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 plant communities and 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, each with one reference and one imazapic treatment plot. The reference plots were chosen to represent the desired final condition for each imazapic treatment site. Imazapic (23.6% a.i.) was applied to cheatgrass post-emergence in 2008 and pre-emergence in 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 more than fivefold to approximately 4% cover between 2008 and 2011 (ANOVA F-test, time and treatment interaction, P=0.04) in all vegetation types. There was no decrease in cover of native grasses, shrubs or forbs in treatment plots. In 2011 an increase in bare ground was detected in treatment plots (ANOVA F-test, time and treatment interaction, P=0.02) in all vegetation types. These results suggest that selectively treating cheatgrass with imazapic can effectively control cheatgrass while avoiding damage to native plant species. The increase in bare ground presents an opportunity to establish native vegetation while cheatgrass populations are diminished, potentially preventing cheatgrass reinvasion and limiting soil erosion and nutrient losses.