly impacted ecosystems of the western U.S., especially in the Great Basin. Bromus has been well-studied in this winter-wet area, but less is known about its ecology in the summer-wet Southern Rocky Mountain region, where it is an emerging problem. Much less is known about the resistance and resilience of this system to its invasion. We set out to explore the responses of Bromus and native species to disturbance at the landscape scale (wildfire) and management at the local scale (herbicide application) in the foothills of the Southern Rocky Mountains. We monitored cover of Bromus, perennial grasses, forbs and shrubs on sites burned by wildfire within 3 years, 3-12 years, or unburned, placing plots on north facing slopes (315 – 45°) and other aspects (hereafter south facing). We hypothesized that landscape position would influence susceptibility and resilience to Bromus invasion. In a separate study on south facing slopes, we applied imazapic herbicide to half the plots and left the others untreated. We measured plant cover as described above and N mineralization rates to indicate resource availability. Here we hypothesized that control of Bromus with imazapic would enhance resilience to invasion.

Results/Conclusions

Cover of Bromus was greater on burned than unburned sites, but vulnerability to Bromus invasion depended on landscape position. In particular, south facing slopes had greater Bromus cover after burning than north facing slopes. Cover of Bromus did not decrease with increasing time since burning. Imazapic application reduced Bromus cover, but this was not matched by a corresponding increase in perennial grass cover. There was a spike in NO₃ mineralization rates in imazapic treated plots in the spring. This indicates increased availability of the resource at a time when Bromus is likely better able to utilize it than slower growing perennials. Overall, certain parts of the landscape were more resistant to Bromus invasion than others, and short term resilience was limited.