Don't quote me on this but I think a lot of the change has to do with fire and flood suppression. Certainly on the Konza prairie and similar areas, small trees (post oaks and eastern red cedar) grew in natural fire breaks like bluffs, and individual trees would live for several hundred years. Floodplains would have large cottonwoods which can withstand seasonal inundation but wouldn't necessarily be thick forest otherwise. And the prodigious lightning storms and (throughout the Holocene) burning by native tribes for hunting kept trees off of the uplands.