The pre-European settlement ponderosa pine forests of the Mogollon Rim consisted of open stands of uneven-aged trees with a significant grass-forb understory. Light surface-fires occurred on an average interval of 2 to 12 years in Arizona and New Mexico (Dietrich 1980). These fires consumed forest floor material, burned most of the young regeneration, and promoted growth of a dense, grassy understory. Catastrophic crown fires were rare due to lack of ladder fuels, and the clumpy, widely spaced ponderosa pine canopy (Sackett 1980). Fine fuels reduction from heavy sheep and cattle grazing and then modern forest fire suppression resulted in the development of dense, overstocked stands.