Are Forest Fires a Good Thing or a Bad Thing?

We have all seen images of raging forest fires that leave scorched bare ground and trees blackened like charcoal. As our climate changes, some wildfires are becoming more severe, killing trees that typically survived past fires. Many people do not realize, however, that fire is an important natural process in a variety of forests. Some ecosystems require fire to remain healthy, and the plants and animals that inhabit these places are adapted to periodic burns [1]. Some trees have thick bark that acts as insulation to protect them from fire, or cones that open and disperse seeds only after experiencing high heat from a burn [2]. After a fire, new trees and plants grow in the area that burned. For ecosystems that are adapted to fire, you can think of it as nature’s way of hitting the reset button. Unfortunately, people did not always recognize the importance of fire.

For many years, people actively suppressed fires. This means people put out all forest fires as quickly as possible. Many of the problems that we see with large, intense wildfires today are the result of years of fire suppression. For example, in the absence of fire, fuel can build up. Fuel can include both living vegetation, such as shrubs and mosses, as well as dead trees that are still standing or have already fallen. Fire suppression over many decades results in conditions that are unhealthy for the forest and dangerous to humans [3]. Fortunately, scientists now see the important role that fire plays.

Types of Fires

Fire ecologists (scientists who study forest fires) classify fires into two main types, although combinations of the two can also happen. Surface fires burn the grasses and broad-leaved plants that grow just above the ground level (Figure 1). Grasses are especially well-adapted to survive surface fires. Crown fires burn the canopy, or the top parts of trees where the leaves grow (Figure 1). Where crown fires occur, trees with serotinous cones release their seeds. Serotinous cones only release seeds after they are exposed to the high heat of a fire. Other traits used to classify fires include severity, frequency of occurrence in a given region, and size. All of these characterize the fire regime for a place, which is closely tied to the local plants and climate or, in other words, the local ecosystem.

Fire Is Important in Many Ecosystems...But Not All!

In the Appalachian Mountains, oaks are the most common trees in the oak-hickory forests that stretch for hundreds of miles along the ridgetops from Alabama to New York. Animals of all sizes depend on the oaks, from the insects that hide in the trees’ craggy bark, to the chipmunks and black bears that rely on acorns (the oaks’ seeds) to survive long, cold winters. Adult oak trees have thick bark that protects them from surface fires, while young oaks have long, thick roots full of nutrients, to fuel the trees’ regrowth after fire. But Appalachian oak forests have a problem. After decades of fire suppression, the oak seedlings sprouting from acorns do not have enough sunlight or space on the forest floor to grow into mature trees [4]. Surface fires create the right environment for oak seedlings to thrive, by creating openings in the forest canopy and removing tree species such as maples that create dense shade. Surface fires allow oak forests to regenerate, and these fires protect habitat for thousands of plants and animals in the Appalachian Mountains.

In savannas of the upper Midwest, black oaks and bur oaks dominate the landscape, while the understory species include not only grasses, but also broad-leaved species that are important to pollinators, and blueberries, which feed wildlife. Here, fire ecologists strive to maintain open conditions so that these important understory species can persist. The goal is to burn often enough so that there is generally not more than 40% canopy closure (you can think of this as 60% open sky). This helps ensure that important understory species receive enough light to survive and reproduce.

Of course, fire is important not only for oaks, but for conifers, or “evergreen” tree species, as well. In the Sierra Nevada mountains of California, giant sequoias are known for their size, long lifespan, and need for fire. Scientists who examine tree-ring samples have shown that fires burned often in the past, occurring every 6 to 35 years [5]. When managers suppressed fires for many decades, the number of young sequoias sharply declined. Why did that happen? Fire plays a key role in the sequoia life cycle. Serotinous cones high up in the trees release many seeds when exposed to heat from a fire. Fire also uncovers bare soil by burning dead branches and leaves. The sequoia seeds, and eventual seedlings, need bare soil to take root, and sunny gaps in which to grow. Adult sequoias have survived most fires because their thick bark resists burning and protects them from a fire’s heat. Since the 1970s, fire managers have used prescribed burns and lightning-ignited fires to restore fire to sequoia forests (Figure 2). Scientists monitoring the forest after fires find young sequoias are more abundant again.

