There are three different types of wildland fires, each with varying intensity. The first of these is a surface fire which is the most common type of wildland fire and burns along the forest floor. The second type of wildland fire is a ground fire. These are normally caused by lightning and burn on the forest floor in the humus layer (the dead organic matter on the floor such as leaves) down to the forest's soil. The final type of wildland fire is a crown or canopy fire. These are high intensity fires that jump along the top of trees and are spread by wind.
Elements of a FireWithin every fire event, there are three elements (known as the fire triangle) that interact to ignite flames. The first of these is heat, which is required for fire ignition. Heat is essential to fires because it removes moisture from fuel and warms the air, allowing the fire to follow an unobstructed path. In wildland fires, the natural heat and ignition source is mainly lightning but people can contribute via such things like campfires.
Fuel is the second element of fire and is characterized as any combustible material. In wildland fires, fuel consists of vegetation; different types of vegetation have different fuel characteristics. Some plants, for example, contain resins or oils that cause them to burn more easily and more intensely than others. Dead plant biomass like leaves is also an important source of fuel for wildland fires. In addition, the drier and more closely spaced the fuel is, the easier it burns. Topography also plays a role in a fire's fuel as they cannot spread if there are large natural breaks in vegetation such as lakes, ridges, or rivers.
Oxygen is the third component of the fire triangle because it supports the chemical processes that take place during a fire. As fuel like leaves burn for example, it reacts with the oxygen in the surrounding air and releases heat, which allows fires to grow and rapidly spread.
In addition to the elements found in the fire triangle, spread and intensity are two important components of wildland fires. The rate of spread is the distance a fire travels during a given amount of time. Fires spread via the advance of the burning front (the front line of fire's movement) but also through the ignition of spot fires by flying embers.
Wildland fire intensity is based on the fire's heat and impact on the area's vegetation. Surface fires for example are low-intensity and burn only ground vegetation while leaving trees intact. Crown or canopy fires are the hottest and most intense wildland fires and can destroy entire forests.
Wildland Fire GeographySince topography and vegetation structure play a large role in the development of wildland fires so too does geography and latitude. For example, areas in the subtropical region often feature grasslands that have low topographic relief, are dry, have low humidity, high temperatures, high winds, and lightning. Due to these factors, portions of subtropical areas like Africa, Australia, and South America experience frequent, large wildland fires.
By contrast, the arctic tundra features a topography with lakes and rivers and has small shrub vegetation. In addition the area's climate is cold, moist, and has few thunderstorms so it experiences few large wildland fires.
Natural Wildland Fires and Plant AdaptationsNatural wildland fires are mainly caused by lightning. Because certain geographic locations experience frequent lightning storms, over time ecosystems in such areas developed to include fire as a natural component and plants have different adaptations that allow them to survive or reproduce during wildland fires. Pines for example have serotinous cones which remain closed and hold in their seeds. When heated by fire, they open and release the seeds, allowing new trees to reproduce post-fire.
Some trees such as sequoias and redwoods have also developed thick fire retardant bark. Because such bark is a poor conductor of heat, these trees can withstand extreme temperatures and survive wildland fires. In addition, these types of trees also normally have high branches so surface and ground fires cannot reach their tops.
Finally, areas prone to natural wildland fires can benefit from fire in that it destroys harmful bacteria and fungi on the ground that can harm seeds before germination and recycles nutrients in the humus layer upon burning.
Human Impacts on Wildland FiresToday, about four out of five wildland fires are caused by people. More importantly, humans have impacted wildland fires through fire suppression. With fire suppression, ecosystems adapted to fire have suffered, because the lack of natural fires reduces clearance of dry brush, causing fires to become more severe when they do start.
For example, in 2008, 1,339,839 acres were burned in wildland fires in California alone. This is because people are moving into undeveloped areas and causing fires but also because fire suppression has increased the amount of fuel. With more fuel, fires become more intense, burn hotter, and turn into canopy fires as high brush (called ladder fuel) allows fire to climb high into trees. Because such fires are more intense, it is more difficult to put them out and firefighters often have to wait until burning front runs out of fuel, causing more acres to burn.
To prevent these intense fires, many agencies are now using controlled burns to clear out brush and re-establish small wildland fires as a natural part of forests and grasslands. Additionally, various governmental entities require property owners to clear a certain amount of brush or forest away from their homes as a defensible fire protection space. The required brush clearance might be 200 feet (61 meters) or more.
To view the current wildland fires burning in the United States visit the US Forest Service's Remote Sensing Applications Center.