For most of the 20th century, U.S. federal fire policy focused on suppressing all fires on national forests. The goal was to protect timber resources and rural communities, but this policy ignored the ecological importance of fire. North American forests have evolved with fire for thousands of years. Fire returns nutrients to soils, encourages growth of older fire-resistant trees, and promotes establishment of seedlings.
Decades of fire exclusion have produced uncharacteristically dense forests in many areas. Some forests, which previously burned lightly every 15-30 years, are now choked with vegetation. If ignited, these forests erupt into conflagrations of much higher intensity than historic levels. Grasses, shrubs, and saplings in the understory now form a fuel ladder, through which flames can climb to the forest canopy, killing entire forest stands.