Forest stands of fire-dependent ponderosa pine cover about 40 million acres (16 million ha) in the Western United States. Ponderosa pine is commonly found in pure stands on dry sites, but in more moist conditions, it is associated with Douglas-fir, lodgepole pine, western larch, and others. Historically, these were often widely spaced stands of large pines with an undergrowth of grasses and forbs. This structure was maintained by frequent surface fires that reduced invading tree species, rejuvenated the understory vegetation, and created a seedbed suitable for ponderosa pine regeneration. Today, by contrast, pine stands are dense, closed canopy stands, often with thickets of small trees in their understories. As a result, these forests are experiencing insect and disease epidemics
and severe wildfires.