fire ecology

FIRE ECOLOGY Perhaps the image most often associated with wildland fire is one of destruction. While it is true that fire has a great potential for the destruction of homes, wildlife, and even human life, fire is just one of many natural forces within ecosystems. Fire Behavior Fire behavior is most often described by intensity and spread. Many factors influence this behavior. Five factors that influence intensity are available fuel, moisture and temperature, fuel composition, wind, and topography. Available fuel is quantified by size and arrangement. The more available fuel, the more intense the fire and cool, moist fuels combust more slowly than hot, dry fuels. Fuel composition can make a fire more or less intense. Oils and resins increase the heat yield of the reaction and cause a fire to burn intensely whereas other chemical factors, such as high concentrations of miner- als, can reduce flammability. Wind increases oxygen supply, convects heat and can produce “spot fires” from fragments that blow down-wind. Finally, topography effects intensity. A fire ignited on the top of a slope is likely to spread slowly as it burns downhill, whereas a fire at the bottom of a slope will start rapidly and gain momentum as it burns uphill because warm air rises and preheats uphill fuels. Many of the factors that affect intensity also affect the rate of spread. For example, fires in dry, windy conditions with abundant fuel spread rapidly. Fuel continuity and topography also play a role in spread. Topographic features such as streams and lakes can create firebreaks, thus influencing the distribution of burns across landscapes. Finally, the composition of plant communities affects spread, as some species are more flammable than others. What is Fire Ecology? Fire is a natural component of many ecosystems, which include plants and animals that interact with one another and with their physical environment. Fire ecology examines the role of fire in ecosystems. Fire ecologists study the origins of fire, what influences spread and intensity, fire’s relationship with ecosystems, and how controlled fires can be used to maintain ecosystem health. The Physical and Chemical Nature of Fire For fuel to ignite it must be heated in the presence of oxygen to the ignition point or kindling temperature. Wood must reach about 800 degrees F to burst into flame. As the wood is being heated to this point it dries as water, oils, and resins are boiled away. The chemical structure of the fuel is broken down and flammable gases are produced. The ignition of these flammable gases is known as flaming combustion. Flaming combustion transforms the surface of the wood to charcoal. At cooler temperatures, glowing combustion con- sumes charcoal, producing ash, water, and carbon dioxide. Many factors such as fuel, weather, topography, and fire his- tory influence the probability of ignition and combustion. Elk near Sula Complex Fire, Montana Photo Courtesy of Alaskan Type I Incident Management Team Ecological Society of America The Effects of Fire on Ecosystems There is much yet to be learned about how wildland fire affects ecosystems. This is in part because each fire and each ecosystem has unique properties. However, some generalities can be made. Mosaic Patterns Wildland fires create a mixture of totally burned, partially burned, and unburned sections called a burn mosaic. The varying degrees of burn are a result of many factors including wind shifts, daily temperature changes, moisture levels, and varying chemical composition of the vegetation. The burn mosaic results in varied regrowth rates that creates a vegeta- tion mosaic. Soil Conditions Wildland fires can be both a detriment and a benefit to soil. The soil can become more nu- trient-rich after a fire due to the high mineral content of the ash and charcoal and also due to the warm, moist conditions that increase mi- crobial activity. The intense heat can also cause soil particles to become water-repellant, causin