Reducing exotic annual grass competition did not improve shrub restoration success during a drought

Authors: Davies, Kirk; Bates, Jonathan - Jon; Svejcar, Lauren

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2022
Publication Date: 9/5/2022
Citation: Davies, K.W., Bates, J.D., Svejcar, L.N. 2022. Reducing exotic annual grass competition did not improve shrub restoration success during a drought. Rangeland Ecology and Management. 85:9-14. https://doi.org/10.1016/j.rama.2022.08.001.
DOI: https://doi.org/10.1016/j.rama.2022.08.001

Interpretive Summary: Restoration of bitterbrush, a wildlife important shrub, is often a management objective on western U.S. rangelands. However, competition from exotic annual grasses can hinder bitterbrush establishment. We evaluated the effects of controlling exotic annual grasses with a pre-emergent herbicide on bitterbrush establishment and other vegetation. Herbicide application reduced exotic annual grass abundance and cover, but did not improve bitterbrush establishment. Drought likely prevented bitterbrush from taking advantage of exotic annual grass control. However, exotic annual grass control did result in greater native bunchgrass cover. The high mortality of bitterbrush seedlings in this study highlights that multiple barriers may need to be overcome for successful restoration. These results are of interest to restoration practitioners, land managers, wildlife biologists, and scientists.

Technical Abstract: Restoration of native shrub species is challenging but direly needed in arid and semiarid rangelands glob- ally as native shrubs provide critical habitat for wildlife and livestock forage. Restoration of antelope bitterbrush ( Purshia tridentata Pursh DC), a wildlife-important shrub, is often a priority on western US rangelands. One challenge to bitterbrush restoration is competitive exotic annual grasses. Exotic annual grasses can be successfully controlled with pre-emergent herbicides, but the effects of controlling ex- otic annual grasses with pre-emergent herbicides on bitterbrush survival and growth are unknown. We evaluated the effects of applying a pre-emergent herbicide, imazapic, to control exotic annual grasses on planted bitterbrush seedlings and existing vegetation for 2 yr post treatment at five sites in southeastern Oregon. Imazapic application reduced exotic annual grass cover and density but did not improve bitter- brush establishment. Exotic annual grass control did lead to an increase in native perennial bunchgrass cover. We suspect the lack of treatment effect was caused by high mortality of bitterbrush seedlings from drought in the first year. By the second year, bitterbrush was largely lost across the study sites with only four individuals surviving. The high bitterbrush seedling mortality observed in this study high- lights that multiple barriers to restoration success likely exist in arid and semiarid rangelands. For suc- cessful restoration, land managers and restoration practitioners need to have a plan and resources for overcoming multiple barriers, which may require several restoration attempts should initial attempts be unsuccessful.