From what I've read about the period the vegetation was what has been called mosaic, patchy. Clumps of trees provided micro-climates where warm-loving species could survive on the south side or underneath. Fire and the extinction of the megafauna destroyed this mosaic vegetation so now we have huge expanses of mono-culture. I've even read that the vast treeless praires of North America didn't exist humans arrived and performed the above. Maybe I'll look for a link.
The American prairies were formed by huge herds of buffalo grazing. On a side note sage brush is now taking over land that once had waist high grass. Why? We have removed the cattle that took over for the buffalo. The grasslands were formed under intensive grazing. Some studies have been done on how to graze cattle to bring the land back to that. Sadly enviros pay no attention to the science, their bias lives on.
The huge herds of buffalo were possibly that size because the Aboriginal people that hunted them had died off do to small pox spreading after first arrival.
I just found some science to back up what I was saying about the northern climates. I can also point to other discussions that are taking place on this subject.
