Influence of aridity and fire on Holocene vegetational patterns in the tallgrass Prairie Peninsula

Retrospective studies of middle-Holocene environmental dynamics in the midwestern United States are important for anticipating future changes, because prevailing climatic conditions were warm and dry, similar to future projections. However, the environmental history of the middle Holocene remains poorly understood. This dissertation uses proxies preserved in the sediments of four lakes in the midwestern United States to investigate climatic, vegetational, and fire dynamics during the middle Holocene.; Results from West Olaf Lake and Steel Lake in Minnesota reveal the response of C3 and C4 plants and fire regimes to middle-Holocene climatic fluctuations. In contrast to the generalization that C4 plants are adapted to moisture-limited habitats, results indicate that severe aridity suppressed C4 plants in favor of weedy C3 plants. Aridity limited fuel availability and fire occurrence at West Olaf Lake, while at Steel Lake biomass flammability was lower than at West Olaf Lake, resulting in fires being less important at Steel Lake.; Results from Chatsworth Bog and Nelson Lake in Illinois address long-standing hypotheses about the eastern Prairie Peninsula's vegetational history and assess the influence of aridity and fire on prairie development in the region. Fire-sensitive woody species declined and prairie taxa expanded as aridity increased after 10000 BP. Between ∼8500 BP and ∼6200 BP, aridity declined and prairie coexisted with fire-sensitive and fire-tolerant woody species. After ∼6200 BP prairie taxa became dominant, although aridity was not more severe than it was ∼8500 BP. Along with aridity, fire appears to have played an important role in the establishment and maintenance of prairie communities in the eastern Prairie Peninsula.; Because of limitations inherent in delta13C analysis of substrates such as charcoal, new techniques are needed to understand C 3 and C4 grass fluctuations. A method of isolating fossil grass pollen for delta13C analysis was developed, and the delta 13C composition of modern and fossil grass pollen was evaluated. Results suggest that modern pollen from C3 and C4 grass species can be distinguished by delta13C Furthermore, delta 13C analysis of fossil grass pollen indicates that this proxy can provide information on C3 and C4 grass variations unavailable from other proxy indicator.