Theses and Dissertations at Montana State University (MSU)
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Item Soil health response to cropping systems in semi-arid Montana(Montana State University - Bozeman, College of Agriculture, 2024) Ashford, Zane Ann; Chairperson, Graduate Committee: Catherine A. ZabinskiTraditional cropping systems in the northern Great Plains (NGP) were dominated by cereal-fallow rotations until the 1970s, resulting in increased soil erosion, decreased soil organic matter (SOM) accumulation, and declines in soil biological activity. Recent shifts toward continuous and more diverse no-till crop production attempt to increase sustainability, diversify economic opportunities, and keep up with the growing food demand without converting more land into agriculture. With a two-year study, I explored the effects of crop types in diverse, no-till, crop sequences on soil health in dryland and irrigated systems on one farm in semi-arid Montana, using biological indicators of potentially mineralizable nitrogen (PMN), soil enzyme activity (beta- glucosaminidase, beta-glucosidase, arylsulfatase, and acid and alkaline phosphatases), and permanganate oxidizable carbon (POxC), a measure of labile carbon. Crop sequences included four crop types -- cereals, oilseeds, legumes, and root crops. Root crops, namely sugar beet, drove soil responses in PMN, evident by increased plant-available N in soils following sugar beet. Soil enzyme activity, an indicator of nutrient cycling capacity, was strongly correlated with SOM, but did not follow a pattern based on crop type. Labile carbon changed in soils between years but did not respond consistently to crops. This research also explored the soil health gap by comparing soil health in cultivated systems to nearby grasslands. In a paired-site comparison on two farms in Montana, biological health indicators were 45% lower, on average, in cultivated soils compared to adjacent uncultivated soils. This difference was consistent with lower SOM averages, offering a simple assessment to quantify the maximum attainable soil health capacity within a specific agroecosystem. Soil acidification, a growing concern for producers across the NGP, contributed to 42% lower soil enzyme activity, based on four enzymes, compared to adjacent neutral pH cultivated soils. Enzyme activity was the only soil health parameter that was lower in acid soils compared to neutral pH soils, demonstrating the sensitivity of soil enzymes. Overall, these results indicate that biological soil health indicators are sensitive to changes in crop production, changing yearly, and provide farmers with the opportunity to fine- tune their management practices to meet their soil health goals.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 Evaluating grazing and defoliation effects on ponderosa pine grassland following the lodgepole complex wildfire(Montana State University - Bozeman, College of Agriculture, 2020) Williams, Amanda Rae; Chairperson, Graduate Committee: Clayton B. Marlow; Lance T. Vermeire, Richard C. Waterman and Clayton B. Marlow were co-authors of the article, 'Evaluating grazing effects on ponderosa pine grassland following the lodgepole complex wildfire' submitted to the journal 'Rangeland ecology and management' which is contained within this thesis.; Lance T. Vermeire, Richard C. Waterman, Clayton B. Marlow were co-authors of the article, 'Season of defoliation effects on ponderosa pine grassland following the lodgepole complex wildfire' submitted to the journal 'Rangeland ecology and management' which is contained within this thesis.Previous research indicated rangelands need rest from grazing after fire, while others show grazing the first year following fire has no negative effects on the plant community. This caused uncertainty around post-fire grazing management. In July 2017, the Lodgepole Complex fire burned ponderosa pine grasslands of the northern Great Plains, including areas burned in 2003 for fuels mitigation. We examined effects of post-fire grazing and season of defoliation. For the grazing study we wanted to determine 1) plant community response to grazing or rest the first growing season after fire in ponderosa pine grassland communities, and 2) whether prescribed fire alters plant community response to subsequent wildfire. For the season of defoliation study we wanted to determine 1) timing of defoliation effects on the plant community one growing season after fire and 2) whether defoliation effects are altered by prescribed fire preceding the fire. Eight exclosures (25 x 15 m) were built, 4 reburned sites and 4 wildfire sites. A non-grazed (15 x 10 m) section inside each exclosure was paired with a grazed section outside the exclosure. Plots (5 x 10 m) were mowed in the exclosure to 10 cm in June, July, or August, or not mowed during 2018. In 2019, biomass samples were clipped at peak production, with species composition and diversity measured by point-intercept transects. We observed a trend for reduced cool-season grasses (P =0.0675) and annual grasses (P =0.0071) if defoliated earlier; a trend for reduced forbs (P =0.0699) if defoliated later; and reduced total current-year biomass (P =0.0362) if defoliated. Functional group composition was not changed, but some individual species were shifted due to fire history. The grazing study only showed a trend for greater old dead biomass on non-grazed sites (P = 0.0600), higher composition of forbs on reburn sites (P = 0.0324), and a trend for a higher composition of Psoralidium tenuiflorum (Pursh) Rydb. on reburn sites (P = 0.0559). Results indicate mowing the first year following fire shifts the plant community, but the community is resistant to post-fire grazing. Prescribed fire 14 years before wildfire had small impacts on community composition.