Graduate Defense: Madelyn Sorrentino

Thesis Information

Title: Ecosystem Effects Of Targeted Grazing With Sheep To Manage Invasive Cheatgrass

Program: Master of Science in Biology

Advisor: Dr. Kelly Hopping, College of Innovation and Design

Committee Members: Dr. Marie-Anne de Graaff, Biological Sciences; Dr. Allison Simler-Williamson, Biological Sciences; and Dr. April Hulet, Plant and Wildlife Sciences

Abstract

Cheatgrass (Bromus tectorum) invasion is widespread across the Great Basin region of the United States and is expected to expand into higher elevations as temperatures increase with climate change. Cheatgrass degrades ecosystem functioning, increases the risk of wildfire, and negatively affects rangeland-based agriculture and other land uses. Targeted grazing—the use of livestock to accomplish a vegetation management objective—has arisen as a management practice that could combat cheatgrass invasion at the landscape level. However, our knowledge of targeted grazing at a management-relevant scale and its effects on other vital ecosystem components, such as non-target plant species and soil properties, remains limited. To test the efficacy of sheep targeted grazing to manage cheatgrass and its effects on other ecosystem components and working in conjunction with researchers, U.S. Forest Service staff, and ranchers, I established a targeted grazing experiment in the Sawtooth National Forest, Idaho. The experiment consists of five treatments: higher intensity targeted grazing when cheatgrass is green in (1) spring, (2) fall, and (3) spring + fall; (4) low-intensity summer grazing that is the norm for this region; and (5) no grazing, which represents a grazing exclusion scenario. I used generalized linear mixed effects models to assess vegetation and soil responses from the experiment’s before-after control-impact design. I found that after one year of grazing treatments, spring targeted grazing significantly decreased cheatgrass cover, but also decreased non-target forbs and other graminoids. Total soil carbon and nitrogen concentrations did not change, but mineral nitrogen concentrations did, which could have longer-term feedbacks to aboveground vegetation. While these results are promising as an early indicator of targeted grazing’s ability to help reduce cheatgrass and associated risks, results also provide insight that management practices should take into account non-target plants and changes to soil nitrogen. Additional years of grazing treatments and data collection are ongoing and will be needed to better understand the potential of sheep grazing as a long-term cheatgrass management tool.