Woody plant expansion in the northern Great Plains: a multi-scale assessment of the drivers and ecological implications of increasing woody abundance in a temperate open ecosystem

Abstract

Open ecosystems (i.e., non-forested ecosystems) are inherently dynamic ecosystems that are essential to the persistence of humankind; yet many are being altered, degraded, or lost. One of the largest changes to open ecosystems globally is an increase in greenness, driven by increasing plant photosynthetic capacity or altered species composition. Often this transition in species composition occurs when woody species (i.e., trees and shrubs) increase in dominance or replace contemporary herbaceous species, a phenomenon referred to here as woody plant expansion (WPE). WPE has been proposed as a possible solution to climate change, yet ultimately has the potential to alter grasslands into a novel state. This dissertation improves the understanding of how open ecosystems have been altered by WPE. The chapters within this document offer a multi-scalar examination of the drivers and ecological implications of WPE in the Northern Great Plains (NGP) of North America, one of the largest grasslands remaining globally. Specifically, I answer three main questions: (1) What is the extent and magnitude of WPE across the NGP? (2) What are the drivers, ecosystem implications, and biogeochemical impacts of WPE? and (3) Moving forward, how should research be prioritized in ecosystems like the NGP, particularly concerning climate mitigation and management? I begin with an introduction to the NGP, the implications of WPE, and an overview of this dissertation. I then examine the drivers and interactions of WPE and the recent increase in vegetative productivity. Next, I couple high-resolution estimates of WPE from the heart of the NGP with data from sampling plots to examine the impact that WPE has on ecosystem biogeochemistry. Next, WPE is examined against agricultural expansion and I propose that future work examining large-scale changes and the subsequent management of open ecosystems be reframed towards weighing the impacts on biodiversity, carbon storage, and ecosystem resilience. Finally, I summarize all findings with implications for future research. The NGP has the potential to represent the future of North American grasslands and, while this dissertation answers many questions, the future remains highly uncertain.