Just for the record, I did admit to a bit of hyperbole. This explains what I was getting at:

Jonah,
“The great plains used to be a giant forest. The Indians burnt it to the ground to hunt buffalo.”
I suspect the only legitimate criticism here is one of degree. Much of the Great Plains has always been grassland. Outside of river valleys trees haven’t grown in recent centuries in western KS or eastern CO, for instance. Not enough rainfall.
However, the Indians expanded this area of grassland over large areas of what we now call the Midwest and upper South: (eastern KS and NE), IA, MO, IL, IN, OH, KY, etc.
So the Indians didn’t create ALL of the grassland, they just probably doubled it in size.