Forest fires can start anywhere in a forest where there is ample combustible biomass (such as wood, leaves, and brush), as well as an ample supply of oxygen and a source of heat that will ignite a flame. If all of these elements are present, and the combustible materials reach a temperature of 572℉ (300°C), the gas in the steam that is produced will react with oxygen to create a flame that starts a fire[sc:1]. Given the right conditions, the fire can then spread wherever there is ample fuel materials to burn and the oxygen needed for burning.
Natural causes of forest fires
Under natural conditions, forest fires can start though lightning during dry conditions. In fact, lightning is the most common natural cause of forest fires. Because lightning strikes our planet an estimated 100 times every second (this translates to about 3 billion lightning strikes per year), and because the lightning strikes often occur in isolated locations which have limited access to them, it is not uncommon for lightning to cause fires in a forest ecosystem[sc:2].
In general, forests have a natural resilience to natural disturbances such as fires that are started by lightning strikes. Some tree species even require fire as part of their lifecycle to create the necessary conditions that are needed for their seeds to germinate. Periodic fires in a forest can also contribute to overall forest health by returning organic matter to the soil and by removing excess organic debris from the forest floor. Natural forest fires also help to open up habitat for those species that need open spaces.
During drought conditions, forests become increasingly at risk of fire during lightning strikes.
Man-made causes of forest fires: Historical and present
Forest fires have been used by native people groups for a large part of human history. For example, Native Americans traditionally used fires for reasons such as to increase habitat for game species, to capture game more easily, and to increase the ease of travel[sc:3].
Fires have also been used traditionally around the world to clear forests and other natural ecosystems for agricultural purposes, and such land clearing techniques are still widely used in many countries around the world today. However, in today’s world, deforestation by fire or otherwise, is a practice that we do not have the luxury of continuing on a broad scale. Not only is the burning of our global forests a large source of carbon dioxide emissions, such broad-scale deforestation diminishes the future potential of forests to store global carbon emissions from the atmosphere.
Other human-caused forest fires occur as a result of accidents during activities such as camping, hiking, or by burning debris and garbage. Sadly, forest fires are also intentionally set from time to time through arson activities.
Still other forest fires are intentionally set by humans through controlled prescribed burning activities in efforts to reduce ignitable fuel and debris from building up on the forest floor or to manage forest ecosystems and wildlife habitat. In the case of reducing the fuel buildup on the forest floor, the conditions that would have lead to even more intensive and out-of-control forest fires are reduced significantly.
Prescribed burning is now a common approach to forest management that was developed after forest managers determined that long-term forest fire suppression actually led to an increased number and intensity of wildfires.