I believe the trees and woodlands were kept in check by the bison more than fire.
Improper grazing, burning and mechanically clearing juniper and mesquite are probably what are provoking them out of their original habitats - which were considerable.
I would also conjecture that humus rich soils, several feet deep broke down plant litter rapidly and that even when lightning struck, fires were much harder to get going than today because the land is now so much more brittle and tender than it was 500 years ago.Keep in mind that there were hardly any earth worms in NA at the time of European settlement. The duff layer may have had more staying power rather than less. My experience is that areas that do not get burned frequently have very hot fires when they do burn as the years of accumulated fuel are there to support strong fires. Grazing animals could limit this considerably of course.
It may be that there were vast treeless expanses in the Great Central Plains or Prairies but I believe that such treeless-ness ends at the line between the Texas High Plains and Rolling Plains. One of Texas largest contiguous forests is reported to have been in the Rolling Plains. Even supposed desert bioregions such as the Trans Pecos are famous for dwarf oak forests.
Kirby Fry
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