Effects of selective imazapic application on Bromus tectorum and non-target species in Rocky Mountain National Park

Background/Question/Methods

Cheatgrass (Bromus tectorum), a winter annual grass introduced from Eurasia, has invaded much of the Western United States over the last century. More recently, cheatgrass has become a threat to the montane and subalpine ecosystems of Rocky Mountain National Park (RMNP). Cheatgrass aggressively invades disturbed sites and competes with native plant species by rapidly establishing a root system in early spring; depleting soil moisture and available nitrogen before many native species germinate. These characteristics make control of cheatgrass of primary importance when restoring disturbances within RMNP. The purpose of this study is to determine the effectiveness of imazapic for cheatgrass control and its effects on non-target native species in grassland, shrubland and forest habitats. In 2008, 12 permanent monitoring plots were established in six imazapic treatment sites in RMNP. Reference plots were chosen to represent the desired final condition for each imazapic treatment site. Imazapic (23.6% a.i.) was applied to cheatgrass infested soils in 2008, 2009 and 2010. Cheatgrass was treated selectively, avoiding application to native species as much as possible. Plant species and functional group cover were estimated using modified Daubenmire cover-classes. Data were analyzed using a repeated measures analysis of variance (ANOVA) model.

 Results/Conclusions

Cheatgrass was reduced nearly fivefold to approximately 5% cover in treatment plots (ANOVA F-test, treatment main effect, P<0.001). There was no change in cover or species richness of native grasses, shrubs or forbs in treatment plots. These results indicate that selectively treating cheatgrass with imazapic can effectively control cheatgrass while avoiding damage to established native plant species. The apparent lack of native plant recruitment in plots where cheatgrass cover has been reduced suggests that imazapic may affect seedling survival in treatment plots, slowing the recovery of these sites following treatment.