The ability of nonfederal forests to provide a variety of benefits to the nation's citizens depends on the biological and physical conditions of these forests. For example, the U.S. Department of Agriculture's (USDA) Forest Service has identified potentially deteriorating and serious conditions of the nation's forests, which may be of concern when considering the future sustainability of forests, including nonfederal forests (Box 5-1). These conditions include loss of biological diversity, diminished water quality, effects of global climate change, and increased timber mortality. An assessment of these ecological trends and conditions is provided in this chapter. In addition, the effects of management intensity, forest fires, air pollution, climatic change, insects and disease, alien plants, and watershed characteristics on forest conditions are discussed.
The term "forest" encompasses an enormous diversity of forest types and structures. The United States contains some of the most magnificent and biologically diverse forests of the world (Box 5-2). The particular kind of forest that occupies an area is determined by the interactions of several elements: the frequency and intensity of natural and anthropogenic disturbances, local environmental variability, and the available gene pool. Interactions among these elements