Changes to nutrient availabilities and cheatgrass (Bromus tectorum L.) metrics following non-surface disturbing restoration treatments in a sagebrush-steppe ecosystem

Background/Question/Methods

Cheatgrass (Bromus tectorum L.) invasion into sagebrush (Artemisia tridentata) shrublands in the Great Basin has resulted in increased wildfire frequencies and a decline of native ecosystem components. Our research, conducted in Box Elder County, Utah, is aimed at testing methods for re-establishing native, fire-resistant perennial grasses into cheatgrass-invaded sagebrush ecosystems and cheatgrass near-monocultures to interrupt the cheatgrass-wildfire cycle. Our treatments, intended to increase the competitive advantage of seeded perennial grasses in cheatgrass-invaded areas without surface-disturbing seeding mechanisms, were: 1) pre-emergent herbicide (Imazapic) application, 2) soil amendment treatments (sucrose addition, activated carbon [AC] addition), and 3) vegetation manipulation treatments (burning of sagebrush overstory and/or vegetative thatch, and sagebrush overstory thinning or clearing), arranged in a 3-way factorial design. This stage of research specifically addresses main effects and their interactions on measures of cheatgrass vigor and reproduction and soil nutrient bioavailability for two years after treatment.

Results/Conclusions

Pre-emergent herbicide was effective at reducing densities, mean weights, numbers of tillers, and numbers of spikelets of cheatgrass individuals for one year after treatment; results of herbicide were stronger when vegetative overstories were removed or burned. However, cheatgrass in herbicide-treated plots had greater mean weights and numbers of tillers and spikelets the second year after treatment as compared to untreated plots. Sucrose treatment reduced soil NO₃^- and H₂PO₄^- during the first winter after treatment, and sucrose-treated cheatgrass had smaller mean weights, tiller numbers and spikelet numbers during the first growing season. An increase in NO₃^- availability was detected the second winter after treatment as compared to control plots, and sucrose-treated cheatgrass also had greater weights, tiller numbers, and spikelet numbers than those in control plots during the second summer. Sagebrush thinning and clearing had no significant effects on soil nutrients; however, cheatgrass in these plots were larger and with greater tiller and spikelet numbers than in control plots both years after treatment. Burning increased soil NO₃^-, H₂PO₄^-, SO₄^-2, Mn²⁺, Ca²⁺, Al³⁺, and K⁺, and resulted in larger cheatgrass with more tillers and spikelets than those in unburned plots for both years after treatment. AC treatment did not change soil chemistry or directly affect cheatgrass, but there is some indication that it sequestered and thereby lessened the negative effects of herbicide. Treatments that initially reduce cheatgrass density and vigor may result in increases in these metrics in subsequent years; concerted efforts to establish desired species should occur within the first season following these treatments.