Landscape Vegetation Pattern, Composition, and Structure of DuPage County, Illinois, as Recorded by the U.S. Public Land Survey (1821-1840)
Date createdDecember 1998
DescriptionReport to the DuPage County Forest Preserve District, Chicago Wilderness, Fish & Wildlife Service & Max McGraw Wildlife Foundation.
AbstractWe mapped and analyzed the notes and bearing tree data from the DuPage County, Illinois Public Land Survey (PLS), which was conducted between 1821 and 1840. Our primary objective was to describe the pattern and structure of vegetation present at that time, and to determine its relationship to landscape fire processes. The vegetation described by the PLS was 80% prairie. The predominant woody vegetation was described as timber,
with smaller amounts of scattering timber, barrens, brush, and hazel thickets. Timber was situated along major
streams, and was dominated by white oak, bur oak, red oak, and hickory, which are considered fire-tolerant. Maple,
basswood, ash, and elm, which are less fire-tolerant, were infrequent and essentially restricted to areas of timber on
the eastern sides of water courses. More than 50% of the woody vegetation also had woody undergrowth in which
American hazelnut was the most frequent species. Tree density and frequency of woody undergrowth were inversely correlated, with low tree density and high frequency of undergrowth in barrens, and the reverse in timber. We also classified vegetation woody types based on bearing tree densities into open savanna (>0-10 trees/ha) savanna (>10-50 trees/ha), woodland (>50-100 trees/ha) and forest (>100 trees/ha). Among these groups, savanna was the predominant and most widespread type of timber, and was oak-dominated. Woodland and forest were much less abundant and tended to occur on the lee side of landscape firebreaks. They were also oak dominated, but supported maple, basswood, ash, and elm at low densities.
These vegetation descriptions fit an expected landscape fire model, in which timber persisted in landscape positions protected from eastward moving prairie fires driven by prevailing winds. These fires also converted timber into scattering timber and then into barrens by reducing tree densities and promoting post-fire sprouting of woody vegetation. This process restricted forests with high tree densities and fire-intolerant species to landscape positions afforded protection from prairie fires. Prairie and savanna with fire-tolerant oaks developed in areas with little fire protection, primarily on the western sides of landscape fire barriers.
Recommendations for managing and restoring woody vegetation based on pre-European structure and composition must take into account the single time period of the PLS data. A general management objective should be to restore fire processes and oak-dominated vegetation in order to maintain biodiversity associated with oak ecosystems. Landscape features can be used to help focus restoration objectives, with management for greater woody undergrowth and forest species diversity in areas with more fire protection, and management for greater prairie and savanna species diversity in areas with less fire protection. However, the landscape scale and processes in operation prior to settlement are difficult to replicate in modern fragmented ecosystems, and experimentation is needed to understand how to implement many of these objectives.