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What Causes Forest Fires? Its Not as Obvious as You Think

What Causes Forest Fires
A forest fire refers to an uncontrolled fire that erupts in wilderness. It can be caused by many factors, including lightning, volcanic eruptions, and some human activities.
ScienceStruck Staff
Last Updated: Feb 20, 2018
Fuel, oxygen, and a source of heat―these three are the prerequisites for a fire, and are commonly known as the 'fire triangle'. The availability of these three elements can unleash an intense fire in the forest, which is usually termed as forest fire, bush fire, or wildfire. Wildfires can be witnessed throughout the world, and they usually occur in cycles.
Though mild forest fires are considered beneficial for many plant species, large and widespread wildfires can wreak havoc on the ecosystem, causing the loss of some important flora and fauna. Mild wildfires are crucial for the reproduction of certain plant species. The exposure to smoke produced by wildfires, coupled with charred wood and heat, can stimulate the germination of seeds.

What makes forest fires astounding is their extensive size, as well as the speed at which they spread. Due to their ability to change direction and overcome barriers like rivers, roads, and firebreaks, wildfires can easily spread and engulf a vast area. A forest fire can be ignited by several factors, including both natural factors and human activities. But before discussing the causes of forest fires, let's find out more about their types.
Types of Forest Fires
Forest or wildfires can be broadly classified into three types - ground fires, surface fires, and crown fires, depending on the type of fuel involved and its vertical arrangement. These two factors not only determine the intensity of the fire, but how fast it spreads as well.
Ground fires are usually fueled by subterranean roots, buried organic matter, and dead vegetative parts like leaves, branches, and bark and stems of trees that exist on the soil surface at various stages of decomposition. Though quite infrequent in nature, ground fires can burn slowly for days to months. They basically burn by smoldering, and can literally destroy all vegetation leaving behind only bare earth.
ground fires
Surface fires are fed by low-lying vegetation, shrubbery, leaves, grass, and other debris. A surface fire is usually less intense as compared to a ground fire, and it does not pose major risks to mature trees and their roots. But factors like the buildup of fuel over a period of time, and drought or dry spells can increase the intensity of the surface fire, and cause it to spread rapidly to become a ground fire.
surface fires
Crown or aerial fires can reach the level of tree canopy and burn suspended materials. The intensity of the fire is determined by the density of the suspended material, canopy height, and canopy continuity. The rising heat from a ground fire can cause it to climb up and engulf tree crowns. A crown fire can blow embers and tree branches to unburned areas, and thus, spread the fire rapidly.
crown or aerial fires
Causes of Forest Fires
Right from lightning and volcanic eruptions to unattended campfires can cause forest fires. Sometimes, a lighted cigarette left in the forest can also ignite a wildfire. The following are some of the most important factors that can cause forest fires, or increase their intensity to the extent that they can wreak havoc on the flora and fauna of the affected area.
Lightning and Volcanic Eruptions
It has been estimated that lightning strikes the earth about 100 times in a second, and is responsible for causing almost 12% of the total forest fires in the United States.
forest fire caused by lightning
Forest fires are usually caused by dry lightning or lightning not accompanied by rain. They often occur in isolated areas, and this is the reason why wildfires caused by lightning burn more areas than fires caused by human activities. Similarly, volcanic eruptions can also ignite forest fires, as the hot lava or magma burns everything that comes in its way.
Underground Coal Fires
underground fires
Underground coal fires or the smoldering of coal deposits is another important contributory factor in reigniting, as well as spreading forest fires. Generally caused by lightning or a forest fire, an underground coal fire can continue to smolder for a long time after the ground fire has been extinguished, and thus, it can reignite a forest fire.
Spontaneous Forest Fires
spontaneous forest fires
At times, wildfires can be spontaneous, especially when the weather is extremely hot and dry to create enough heat that can induce spontaneous combustion. Everything including wood has a temperature at which it burst into flames, which is called its flash point. For wood, the flash point is 572°F or 300°C.
The accumulation of dead organic matter such as leaves, twigs, and dry branches on the ground can increase the heat. At high temperatures, wood can also release hydrocarbon gases that react with oxygen to create a fire. Thus, wood can reach its flash point to ignite spontaneously in extremely hot and dry climatic conditions.
Spontaneous forest fires have been mostly observed in climates that are moist enough to promote the growth of vegetation, but are also characterized by extended hot and dry periods. The vegetated areas of Australia and Southeast Asia, the forested areas of the United States and Canada, and the Mediterranean basin are some areas where such spontaneous wildfires are quite common.
Spontaneous forest fires usually occur in summer and fall, and also during drought, when fallen leaves, twigs, and other organic matter become dry and highly combustible. Strong winds can spread such forest fires to a large area, and make it difficult to contain them.
Human Activities
Human activities, or to be more specific, human carelessness is responsible for causing more than 80% of all wildfires.
lighted cigarette in the forest
The majority of forest fires caused by human activities can be termed as accidental fires. One of the most important human activities that can cause forest fires is the use of arson for clearing land, which can sometimes go out of control. Sometimes, campers and hikers can leave behind a potential source of ignition like campfires or lighted cigarettes in the forest, which can ignite a wildfire.
Other Causes
rockfalls cause fire
Another important cause of wildfires is the sparks from rockfalls. Forest fires can be frequent during the dry summer months, and the periods of droughts and strong winds. Even global warming is believed to play a role in creating frequent forest fires, by increasing the frequency and intensity of droughts.
Control and Prevention
The preventive measures for forest fires include some preemptive methods that can help reduce the risks of fires and control their severity and spread, and thus, maintain ecological balance and protect resources. Close monitoring of forest fires caused by natural factors can help prevent their spread to a large extent. To contain forest fires, fire departments usually use water and chemical fire retardants that can be dropped from planes and helicopters.
fire retardants dropped from planes and helicopters
To control the spread of a wildfire, firefighters usually create a control line by removing all fuel from an area so that the fire cannot travel across it.

Controlled or prescribed burns are often employed by government authorities to reduce fuel buildup, and clean up the debris, and thus, prevent the spread of forest fires to a vast area. Periodic prescribed burns can also help maintain biodiversity, as the smoke and heat produced in the process can facilitate the germination of seeds of certain plant species. Apart from these, responsible human behavior can help reduce the number of forest fires to a great extent. In fact, we can reduce the number of wildfires dramatically by not leaving behind any source of ignition in the forest.
firefighters
Forest fires can affect climate and weather to a great extent, besides causing severe damage to valuable trees. Wildfires can increase the level of greenhouse gases (water vapor, carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons), and thereby increase pollution and global warming. However, they are also an important part of the ecosystem, and many plants depend on the heat and smoke generated by wildfires for their growth and reproduction. But large wildfires can cause extensive damage to the ecosystem, which again highlights the importance of effective control and prevention of forest fires.
Fire Fighters Fighting Wildfire
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A high res photo of a forest fire in progress
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