Shades of Green: Earth's Forests -- Fire

Fire is one of the natural disturbances that forests have evolved to deal with over time. Periodic fires caused by lightning have shaped and altered forests for millions of years. For example, the heat generated in fires actually enables Lodgepole pinecones to release their seeds. In this century, humans have made serious efforts at fire suppression. Ironically, this can lead to even worse fires, due to a build-up of needles and branches that would have been burned in more frequent small fires.

Fire kills trees directly, and the smoke it creates also contributes to air pollution. However, small natural fires actually benefit certain forests, and are necessary to the long-term survival of dry temperate coniferous forests. Photo credit Corel Photo Clipart CD.

The relationships between forests, fire, insects, disease, and climate are only recently beginning to be understood. The huge fires in Yellowstone National Park in 1988 caused many people to wonder whether a more natural approach to fire might be better than complete suppression. There is no doubt that fire destroys trees, which humans consider a waste. But the hidden benefits of natural fires might be greater than were previously thought.

Fires are often set intentionally in a process called "slash-and-burn" agriculture. A farmer clears the land by setting fire to the trees, and is left with a charred patch of open land after the flames subside. Though this may fertilize the soil for a while, it opens up a whole new set of problems, and carbon dioxide is released into the atmosphere, which contributes to the greenhouse effect, providing even more problems. The "haze" in Indonesia and Malaysia during 1997 was the product of huge fires burning in the rainforest, many of which were originally set to clear land and then got out of control.

Now mostly in tropical regions, fires are set to clear land for agriculture and pastures. Photo by Maya Walters.

Effects from the haze will continue to be seen well into the future, as many of the millions of people exposed to the pollution will develop health problems associated with such exposure. The haze was particularly bad because many peat bogs were in the burning areas, and large quantities of carbon particles were being released into the air. These peat fires are particularly dangerous. Peat has been accumulating in lowland areas for thousands of years and the bogs can be 20 feet deep. They are usually moist and excellent at soaking up and storing rainwater, preventing flooding. When it is dry, however, the peat burns easily. Fire moves deep underground in the dry peat bog, where it is almost impossible to put out. Fires that began in 1983 are still burning underground in peat bogs today. One of the major sources of air pollution in the 1997 haze was from a one-million hectare peat bog on the island of Borneo, which the government was draining and it began to burn.

An aerial view of a recently deforested and burned tract of land in the Guatemalan rainforest. This land will soon be used for cattle grazing. Photo courtesy of Dana Slaymaker.

Forest fires move in varying and often unpredictable ways. Ground fires creep through the duff, and fires may smolder below the surface for long periods of time. Surface fires are another form of forest fire, and move along at up to 1.3 meters in height. Crown fires occur higher in the trees (in the upper branches known as the "crown" of a tree), and can be of several different varieties. These include "dependent crown fires", which use convection* to pre-heat the crown, and most dangerous of all, independent crown fires which leap from tree to tree.

Most forest fires, especially those near houses, are now suppressed. Once the pattern of fire suppression begins, it becomes necessary to continue fighting fires. This is because dead branches and needles collect on the forest floor for many years, and when a fire does occur eventually, it will be much larger and have the potential to destroy a vast area. The smoke released in huge fires contributes to global warming and climate change.

Many fires are set by lightning. But lightning is usually accompanied by rain, so most of these small fires are put out quite quickly. Photo by Maya Walters.

Suppression, or actually fighting a fire, can be done in hundreds of ways, depending on the spread rate, intensity, and many other factors. Ground fires can be contained by digging trenches around the fire, surrounding it and removing it from other fuels. The most common type of fire is the surface fire, and it is battled by creating fire lines, utilizing back pack water pumps, and calling in aerial water bombers for inaccessible areas. Crown fires are very dangerous and are most often fought from a distance. Fire-retardant chemicals may be sprayed on buildings in the fire's path.

Several other factors influence the way fires move. Wind can turn fires in unpredictable directions, surrounding or engulfing fire fighters in flames. It can increase the rate of spread and move flaming particles through the air, causing blazes to flare up in other places.

Topography is a variable that can be considered in simple terms or in advanced ways. For instance, a hill places the fire below closer to the fuels above, therefore making it easier for the fire to spread. Topography also influences the movement of air, and since warm air rises, an uphill slope pre-heats the trees above, allowing fires to move faster uphill than down.

Fires affect forests in various other ways. In intense fires, nitrogen, calcium, phosphorus, and potassium can be vaporized; and soil microorganisms can be killed up to 3 inches under the soil, but recover quickly. Mycorrhizal fungi growing on tree roots can also be killed by fire. Recovery of trees takes longer than other organisms. A year after the Yellowstone fires of 1988, grass and inch-tall lodgepole pine seedlings were spotted emerging from the soil. Certain plants have adaptations which help them recover more quickly after a fire. Chaparral shrubs benefit from their large root systems, as their taproot can grow to 3.5 feet in length after only three months. Other plants sprout from roots instead of seeds, giving them an advantage.

Animals can be severely affected by fires. Their habitats are most often destroyed, including their food source which means that even if they escape the flames, they have to find new territory. Many animals do escape from fires, and move to another area. Other animals which live in the surrounding forest come to feed on the new seedlings and young plants growing in a recently burned area.

view the regular version of the fire article for faster load time

return to the list of condensed articles