It seems to me that if the plains were once forested, lightning strikes took out the trees and the Indians had no way of stopping the fires. Lightning is more rare in the Pacific Northwest, and accompanied with heavy rain that lasts a long time, so the forests there were for the most part preserved for hundreds of years, but the lightning in the midwest is a different story. Even in the Northwest, millions of acres of forest burned from lightning.
I find it hard to believe the plains were ever forested. Going back 30,000 years to the Wooly Mamouths the area has been populated by plains animals, not forest animals. The land is largely more suited to grass. In fields that have been essentially abandoned since the depression, trees grow in all the low areas along creeks and rivers, but after 75 years there are still no trees growing on the prarie.
There is a lot of evidence of fire, probably lightning caused, clearing forested areas all round the margins of the prarie.
On the other hand the evidence keeps improving that indian populations were much larger than we thought only a few years ago, and that European disease ran ahead of the European settlers and decimated them.
We won, they lost, end of story.
So9