Author: Kevin G. McKeehan
Publisher:
Published: 2022
Total Pages: 0
ISBN-13:
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This dissertation attempts to fill a gap in knowledge regarding conditions amongst the dunefields of Lake Michigan's eastern shore. Much is now known about the evolution and geochronology of these unique freshwater dune systems. The region's coastal dunes began forming during the Nipissing high stand phase (~5.5 ka) of ancestral Lake Michigan. Since then, according to the chronology constructed from several studies, the coastal dunes then underwent several periods of stability and instability along the entire shoreline. However, questions remain regarding dune conditions and variability since ~1900. The goal of this dissertation was to determine if changes have occurred to the region's coastal dune systems in the last ~120 years and what might be driving those changes. Given that dune systems are sensitive to biotic and abiotic variables, examining the last ~120 years of dune behavior could potentially reveal how Lake Michigan coastal dunes are responding to anthropogenic climate change and human development.Three studies, each comprising a dissertation chapter (Chapters 2-4), were conducted to help close this knowledge gap. Each chapter is broadly linked through an ecogeomorphic lens, particularly through the relationship between dunes and vegetation, which are interconnected in important ways. In Chapter 2, changes in dunefield vegetation and morphology were determined at several locations along the eastern Lake Michigan shoreline through the use of ground-level repeat photography. The second dissertation study-Chapter 3-concerns the spatiotemporal analysis of historical changes of blowouts, which are important indicators of significant disturbance in the dunes. In this chapter, blowouts were mapped from aerial images at three timestamps-1938, 1986-8, and 2018-and the changes quantified. Chapter 4, the final dissertation study, explores the relationship between terrain ruggedness and vegetation in a coastal dunefield along Lake Michigan by calculating two terrain indices-Riley's Terrain Ruggedness Index (TRI) and Sappington's Vector Ruggedness Measure (VRM)-and the Soil-Adjusted Vegetation Index (SAVI). Through a land systems framework, the results were compared to determine if any correlation exists between the ruggedness of dunes and vegetation.In the first two dissertation studies, the results show a clear expansion of vegetation at the expense of previously bare sand. In the final study, the values from TRI and VRM and the values from the Soil-Adjusted Vegetation Index (SAVI) were not correlated overall, especially where one type of vegetation was dominant. However, within one land system-the dune barrens -- a moderate-to-strong negative correlation existed between terrain ruggedness and vegetation. Moreover, evidence suggests that vegetation has transformed the dune barrens land system area within the modern period. Overall, the results of these three studies demonstrate that vegetation is expanding over previously bare surfaces in coastal dunes along the eastern shore of Lake Michigan and has a considerable influence on regional dune conditions. While the precise driver(s) of this transformation is unclear, the regional-scale nature of these results suggests a uniform control is affecting these changes. As described in this dissertation, it is possible that an increase in precipitation since the 1930s, elevated atmospheric CO2 and N concentrations, a reduction in wind power, some other change in climate drivers, or a combination of many factors is responsible for the expansion in vegetation. It is also possible the trend in vegetation growth in Lake Michigan's coastal dunes is a lagged response to an earlier climate event.
