Date of Award

Fall 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Forestry and Environmental Studies

First Advisor

Lauenroth, William

Abstract

Dryland ecosystems cover over 40% of Earth’s terrestrial surface and are critical to both biodiversity and human livelihoods, yet they are increasingly vulnerable to the interacting pressures of climate variability and land use. Effective management of these systems requires a deeper understanding of how chronic pressures and acute disturbances shape plant communities. This dissertation examines the interplay between climate variability, livestock grazing, and plant community dynamics in big sagebrush ecosystems of the Upper Green River Basin, Wyoming. Through three complementary studies, I demonstrate that while livestock grazing influences plant structure, its effects are consistently subordinate to climatic and edaphic factors, supporting the central role of environmental filtering in dryland systems. Chapter 1 quantifies how grazing intensity and abiotic variability jointly shape sagebrush steppe plant communities. I find that grazing modestly decreases perennial bunchgrass cover and increases sagebrush abundance, but precipitation and soil texture emerge as stronger determinants of community structure. Chapter 2 investigates recovery trajectories following a climate-induced mortality event in sagebrush communities. Results reveal that resilience varies across the landscape, with upland sites showing greater native species retention and sagebrush recruitment, while lowland sites are more vulnerable to non-native annual invasion and biodiversity loss. Chapter 3 uses a factorial experiment to disentangle the effects of grazing intensity, interannual precipitation variability, and extended growing seasons on forage species phenology and productivity. Higher precipitation delayed peak biomass for C₃ grasses, and species-specific responses suggest that grazing-resistant species like Poa secunda may gain an advantage under future climate scenarios. These findings demonstrate that abiotic conditions, especially water availability, consistently outweigh biotic pressures in structuring dryland plant communities. The work highlights critical limitations of grazing management frameworks that fail to account for climate variability and landscape heterogeneity. Adaptive, climate-responsive grazing strategies that incorporate seasonal forecasts and functional group sensitivities are necessary to maintain rangeland resilience under changing environmental conditions. By integrating theoretical and applied perspectives, this dissertation advances understanding of how dryland ecosystems respond to compounded environmental stressors and offers practical insights for sustainable land management in an era of accelerating climate change.

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