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Item 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(Montana State University - Bozeman, College of Agriculture, 2022) Currey, Bryce Alan; Chairperson, Graduate Committee: Jack Brookshire; This is a manuscript style paper that includes co-authored chapters.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.Item Change in deciduous woody vegetation, implications of increased willow (Salix spp.) growth for bird species diversity, and willow species composition in and around Yellowstone National Park's Northern range(Montana State University - Bozeman, College of Letters & Science, 2009) Baril, Lisa Marie; Chairperson, Graduate Committee: Andrew J. HansenDeciduous woody vegetation (DWV) in Yellowstone's northern range is a rare, but important habitat type. Including willow, alder, aspen, and cottonwood, DWV was limited in height and areal extent since the early 1900s. Since the mid-1990s; however, DWV has increased in height and areal extent in some locations. Previous studies regarding DWV change have focused on quantifying change in vegetation growth in areas of limited extent and none have quantified increased DWV growth at the scale of the landscape. Furthermore, no studies have evaluated the implications for increased growth of DWV for wildlife in the region or how DWV species themselves may influence rates of growth. In this thesis I evaluate change in DWV within wetland and riparian areas using aerial photographs from 1991 and 2006. I also investigate how an increase in willow, the dominant component of DWV in the region, has likely influenced bird diversity. Finally, since willow species vary with respect to genetic limitations on growth and reproduction that in turn may influence observed patterns of growth; I examine willow species composition across the northern range. Results of the air photo interpretation revealed a 170% increase in DWV. Most plots that increased did so by recruitment rather than densification; however, DWV was difficult to detect below 100 cm indicating that the number of plots increasing by recruitment may have been overestimated. Bird community variables were examined in three willow growth conditions: height suppressed, recently released, and previously tall (tall prior to observed increase in growth). I found that willow structure generally increased in complexity from structurally simple height suppressed willows to structurally complex previously tall willows and that this allowed for greater bird richness, abundance, diversity, and the abundance of several willow dependent bird species. I also found that willow species composition was similar and diverse between the released and previously tall condition, but that the suppressed condition was dominated by a singe species. I conclude that increased DWV has allowed for greater bird diversity in the northern range and that increased willow growth in the suppressed condition may be influenced by species composition there.