Wildfire

Wildfire is especially frequent in ecosystems that experience seasonal drought, for example, boreal forests, temperate pine forests, tall-grass prairie, chaparral, and savannah. Wildfires can be very extensive, and in aggregate they affect tremendous areas of landscape each year. For example, an average of about 8 million acres (3 million ha) of forest burns each year in Canada, and in some years it exceeds 25 million acres (10 million ha). Most of those fires are started by humans, although most of the actual burned area is ignited naturally by lightning. The natural fires predominantly affect northern, non-commercial forests, where most fires are not actively quenched by humans, so that individual burns can exceed 2.5 million acres (one million ha) in area. However, even vigorously fought fires can be enormous, as was the case of the famous Yellowstone fires of 1988, which burned more than 1.2 million acres (0.5 million ha), including 45% of Yellowstone National Park. Even moist tropical rainforest will occasionally burn, as happened over more than 7.4 million acres (3.0 million ha) of Borneo during relatively dry conditions in 1982-1983.

Fire causes a number of changes in soil quality. Depending on the intensity of the burn, much of the organic A climax forest (1) destroyed by wildfire (2) and its recovery (3, 4). Illustration by Hans & Cassidy. Courtesy of Gale Group.
matter and litter of the forest floor may be consumed, and mineral soil may be exposed. The combustion of organic matter results in a large emission of carbon dioxide to the atmosphere, along with gaseous oxides of nitrogen derived from the oxidation of organic nitrogen, and sooty particulates. The layer of ash that deposits onto the soil surface is of a basic quality, so soil acidity is temporarily decreased after a fire. The ash also supplies large quantities of certain nutrients, especially calcium, magnesium, potassium, and phosphorus, some of which leaches from the site. Often, post-fire soils are relatively fertile for several years as a net effect of these physical and chemical changes, and plant growth can be rather lush until the intensity of competition increases when the canopy closes again.