Greetings Brian:
The landscape we envision as pre-settlement Illinois consisted of 38-40% forest (13.8 million acres of forest), 60% prairie, and 1% water. By 1924, forest cover was reduced to a mere 3.0 million acres. Therefore, the forest landscape has changed dramatically over the last 100 years--agriculture; urban development, transportation and industrialization; wood for fuel, railroads, and construction, etc..
However, Illinois has gained 1.5 million acres of forest land since Telford's 1924 forest inventory of the state. Illinois now has approximately 4.5 million acres of forest land due in large part to state and federal conservation programs, professional forestry assistance, and landowner stewardship efforts.
Additional reading on this subject may be found here:
http://ifdc.nres.uiuc.edu/publications/foresrc02.htm
