Environmental historian Stephen J. Pyne, in his book entitled Fire: A Brief History, suggests that fire and flame can only exist on earth in the presence of the "living world". Our carbon based and flammable environment supplies all of the elements of chemistry for fire. I will review these elements in a moment. So, fire is dependent on, cannot exist without, and must follow the biology of life - those flora and fauna who then and now have adapted to fire for survival.
How Fire Came To Be
It is interesting to note that, of the four billion years of earth's existence, conditions were not conducive for spontaneous wildfire until the last 400 million years. Naturally occurring atmospheric fire did not have the chemical elements available until major several earth changes occurred.
The earliest life forms emerged without needing oxygen (anaerobic) to live about 3.5 million years ago and in a carbon dioxide based atmosphere. Life forms that needed oxygen in small amounts (aerobic) came much later in the form of photosynthesizing blue-green algae and ultimately changed the earth's atmospheric balance toward oxygen and away from CO2.
Photosynthesis increasingly dominated earth's biology by initially creating and continuously increasing the earth's percentage of oxygen in the air. Green plant growth then exploded and aerobic respiration became the biologic catalyst for terrestrial life. Around 600 million years ago and during the Paleozoic, conditions for natural combustion started developing with increasing speed.
Remembering the "fire triangle", fire needs fuel, oxygen, and heat to ignite and spread. Where ever forests grow, the fuel for forest fires is provided mainly by continued biomass production along with the resulting detritus of that vegetative growth. Oxygen is created in abundance by the photosynthesizing process of living green organisms so it is all around us in the air. All that is needed then is a source of heat to provide the exact chemistry combinations for a flame.
When these natural combustibles (in the form of wood, leaves, brush) reach 572º, gas in the steam given off reacts with oxygen to reach its flash point and with a burst of flame. This flame then preheats surrounding fuels. In turn, other fuels heat up and the fire grows and spreads. If this spreading process is not controlled, you have a wildfire or forest fire. Depending on the geographic condition of the site and the vegetative fuels present, you might call these brush fires, forest fires, sage field fires, grass fires, woods fires, peat fires, bush fires, wildland fires, or veld fires.
The Initial Wildfire Problem
Wildfire has been a natural force in North America for hundreds of thousands of years. Forests have developed around fire occurring both naturally and intentionally. Lightning is the most common source of naturally caused fires. Native Americans first used forest fires to encourage and increase game forage potential, to reduce the understory for easy travel and to herd potential prey toward hunters.
With European expansion over the past 400 years, European Americans as a society have grown to fear most forms of uncontrolled fire. This has lead to demands for state and federal agencies to suppress fire as completely as possible. Wildland fires now represent unique challenges to firefighting agencies and require vastly different approaches to its prevention, mitigation, and suppression. As more people choose to leave the cities and build their homes in the "wildland urban" interface, it is critical that these ongoing concerns are addressed.
How Do Forest Fires Start?
Naturally caused forest fires are usually started by dry lightning where little to no rain accompanies a stormy weather disturbance. Lightning randomly strikes the earth an average of 100 times each second or 3 billion times every year and has caused some of the most notable wildland fires in the western United States.
Most lightning strikes occur in the North American southeast and southwest. Because they often occur in isolated locations with limited access, lightning fires burn more total acres than human-caused starts. The average 10-year total of U.S. wildfire acres burned and caused by humans is 1.9 million acres; 2.1 million acres burned are lightning caused.
Still, human fire activity is the primary cause of wildfires having nearly ten times the start rate of natural starts (average 10-year percent U.S. starts: 88% human caused; 12% lightning caused). Most of these human fires result from accidental causes. Accidental fires are usually caused by carelessness or inattention by campers, hikers, or others traveling through wild land or by debris and garbage burners. Some are intentionally set by arsonists.
I want to stress that many human-caused fires are started to reduce heavy fuel buildup and used as a forest management tool. This is called a controlled or prescribed burn and used for wildfire fire fuel reduction, wildlife habitat enhancement and debris clearing. They are not included in the above statistics and ultimately reduce wildfire numbers by reducing conditions that contribute to wildfire and forest fires.
How Does Wildland Fire Spread?
The three primary classes of wildland fires are surface, crown, and ground fires. Each classification's intensity depends on the quantity and types of fuels involved and their moisture content. These conditions have an effect on fire intensity and will determine how fast the fire will spread.
Surface fires typically burn readily but at a low intensity and partially consumes the entire fuel layer while presenting little danger to mature trees and root systems. Fuel buildup over many years will increase intensity and especially when associated with drought can become a rapidly spreading ground fire. Regular controlled fire or prescribed burning effectively reduces the fuel buildup leading to a damaging ground fire.
Crown fires generally result from intense rising ground fire heat and occur in the higher sections of draping trees. The resulting "ladder effect" causes hot surface or ground fires to climb the fuels into the canopy. This can increase the chance for embers to blow and branches to fall into unburned areas and increase the spread the fire.
Ground fires are the most infrequent type of fire but make for very intense blazes that can potentially destroy all vegetation and organic manner, leaving only bare earth. These largest fires actually create their own winds and weather, increasing the flow of oxygen and "feeding" the fire.