The Great Plains had no trees to speak of, but the fertile lands of the Great Lakes states were covered by deep forests. It was states like Ohio Illinois Michigan etc that fed the new Republic until the frontier moved west.
The mid-west extends all the way to the eastern Ohio border and the Ohio Valley was once the homeland of multiple Siouxian tribes before the Huron and other tribes pushed them further west starting in the 15th century.
The Mandan, in particular, were quite adept at agriculture and cleared forests to make fields, using the timber to build earthen lodge villages both in their original homeland of the Ohio River Valley and they new adopted homeland of what is know the Knife and Missouri River Valleys in North Dakota.
By the time the white man came, the massive bison herds were much smaller herds at points in the eastern mid-west and didn't become massive until the grasslands really started about near what is now Lincoln, Nebraska in the more westerly part of the mid-west.