Governmental Land Office records were used to reconstruct the presettlement vegetation of Williamson County, Illinois, about 1807. The vegetation was mapped into four categories, prairie, savannah, and open and closed forest types. Oak-hickory forests were the predominant vegetation occupying 81.2% of the area; savannahs covered 16%, and prairies 2.5%. Savannahs, prairies and open forest were more predominant in the flatter northern 2/3 of the county, while closed forest was most abundant in the southern part where the topography is more rugged. Forest areas supported 109.2 sq. ft. of basal area/acre (24.1M2/ha.) and 64.7 trees/acre (159.8/ha.), and savannah 15.1 sq. ft. of basal area/acre (3.5 M2/ha.) and 6.4 trees/acre (15.8/ha.). Thirty-five tree species were recognized by the surveyors. White and black oaks were the dominant tree species in both forest and savannah. Tests for species association revealed that most species were negatively associated with other species. However, many species were positively associated with themselves as pairs of witness trees.