Disturbance caused by diseases, by themselves or in conjunction with disturbance by insects, abiotic factors such as drought, fire and wind, and, increasingly, human activities, has played a critical role in the dynamics of many forest ecosystems in North America.
In the predominantly coniferous forests in western North America there are considerable areas undisturbed directly by human activities. In these areas, diseases kill trees or predispose them to other agents of disturbance, resulting in gradual change in stand composition and structure. In areas disturbed by forest management practices of harvesting or exclusion of fire, increased disease incidence and severity has increased the damage caused by disease, and consequently, the rate of change.
In the absence of introduced diseases in the predominantly deciduous forests of the Appalachian region of eastern North America, forests are relatively healthy. Here, forests are disturbed significantly by disease only after they are disturbed or stressed by other agents, predominantly defoliating insects and drought. In the eastern montane coniferous forest, chronic wind damage is a major predisposing factor to disease. Past harvesting practices, introduced diseases and insects, and fire exclusion have in some instances resulted in large areas of similar species and relatively similar ages that exacerbate the magnitude and severity of disturbance by disease.
Key words: tree diseases, stress, North America.
Correspondence: Duncan Morrison, Pacific Forestry Centre, Canadian Forest Service, 506 West Burnside Road, Victoria, B.C. Canada V8Z 1M5