Ecology, Fire
Fire has been an agent of change in nearly every terrestrial vegetation type on Earth, shaping both the species composition and structure. The probability of occurrence and the effects of fire vary widely depending upon the amount of fuel present, topography, climate, sources of ignition, and present species composition of the area. Fires may be severe, causing great mortality of existing plants and significantly changing the species compositionof the burned area, or they may have little impact on the composition and consume only the dry dead plant material present. They may burn intensely as fast-moving fires with flame lengths greater than 25 meters, or they may occur as slow-moving fires with flame lengths less than 0.5 meter. These sources of variation influence the effects that fire has on the vegetation and the ecological role of fire.
Prior to human intervention, fires occurred very frequently in some vegetation types. Fire history studies in temperate grasslands in Africa and North America, some ponderosa pine forests of western North America, and longleaf pine forests of southeastern North America indicate that fires occurred at an average of less than every ten years. When fires occur frequently, the vegetation's composition becomes dominated by fire-adapted (tolerant) species.
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