Identifying Structural Thresholds in Annual Grass–Invaded Rangelands

Abstract

Invasive plants have an increasing impact on the ecological and economic functions of natural systems. Cheatgrass (Bromus tectorum L.) is an invasive annual grass that is widely impactful throughout most of the western United States. Describing ecological and management thresholds in cheatgrass-invaded rangelands may help managers conserve native plant communities. The objective of this research was to determine if direct, predictable relationships exist between pretreatment vegetation conditions and post-treatment changes in perennial grass biomass following cheatgrass control. We sampled locations representing a gradient of cheatgrass-to–perennial grass ratios before and following application of two imazapic formulations—liquid imazapic with a water carrier and dry imazapic with sand/granular carrier—at field sites near Saratoga and Pinedale, Wyoming. We monitored vegetation responses 1 yr before treatment and 2 yr following treatment. We observed a general pattern that posttreatment increases in perennial grass biomass were greater where pretreatment relative cheatgrass canopy cover was higher, and magnitudes of increase varied according to site conditions. We tentatively suggest thresholds at ratios of pretreatment cheatgrass-to–perennial grass cover at 4:1, where perennial grass biomass increased following herbicide treatment and > 10:1, beyond which increases in perennial grasses were no longer observed. Such ratios provide a starting point for threshold identification in cheatgrass-invaded rangelands that may be transferrable across plant communities but need to be explored at a broader scale to adequately make management recommendations.