The boreal forest, or taiga, predominates as a vegetation type in northern circumpolar countries, covering in excess of 12 million square kilometres, primarily in Russia and Canada, with lesser amounts in Scandinavia, China, and the United States (Alaska). Lying generally between 45 degrees and 70 degrees N latitude, and accounting for approximately 25% of the world's forested area, the boreal zone contained extensive tracts of coniferous forest, primarily pine, spruce and larch, which provide a vital natural and economic resource in northern countries. Forest fire is the major disturbance in the boreal zone, boreal species having adapted over millennia to large-scale natural fires, to the point where fire is generally required for adequate species regeneration. When viewed on a large scale, the boreal forest is an irregular patchwork mosaic of even-aged, fire-origin stands, and is a classic example of a fire-dependent ecosystem, capable, during periods of extreme fire weather, of sustaining the large, high-intensity, stand-replacement wildfires which are responsible for its existence.