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Research and Innovation

Wildfires: Expert Answers to Your Burning Questions

An image of a wildfire burning trees.

Large wildfires continue to burn across the western United States. In its latest briefing, the National Interagency Fire Center reported that 93 large fires have burned more than 1.8 million acres across 14 states. That includes nearly 1.4 million acres in California alone.

Wildfires not only scorch millions of acres each year but also destroy thousands of homes and cloak cities in unhealthy levels of smoke, according to Joseph Roise, a professor in the Department of Forestry and Environmental Resources at NC State’s College of Natural Resources.

Roise, whose areas of expertise include wildland fire behavior and prevention, currently teaches courses related to fire management, fire ecology and firefighting. He also serves as a member of the Southern Fire Exchange Management Team and the North Carolina Prescribed Fire Council. 

We recently spoke with Roise to answer some of the most frequently asked questions about wildfires, including how they begin and how they can be prevented. Here’s what we learned.

What is a wildfire?

A wildfire is best defined as an unplanned — and uncontrolled — fire that occurs in a natural area such as a forest, grassland or prairie. There are three types of wildfires: Ground fires, surface fires and crown fires.

Ground fires occur when plant roots and other organic matter below the soil surface ignite. These fires can grow into surface fires, which burn dead or dry vegetation that’s lying or growing just above the ground.

Crown fires, on the other hand, burn through the tree canopy. Because the influence of wind is greater in the canopy and the canopy is composed of interconnected vegetation, these fires can spread quickly.

How does a wildfire start? 

Wildfires can occur naturally — ignited by heat from the sun or a lightning strike. But the vast majority of wildfires are caused by humans, according to Roise.

Research estimates that about 85 percent of wildfires in the U.S. result from human causes, including arson, unattended campfires and debris fires and discarded cigarettes.

There are three components needed to start a wildfire: fuel, heat and oxygen. These components are commonly referred to as the “fire triangle,” according to Roise.

In order for a fire to start, there must be fuel to burn. Fuel is any kind of combustible material, including leaves, trees, grasses and even houses. The fuel’s moisture content, size, shape and quantity determine how easily it will burn and at what temperature.

A heat source, whether it’s natural or human-caused, is responsible for the initial ignition of fire. All flammable materials emit vapors which, when heat is present, combust.

The final component — oxygen — supports the chemical processes that occur during a fire. When the fuel burns, it reacts with the oxygen from the surrounding air to release heat and generate combustion.

When are wildfires most likely to occur?

Wildfires can happen any time of the year. But generally, the risk of wildfires increases when an area experiences increasingly dry and warm conditions, such as drought. 

Drought conditions are sometimes preceded by a period of above average rainfall, which encourages more plants and trees to grow. This vegetation, however, usually wilts and becomes dried out when drought conditions occur, reducing moisture content and providing ample fuel for wildfires.

Roise said drought conditions — and wildfires — typically occur in North Carolina in the late fall or early winter, sometimes extending into the spring. In California, on the other hand, the risk of wildfires is highest between August and November when hot, dry winds are most frequent.

“The southeast doesn’t really experience wildfires during the summer, because the ground is so wet from all the rain,” Roise said. “You could dump several gallons of gasoline out in the woods and light it and nothing would happen. It’s a different story out west. They’ve had drought for years.” 

How do wildfires impact the environment?

Wildfires are integral to the health of ecosystems worldwide, according to Roise. In addition to clearing heavy brush and invasive weeds from the forest floor, wildfires also thin the tree canopy and allow more sunlight to hit the ground. This helps to promote the growth of new grasses, herbs and shrubs. 

“After a fire, plants grow back 99 percent of the time,” Roise said.

He added that many southeastern ecosystems actually depend on fire to maintain desired conditions. In fact, without fire, hardwood seedlings would dominate the understory and overshadow longleaf pine and wiregrass communities, which provide critical habitat for rare species such as the red-cockaded woodpecker and gopher tortoise. 

Once the forest floor recovers from a wildfire, the new vegetation can also provide food and habitat for many grazing animals, such as rabbits and deer, as well as birds.

Although wildfires can kill individual animals, they don’t destroy entire populations or species, according to Roise. “Some animals get trapped. But many of them know where to go to escape the heat,” he said.

Wildfires, however, can negatively impact the environment. Due to the loss of understory brush, the ground’s soil can lose its ability to absorb as much water. This allows flooding to carry heavy metals from ash and soil into waterways.

Roise said wildfires also release large amounts of carbon dioxide and other greenhouse gases into the atmosphere – contributing to further climate change.

Why are wildfires so destructive?   

Destruction caused by wildfires in the U.S. has significantly increased in the last two decades. Every year since 2000, an average of 71,300 wildfires has burned an average of 7 million acres of land. That’s more than double the average of 3.3 million acres scorched by wildfires in the 1990s.

In 2015, the largest wildfire season recorded in U.S. history burned more than 10 million acres of land and caused more than $3 billion in property damage and losses.  

Wildfires have become more intense over the years in part due drought and past fire suppression policies that allowed increased fuel loads to accumulate on the ground, according to Roise. 

For most of the 20th century, the U.S. Forest Service and other land management agencies sought to suppress all wildfires, whether they occurred naturally or were caused by humans, in an effort to preserve the timber supply and other natural resources. This led to more living and dead vegetation on the ground, increasing the fuel load and as a result, the risk of large wildfires.

Roise said wildfires have also become more deadly and destructive due to the increasing number of people living in rural and wilderness areas located in high-risk fire corridors. A 2018 study published in the journal Natural Hazards found that nearly 7 million properties in California alone were located in areas prone to wildfires, a nearly 1,400 percent increase since the 1940s.

Climate change also exacerbates the damaging effects of wildfires, according to Roise. While increasing amounts of rainfall cause more vegetation, or fuel loads, to grow on the forest floor, rising temperatures dry it out and make it easier to ignite. In California and other states, the warmer spring and summer temperatures also cause earlier snowmelt, creating longer and more intense dry seasons.

How can we stop wildfires?

Fighting a wildfire once it’s already started is a dangerous and expensive process. But land management agencies can use a variety of tools and techniques to prevent wildfires, including controlled burning.

Controlled burning, also called prescribed fire, involves planning and applying fire to a predetermined area, under specific weather conditions, to help reduce wildfire risk.

These fires not only reduce excessive amounts of brush and other fuel sources on the forest floor but also encourage the new growth of native vegetation, according to Roise.

Research shows that wildfires that burn in areas where fuels have been reduced by prescribed fire cause less damage and are much easier to control. “More prescribed fires mean less fuel on the ground, which makes wildfire less intense,” Roise said.