In the western interior of the United States, more than 73 million acres of national forests are at risk from insect attack and catastrophic fire because of the major change over the years in their vegetative structure and composition. Some of this change has been caused by 50 years of successful fire suppression that has created forests with significantly overstocked stands of small-diameter trees. This dense understory in the forest creates a ladder-type fuel (allowing fire to grow from ground level into the canopy fueled by vegetation at different heights) that can lead to high-intensity crown fires, which spread fast and can cause catastrophic alterations to forested landscapes and watersheds. The western wildfires of 2000 demonstrated the devastating impact these dense, overstocked stands can have on forests.