When you hear the word 'fire,' what comes to mind? Were you ever afraid of it? Mesmerized by it? Comforted by it? No matter where or how you have experienced fire, it is essentially the same. Fire is a chemical reaction.
In order for fire to burn, three elements must be present. Oxygen, fuel and heat combine to make what is called the “Fire Triangle.”
Oxygen, fuels, and heat are around us all the time. So why don't we see fire more often? The answer lies in the details.
Oxygen is pretty easy. Fire needs air that is about 16% oxygen. The earth's atmosphere is 21% oxygen. Fuel is anything that will burn. In the outdoors, that often includes wood, grass, shrubs, pine needles and the like. The presence of heat will vary. Wood needs to be about 617 degrees F to burn.
If any one of these elements is missing or not present in the right form or amount, one “leg” of the fire triangle collapses and no burning occurs. So, once a fire is ignited and there is enough fuel and oxygen for it to burn, the fire will create all the heat it needs to sustain itself. The more fuel, the higher the temperature. The higher the temperature, the faster the fire spreads. The more the fire spreads, the more it “preheats” or heats the fuels around it increasing its size and the temperature around it. And the race is on!
All of us have seen fire. Technically, the fuel you see burning isn't really on fire. Instead, the fuel is being converted into a gas. It's the gas produced by the fuel that's burning. Next time you are watching a log burning in your fireplace, see if you can see a space between the log and the flame. I'll bet you can!
Not All Fires Are Alike
If you listen to the news or if you talk to people who work with fire, you will hear it described in several ways. Here are some terms that will help you understand what is going on.
Wildland fire is one of nature's oldest phenomena. Evidence of free-burning fires has been found in petrified wood and coal deposits formed as early as the Paleozoic Era, about 350 million years ago. Wildland fire is any fire burning in wildlands, including wildfires and all prescribed fires.
A wildfire is a fire is one that is out of control and generally viewed as undesirable by land managers. It needs to be put out or suppressed. An example of a wildfire might be one that is burning the habitat of an endangered animal like the sage grouse, as has been the case in southern Idaho the past few years. Managers would call for fire fighters to suppress this fire.
A prescribed fire is one that is considered desirable by managers because it meets some management objective. They can be naturally ignited, such as those that are started by lightning, or they can by lit by land managers to accomplish a specific task. Burning logging debris following a logging operation would be one example of a time that managers might ignite a fire. Allowing a lightning-caused fire to burn because it is clearing out dead branches and needles on the forest floor of a ponderosa pine forest would be an example of a prescribed natural fire.
Fire in Ecosystems
It is important to remember that fire behaves differently in different ecosystems. The lodgepole pine forest depends on fire to survive because the lodgepole cones need fire to open them so seeds can be released.
Repeated fire in sagebrush-steppe country can destroy the sagebrush, an important part of that system. Ponderosa pine forests benefit from an occasional ground fire to help clear the forest floor of competing grasses and young trees. A healthy ponderosa pine forest has trees that are spaced far apart so that sun can reach the ground and grasses and shrubs can grow.
Fires spread in three general patterns: ground fires, surface fires, and crown fires.
Ground Fires — These fires burn organic material in the soil beneath the litter on the surface. They burn by glowing combustion.
Surface Fires — Surface fires have a flaming front and burn leaf litter, fallen branches and other materials on the ground.
Crown Fires — These fires are the hottest and most intense. They are often difficult to control, need strong winds, steep slopes and lots of fuel to keep burning. Crown fires burn the top layer of foliage on the tree.
Once a wildfire is started, the way it behaves is determined by the current weather conditions, the amount of humidity in the air, the type and amount of fuel available to the fire, and the topography of land the. Because live plants contain so much water, they are less likely to burn than dry logs and branches or stems.
High winds can create small fires out in front of a large fire by blowing embers into the unburned fuel. These fires are called spot fires and may burn some trees and shrubs and leave others untouched.
A large fire can even create its own wind. As the fire heats the air around it, the air quickly rises. Cool air rushes in to replace the hot air, which creates a wind and increases the supply of oxygen to the fire.
Trees can explode if water deep inside the tree turns quickly to steam.
After the Fire
After a fire, the hard work of rehabilitating the landscape begins. Most of it needs to be done quickly because there is often little to hold the soil in place, and erosion can be a big problem. This is especially true if the burn is on a steep slope. This was the case in Boise in both 1959 and 1996 when the Boise Foothills burned.
Stablizing and Rehabilitating the Land
Land managers use a variety of techniques to stabilize the soil and rehabilitate the land after a fire.
Burned trees are cut down and logs are laid horizontally across the hillside. Most of the logs are 8–14 inches in diameter and held in place by wooden stakes. A small trench may also be dug on the uphill side of the log to collect water and store sediment.
In the same area as the contour-felled logs, crews use hand tools to dig more small trenches uphill from the logs. These horizontal trenches catch water and dirt as it flows down the hill.
A mini-excavator is used to dig larger horizontal trenches along the hillside. These trenches are usually 2–3 feet wide and 2–3 feet deep. Every 30–50 feet, dirt is piled into the trenches to create a dike. If a trench breaks, the water and soil will stop at the dike and not continue down the hill.
Wattles are made of straw wrapped in a mesh that will break down in sunlight. They are about 8 inches in diameter, 25 feet long, and weigh about 35 pounds. They are placed horizontally across the hill, with stakes holding them in place. Wattles slow the water and soil moving down the hill and provide a good seed bed for future seedlings.
From time to time horizontal strips of earth will be tilled 6–12 inches deep to allow water running down the hill to soak into the ground. This also provides a good seed bed. Bands of undisturbed earth are left between the tilled rows allowing plants surviving the fire to resprout.
Straw-Bale Check Dams
Dams made of three or more straw bales are built across gullies to slow water and soil as they wash downhill. The straw bales are wrapped in wire mesh to help hold them together. Then they are covered with a strong cloth. The straw and cloth are porous, allowing water to seep through while collecting the sediment behind the dam. There are usually 3–8 dams per gully. As the water runs down the hill, its velocity is slowed as it is routed from basin to basin behind each dam.
Sediment Ponds and Basins
Ponds and basins are built in gentle stream channels or at the base of hills to trap and store water running downhill. As the water sits in the pond, it soaks into the ground while providing a water supply to wildlife in the area.
A rangeland drill pulled by a tractor is used to seed many burned areas. Round disks cut furrows into the ground and seeds are dropped from long tubes behind the discs. Chains dragging behind the tractor help cover the seeds with soil so they sprout.
Some seeds, such as sagebrush seed, is aerially distributed from a bucket carried below a helicopter. It is usually dropped onto snow-covered ground so the seed will stay moist and sink into the ground as the snow melts.
Seeding is sometimes done by volunteers using hand-held spreaders. They are followed by other volunteers who rake the seed into the ground.