Date of Award
Doctor of Philosophy
Ecology, Evolution, and Organismal Biology
Ecology, Evolution, and Organismal Biology; Environmental Science
Lynn G Clark
Vegetation dynamics at regional to subcontinental scales are complex, and our understanding of the critical factors which govern them is far from complete. Temperature operates on a roughly longitudinal gradient across North America, while precipitation gradients are roughly latitudinal. What results is a complex climate space which is then further subdivided by topography, underlying geology, surface and subsurface hydrology, and seasonality, to name but a few, into broad-scale vegetation zones, or ecoregions. I used a combination of approaches to investigate climate and vegetation dynamics at multiple spatiotemporal scales, and to develop new tools to study past climate.
Defining and delineating past ecoregions has long presented a challenge. Fossil pollen distinguishes between major biome types, i.e., grassland versus forest, with great success. However, distinguishing between prairie types at regional to subcontinental scales using the pollen record has been impossible. The ratio of Ambrosia (ragweed) to Artemisia (sagebrush, wormwood, mugwort) pollen has been shown to differentiate between tallgrass, mixed grass, and shortgrass prairie over a small portion of the modern Great Plains of North America. I extended that technique, and showed that the log-transformed Ambrosia to Artemisia ratio can reliably distinguish between subregions within the Great Plains, as well as regions immediately adjacent. In addition, I found that the relationship between Ambrosia and Artemisia pollen is best explained by precipitation rather than temperature, and that it produces reliable precipitation estimates when used to create models. This will allow for better reconstructions of past climate and improve delineations of past ecoregion boundaries.
Pollen data are routinely used in paleoenvironmental studies to understand past climate and vegetation. One existing limitation in working with pollen data is the need to write code in order to execute several of the routine analyses in paleoecological work. I developed GeoPollen, a Shiny Dashboard application, to be a streamlined, user-friendly GUI-based tool for performing these basic analyses. Users are able to utilize more than 3,000 publicly available pollen datasets from the Neotoma Paleoecology Database spanning the last 22,000 years from the United States and Canada. GeoPollen performs a suite of common tasks on demand and generates diagnostics necessary for evaluating results. I developed GeoPollen in order to increase the openness and accessibility of late Quaternary pollen data.
Boundaries between vegetation types are often highly sensitive to perturbations in climate. For example, the tallgrass prairie-temperate forest ecotone in Minnesota shifted rapidly and repeatedly during the mid-Holocene climate optimum, a warm and dry period ~9,000 - 5,000 years ago. A much less well-understood region exists at the southern limit of the prairie in southwest Missouri. This location serves as an excellent model system, as it represents a transitional zone between temperate grasslands and warm forests. I employed a multiproxy approach using soil carbon isotopes (a proxy for the C3:C4 ratio of the vegetation, and thereby hydroclimate), charcoal (a paleofire proxy), and phytoliths (taxonomically significant silica bodies produced by many plants) to examine vegetation dynamics at the tallgrass prairie-Ozark forest ecotone. Forest has not significantly invaded the prairie during the last 20,000 years, but the prairie has shifted from mixed grass (dry-mesic) to tallgrass (mesic) within the last 4,000 years. Fire likely played only a minor role until the mid-Holocene, after which it became frequent. This body of work serves to expand our understanding of the signals produced by vegetation in response to changing climate across the North American midcontinent throughout the late Quaternary.
Hannah M Carroll
Carroll, Hannah M., "Late Quaternary paleoecology of the North American midcontinent" (2020). Graduate Theses and Dissertations. 17846.