Date of Award

2022

Degree Name

Biology

College

College of Science

Type of Degree

M.S.

Document Type

Thesis

First Advisor

Dr. Kyle Palmquist, Committee Chairperson

Second Advisor

Dr. Anne Axel

Third Advisor

Dr. John Bradford

Abstract

Climate change is increasingly affecting the plant communities across the world. In the Western United States, these changes are affecting plant communities and their risk of invasive species, as well as the frequency and severity of wildfire. Big sagebrush (Artemisia tridentata Nutt.) ecosystems of the Western United States are facing threats of increasingly frequent wildfire and invasion of the annual grass cheatgrass (Bromus tectorum L.), which may be exacerbated by global climate change. Past studies have found cheatgrass to exhibit a positive feedback loop with wildfire, and climate change is expected to facilitate expansion of the cheatgrass range. Livestock grazing, particularly overgrazing, can also promote cheatgrass invasion and subsequent decline of native species. The goal of this work is twofold: (1) to simulate where cheatgrass invasion is most likely to occur, and what the resulting impacts on fire frequency and the native plant community will be under future climate scenarios, and (2) to determine the response of cheatgrass and native plant functional types to different grazing treatments. We modeled changes in plant community composition and fire frequency under future climate conditions and livestock grazing intensity treatments using an individual-based plant simulation model, STEPWAT2, for 200 sites across the big sagebrush ecosystem. We used global circulation models for established representative concentration pathway (RCP) emissions scenarios 4.5, and 8.5 for mid-century, and end-century conditions, and implemented livestock grazing treatments as a predetermined portion of biomass removed for each functional type for each simulation year. We found that sites within the northeastern ecoregions of our study area, including the Snake River Plain, the Wyoming Basins, and the Great Plains, were the most likely to become invaded by cheatgrass in the future as climate becomes more suitable for cheatgrass. On average across all sites, we simulated declines in biomass for native perennial forbs, big sagebrush, and C3 perennial grasses under future climate scenarios, and simulated increases in cheatgrass and C4 perennial grass biomass. Sites which had a simulated increase in cheatgrass invasion burned more frequently, which resulted in lower simulated big sagebrush biomass than uninvaded sites. We found that functional types which are most sensitive to climate change had similar responses to climate regardless of livestock grazing treatment, and the implementation of livestock grazing in any intensity did not fully counteract the effects of climate on native or invasive plant functional types. Livestock grazing had a larger effect on plant communities than climate for all plant functional types except big sagebrush, and in the case of native C3 perennial grasses livestock grazing exacerbated the effects of climate on biomass loss. These results suggest that climate change may lead to the increased spread of cheatgrass and increased wildfire in big sagebrush ecosystems and a reduction in biomass for most native plant functional types, irrespective of livestock grazing intensity. These findings may help to inform land management decisions and conservation planning within the big sagebrush region in light of a changing climate.

Subject(s)

Climatic changes – West (U.S.) – Evaluation.

Climatic changes – Research – West (U.S.)

Cheatgrass brome – West (U.S.)

Fire ecology – West (U.S.)

Introduced organisms – Environmental aspects – West (U.S.)

Rangelands – Fire management – West (U.S.)

Big sagebrush – Ecology – West (U.S.)

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