Presettlement Vegetation of Two Prairie Peninsula Counties

Based on the original General Land Survey records (1821-24), the presettlement vegetation of McLean and Mason Counties, Illinois, included four vegetation types (prairie, savannah, open forest, and closed forest), separated on the basis of tree density. Density, dominance, frequency, and Importance Values were estimated for tree taxa in each vegetation type by applying a modification of the quarter method to the information in the survey records. The presettlement vegetation of McLean County was mostly prairie (89.5%). Savannah, open forest, and closed forest, dominated by white oak (Quercus alba), comprised 5.4%, 1.8%, and 3.3%, respectively, of the total vegetation. About 0.9% of the county is still forested today. The presettlement vegetation of Mason County consisted of 67.7% prairie, 14.4% savannah, 4.3% open forest, and 9.0% closed forest. Lakes and swamps covered 4.6% of the county. Presettlement forests were dominated by black oak (Q. velutina). Presently, about 2% of the county is forested. In both counties, forested areas occurred on rugged topography associated with streams, rivers, glacial moraines, and sand dunes. Significant associations were found between vegetation types and soil associations. Generally, the soils in the forests differed from those in the prairies in presettlement times.