OBSERVING PATTERNS IN PITTSBURG BASIN, ILLINOIS POLLEN DATA USING DETRENDED CORRESPONDENCE ANALYSIS

Pittsburg Basin, central Illinois, contains a long pollen record; dating back at least 130,000 years. The basal layer is composed of late Illinoian sands. The site includes assemblages indicating a period as warm as the present last interglacial. Pollen indicates considerable change throughout the record. Detrended Correspondence Analysis (DCA) was used to summarize the pollen data (from Teed, 2000) in order to analyze the relationships among climate, vegetation, and fire frequency. The first two eigenvectors account for 82% of the variation in the terrestrial pollen data. Sample loadings on both eigenvectors allow comparison of previous and current interglacials and of the Wisconsinan and the Late Illinoian glacial stages.

Boreal trees such as spruce and pine have high positive loadings on the first eigenvector. Conversely, the loadings are negative for most herbs and temperate trees. Pollen taxa with higher positive first-eigenvector loadings contain more boreal species, indicating a cooler climate, possibly during the summer. For eigenvector two, the boreal-tree loadings are neutral while the temperate-tree loadings are positive. All of the loadings for herbs are negative for eigenvector two. Samples with positive second-eigenvector values were deposited under more densely forested conditions, while the samples with negative values reflect a dominance of prairie. This second eigenvector is negatively correlated with the pollen: charcoal ratio for most zones. Increases in the abundance of prairie taxa appear to be driven by increases in fire frequency in these zones.