Elimination of the historic pattern of frequent low-intensity fires in ponderosa pine and pine-mixed conifer forests has resulted in major ecological disruptions. Prior to 1900, open stands of large, long-lived, fire-resistant ponderosa pine were typical. These were accompanied in some areas by other fire-dependent species such as western larch. Today, as a result of fire exclusion, most stands have dense thickets of small trees and are experiencing insect and disease epidemics and severe wildfires. These forests cover about 40 million acres in the Western United States and are the focus of concerns about declining forest health (American Forests 1995; Phillips 1995).