Typically, for wildfires in conifer forests to become large, some degree of crowning must occur. A common axiom in wildland fire management is that approximately 95 percent of area burned is generally caused by less than about 5 percent of the fires. A forest fire at the very minimum doubles its spread rate after the onset of crowning, and the area burned for a given period will be at least four times what would have been covered by a surface fire. In other words: the area burned is proportional to the rate of spread increase (following the transition to crowning) to the power of 2. Thus, if a fire triples its rate of advance after crowning, the area burned will be nine times greater than had it remained as a surface fire.