Fires are also common in other western conifer forests. Many people are aware of fires in the western United States; fewer people realize how important fire is in Alaska. The summers in interior Alaska can be warm and lightning is common. The black spruce forest, or boreal forest, is adapted to both surface and crown fires and is designed to burn. Low-hanging branches drape into the flammable moss layer and ground vegetation. When a fire starts, it spreads across the ground and up to the crown of the tree—a crown fire! Although the trees are not well-protected from fire, the seed cones at the top of the trees are serotinous, so seeds are released, rejuvenating the forest. Young black spruce seedlings often begin to grow in the first 5 to 10 years after a fire (Figure 3). Shrubs and leafy trees such as birch and aspen come back quickly after fire, either from seeds or by re-sprouting from below-ground roots. These young forests are important for wildlife. Moose browse the leafy shrubs and trees and voles eat the new green plants. Voles are prey for other animals, including owls, fox, and marten. Fire is a necessary part of the landscape in Alaska. Rather than being suppressed, many fires are managed, not only to protect people and communities, but also to help the forests and wildlife.

In contrast to Alaska, Hawai’i is quite different. Fire is usually very harmful to Hawai’i’s native vegetation. Before people settled on these islands, fire was uncommon, mainly because lightning is not common [6]. Hawai’i has fewer native plant species than does the United States mainland. The plants that do grow there have adaptations that helped them to disperse across the ocean (such as seeds that could float, or had “wings”), but not to survive fires. Fires are now common in Hawai’i—most are accidentally started by people. These fires destroy the native vegetation, and the native plants are not good at recovering after this happens. One of the biggest problems is non-native species. These are brought over from elsewhere—usually accidentally—and they are sometimes much better at dealing with fire. After a fire, these non-native species often grow so quickly that the native plants cannot keep up with them, and do not ever grow back. Because of this, we try to put out fires in Hawai’i as quickly as we can!

Managing Fire Is a Balance Between Risks and Benefits

Most national parks have developed fire-management plans that identify priorities and approaches for managing wildfires. Unplanned, human-caused fires are nearly always suppressed as soon as possible, since they usually occur in places or under conditions where they may threaten human life or property, as well as natural or cultural resources like wildlife, plants, or historic buildings. At higher elevations or in remote wilderness, lightning-caused fires may be managed and monitored if they do not cause threats to human life or property. These fires may benefit forests, meadows, shrublands, or other park ecosystems.

Some national parks also conduct prescribed burns. Each year, park managers identify the type, number, and timing of prescribed burns they hope to conduct. For a prescribed fire to be approved, certain conditions must be met. The weather must be favorable (without extreme winds or very dry conditions) and enough fire staff must be available to manage the fire.

We Need Fire

Fire is a natural, important process in many ecosystems. Without it, fire-dependent forest types will be lost from national parks, eliminating important habitats for animals and plants that depend on these forests. Decades of fire suppression and a changing climate can contribute to extremely severe fires that convert forests into other vegetation types, such as shrublands or grasslands. Therefore, for forests adapted to periodic fire, it is important that the National Park Service and other land management agencies allow fires that start from natural sources to burn when possible, in addition to conducting prescribed burns. This will result in healthier forests, improved wildlife habitats, and safer conditions for people who live near or spend time in forests.

Glossary

Surface Fire: ↑ A fire that burns just above the ground. This typically burns old leaves and needles as well as living grasses and plants and allows more sunlight to reach the ground.

Crown Fire: ↑ A fire that burns the top part of the tree, where the leaves or needles are.

Canopy: ↑ The highest part of a forest, including the leaves or needles and upper branches.

Serotinous Cone: ↑ A cone of a conifer tree that is closed shut by a substance called resin. When a crown fire happens, the resin burns off, and the cone releases its seeds.

Fire Regime: ↑ The pattern of naturally occurring fires. The fire regime of an area includes such information as the length of time between fires and whether they are surface or crown fires.

Understory: ↑ The lowest layer of the forests. This includes grasses, herbs, shrubs, and very small trees.

Prescribed Burn: ↑ An intentional fire that fire managers conduct for the benefit of plants, animals, and soil. Managers ensure that the burn is carried out in a safe and controlled manner.

Boreal Forest: ↑ The northernmost, and most extensive, forest type in the world. In North America, boreal forest extends from Alaska across Canada and into the Great Lakes region of the United States.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.