Recurring fires are part of the natural environment--as natural as rain, snow, or wind (Heinselman 1978). Evidence of past fires and their periodicity is found in charcoal layers in lakes and bogs; and in the fire-scarred cross sections of trees. Heinselman indicated that fire-adapted ecosystems in North America are termed fire-dependent, if disturbances by fire are essential to the functioning of these systems (1978). Fire affects the functioning of ecosystems in numerous ways:
• Regulating plant succession
• Regulating fuel accumulations.
• Controlling age, structure and species composition of vegetation.
• Affecting insect and disease populations.
• Influencing nutrient cycles and energy flows.
• Regulating biotic productivity, diversity and stability.
• Determining habitats for wildlife.
Lightning, volcanoes and people have been igniting fires in wildland ecosystems for millennia. The current emphasis on ecosystem management calls for the maintenance of interactions between such disturbance processes and ecosystem functions. It is incumbent, therefore, on resource managers and fire managers to understand the historic frequency, intensity and areal extent of past fires. Such knowledge provides a frame of reference for prescribing appropriate management practices on a landscape scale. Many studies have described the historical occurrence of fires. Swetnam (1993), for example, reported on 2000 years of fire history in giant sequoia groves in California. He found that frequent small fires occurred during a warm period from about A.D. 1000 to 1300, and less frequent but more widespread fires occurred during cooler periods from about A.D. 500-1000 and after 1300. Swain (1973) determined from lake sediment analysis in the Boundary Waters Canoe Area in Minnesota that tree species and fire had interacted in complex ways over a 10 000 year period.
An even larger body of science details the numerous effects of wildland fires on ecosystems. It is this knowledge of fire history, fire regimes and fire effects that allows managers to develop strategies for the management of fire in various ecosystems. For those ecosystems that are fire-adapted, fire prescriptions and prescribed fire programs are developed to achieve a variety of resource management objectives. Other ecosystems in the world that are fire sensitive are better served by excluding wildland fires through well organized and staffed fire protection programmes.
