Maybe those "natural fires" happened pretty often. Lightning and human intervention, campfires and such.  When my grandparents came here in 1903 there was lots of bush land but no big trees. Apparently prairie fires frequently burned through the area and thus no trees survived to grow to any great size. With cultivation of the prairie it formed fire breaks and that started a change. There used to be abundant sharp tailed grouse and they were "harvested" for meals sometimes. We would starve now on the number that are left.