and disease: (1) fragmentation of forest ownership and substantial increases in the number of owners of nonfederal forestland, making even minor insect or disease epidemics a substantial problem for individual forest owners; (2) inadequate management by forest landowners and unthrifty forests, resulting in substantial areas of susceptible forest; and (3) introduction of alien insects and diseases with no natural controls that attack native species. Many nonfederal-forest landowners, particularly nonindustrial forest landowners, do not have the technical knowledge or assistance to design prescription to protect against native insects and diseases.
In the West, native forest insects and diseases are increasingly attacking old forests at epidemic levels, which, in many cases could be protected against through appropriate management. Because many organisms are species-specific or group-specific and attack trees of low vigor, selection of appropriate species or management of a stand to provide adequate vigor might prevent epidemics. Thinning is often effective at reducing competition among trees and results in increased vigor of the residual trees (Waring and Pitman 1980). High-vigor trees are often successful at repelling attacks by insects, such as pine beetles. High-vigor trees that have adequate nitrogen also have been shown to be more resistant to pathogens, such as laminated root rot (Matson and Boone 1984). In such cases, active management can increase protection against insects and disease. For example, in the South, early cultural practices intensified fusiform rust incidence through the planting of infected seedlings, intensive site preparation, fire control, selection for fast-growing genotypes without consideration of disease resistance, and expansion of the range and extent of susceptible species. Today this particular problem has been mitigated partially through the development of rust-resistant tree genotypes and improved stand management (Schmidt 1978).
During the twentieth century, numerous insects and diseases have been introduced in the United States. Many did not find appropriate ecological niches and disappeared. Others found ideal conditions to flourish, at the expense of native species. Among the worst have been the European gypsy moth, Dutch elm disease (American elm), white pine blister rust (white pines), pine shoot beetle (conifers, especially pines), phytophthora root rot (Port Orford cedar), and chestnut blight (American chestnut).
Biotic diversity and wildlife habitat are seriously impaired by these organisms. Some, such as chestnut blight, have resulted in near extirpation of native species by killing the host. Other organisms will affect future losses: white pine blister rust damages mature and kills young whitebark pine, the seeds of which are a critical source of food for grizzly bears in the Rocky Mountains.
Plants that are nonindigenous to a geographic locality are called "alien," as well as "exotic," which does not convey the ecological risk posed by the more