The combination of human-caused and natural fires kept the fuel-load low enough to prevent intensely hot fires from which the eco-system cannot recover. A simplistic model is that low temperature burns allow for a cycle of wildflowers, replaced by stump-sprouting brush, replaced by trees, along with the variety of animal life supported by this ecosystem, then fire ignites and the cycle repeats. Suppressing fire results in an overgrowth of woody vegetation which results in hotter fires which incinerates the soil, thereby destroying all plant and animal life that would normally survive below the surface of the soil, and the cycle cannot repeat.