National Geographic : 1996 Sep
Nature's ancient alliance with fire can be seen in ecosystems all over the U.S. The West's vast ponderosa pine forests are among the most fire dependent. Denied a fire every 5 to 25 years, they become primed for destruction. Density and diversity A healthy ponderosa forest is made of widely spaced, fire resistant trees. With over protection (bottom), young trees and competing species make a flammable understory so shaded that ponderosa seedlings can't grow. Forest floor If burned often, a forest floor is a shallow layer of duff on soil that accepts ponderosa seeds (inset below). Over protected, the floor builds deep layers of debris (inset bottom). Seed germination declines; fires burn hotter. The gift of fire Potential natural vegetation I Broadleaf forest Needleleaf forest Mixed forest Forest and grassland I] Grassland L Grassland and shrub I Shrub Scale varies in this perspective. NGSCARTOGRAPHIC DIVISION California chaparral: Densely growing mass of many species of shrubs and low trees. Explosive fires scour hillsides bare every 20 to 50 years. Ponderosa pine: Spa cious forests of trees hundreds of years old. Frequent fires (5to 25 years) clear ground but seldom kill large trees. Lodgepole pine: Domi nant in Yellowstone National Park; grows in dense stands. Sections burn wholly every 200 to 400 years.