Fire ecology is the study of how fire affects natural environments. Many ecosystems, such as prairies, savannas, chaparral, and coniferous forests, have adapted to live with fire. Fire helps these areas stay healthy and renew themselves. Some plants in these environments rely on fire to grow, spread, or reproduce. Preventing wildfires can harm these plants and the animals that depend on them.
In the United States, efforts to stop wildfires have led people to believe that fires are always bad for nature. However, scientific research shows that fire is an important part of many ecosystems. The plants and animals in these areas have adapted to survive and even benefit from natural wildfires. Fire is now seen as a "natural disturbance," like flooding, strong winds, or landslides, that has shaped the development of species and influenced ecosystem features.
Stopping wildfires, along with other human actions, may have caused unexpected problems for ecosystems. Some large wildfires in the United States have been linked to years of fire suppression, the growth of human populations into areas that naturally handle fire, and changes in climate. Land managers must decide how to restore natural fire patterns. In many cases, allowing wildfires to burn naturally is the most cost-effective and successful solution.
History
Fire has helped shape the plants on Earth. A natural process called photosynthesis started increasing the amount of oxygen in the air during the Devonian period, about 350 million years ago. Later, around 125 million years ago, fire began to affect the environment where land plants lived.
In the 20th century, ecologist Charles Cooper argued that fire is an important process for ecosystems.
Fire components
A fire regime describes how fire behaves in an ecosystem and how it affects the plants and animals there. The "severity" of a fire refers to how much damage it causes to the environment. Scientists study this using tools like satellites or planes, which help them measure burned areas, how bad the fire was, and the risk of fire in a place. Scientists can describe severity in many ways, but one common method is by looking at how many plants die after a fire.
Fires can burn in three different ways depending on where they are. Ground fires burn underground through soil with lots of organic material. Surface fires burn through plants and leaves on the ground. Crown fires burn through the tops of trees and shrubs. Most ecosystems experience all three types of fires at different times.
Fires often start during dry seasons, but in some places, they also happen when lightning is common. The frequency of fires in an area over many years shows how often wildfires occur in that ecosystem. This can be measured as the average time between fires in a specific place or in a certain area.
Wildfire intensity is a measure of how much heat a fire produces along its path. It can be estimated in two ways:
• By multiplying how fast the fire spreads (in meters per second), how much energy is in the burning material (in kilojoules per kilogram), and how much fuel is burned per square meter.
• Or by measuring the length of the flames.
Abiotic responses
Fires can change soils by heating and burning. The effects depend on the temperature during the fire. At lower temperatures, water in the soil may evaporate. At higher temperatures, organic matter in the soil can burn, creating substances like charcoal.
Fires can change soil nutrients in several ways, such as breaking down materials, turning them into gases, washing them away, or carrying them away with water. These changes usually require very high temperatures to cause major losses of nutrients. However, fires often increase the amount of nutrients available to plants because ash from the fire releases nutrients quickly, compared to the slow release from decaying plants. Heat can also cause rocks to crack, speeding up the breakdown of rocks and possibly releasing nutrients.
After a fire, the soil often becomes more basic (higher pH). This likely happens because calcium carbonate in the soil turns into calcium oxide when temperatures are very high. It may also occur because ash adds more positive ions to the soil, temporarily raising the pH. Fire can also increase microbial activity in the soil due to heat and more nutrients, but some studies show that fires can completely destroy microbes in the top layer of soil. Overall, fires make soil more basic because of the chemical reactions caused by high heat. Fire can also change soil texture and structure by affecting clay content and how porous the soil is.
Removing plants after a fire can affect the soil in several ways. Without plants, the soil may get hotter during the day because more sunlight reaches the surface. At night, the soil may cool faster because plants are no longer holding heat. With fewer plants to block rain, more water may reach the soil. However, if the soil has ash that is dry, the ash might repel water, so the amount of water available to plants might not increase.
Biotic responses and adaptations
Fire adaptations are features that help plants and animals survive wildfires or use resources left after fires. These traits can help them stay alive during a fire or grow new plants after a fire. Both plants and animals use different methods to survive and reproduce after a fire. Plants in areas where fires happen often have traits that match the fire patterns in their environment. These traits include protection from heat, faster growth after a fire, and materials that catch fire easily, which can remove competing plants.
For example, plants in the Eucalyptus genus have oils that catch fire easily and tough leaves that resist heat and dry weather. This helps them grow better than plants that are not as fire-tolerant. Thick bark, branches that fall off near the ground, and water stored in parts of trees can also protect trees from heat. Some plants have seeds or growth parts that survive fires and sprout afterward, helping their species continue. Smoke, burned wood, and heat can cause some seeds to grow, a process called serotiny. Smoke from fires can also help other plants grow by causing a chemical called orange butenolide to form.
Fires have led to changes in how plants and animals survive after fires. These changes help them live better after fires and grow new plants. In boreal forests, for example, trees have thick bark and flower after fires. Other plants and animals also adapt to survive and regrow after fires, which helps them spread and create more stable environments. Plants in burned areas often have more genetic diversity compared to areas that have not been burned.
Plants have developed many ways to deal with fire. One well-known method is when seeds are released after fire or smoke. This is sometimes called serotiny, but it includes more than just fire. Some plants use smoke, water, or light to release seeds. Germination triggered by fire is different from seed release and is called physiological dormancy.
In chaparral areas of Southern California, some plants have leaves with oils that catch fire easily, causing strong fires. The heat from these fires helps their seeds grow, and the young plants can grow without competition in burned areas. Other plants have seeds that grow after smoke or fire, or buds that sprout after fire. Lodgepole pine trees have cones sealed with resin that melts during fire, releasing seeds. Some plants, like giant sequoias, need fire to create gaps in the forest canopy, letting sunlight reach the ground so their seedlings can grow. Plants cannot move, so they are either destroyed by fire, can survive some fire damage, or are very resistant to fire.
Plants that are destroyed by fire are often very flammable and are completely burned. Some may not return after a fire, while others have seeds that grow quickly after a fire. These plants have large seed stores that sprout, grow, and produce new seeds before the next fire. Some seeds have a protein called KAI2 that reacts to chemicals from fire, helping them grow.
Plants that can survive some fire damage are called "resprouters." These plants store energy in their roots to help them recover after a fire. For example, after a fire in Australia, Mountain Grey Gum trees grow new leaves from the base of the tree up the trunk.
Plants that are very resistant to fire suffer little damage during typical fires. Large trees with flammable parts high above the ground are often resistant. Mature ponderosa pines, for example, lose their lower branches as they grow, so they are not damaged by low-intensity fires.
Like plants, animals have ways to survive fires, but they usually need to escape the fire. Birds can often fly away from fires and may find prey that is fleeing. Many animals depend on fires to create habitats. Some birds may intentionally spread fires to find prey. Mammals can run or dig into the ground to avoid fires. Amphibians and reptiles can hide in the ground or use other animals' burrows. Amphibians may also hide in water or wet soil.
Some insects hide during fires, but others are attracted to heat and smoke, which can be dangerous. Microbes in the soil survive better if they are deep underground. A quick fire and dry soil help microbes survive. After fires, more nutrients are available, which can increase microbial populations. Bacteria are more heat-tolerant than fungi, so fires can change the types of microbes in the soil. Some fungi, like Cylindrocarpon destructans, are not affected by fire chemicals and may outcompete other fungi after fires.
Fire and ecological succession
Fire acts differently in each ecosystem, and the plants and animals there have adapted to these differences. A general rule is that fire creates a mix of different habitat areas, from places that were just burned to those that have not been touched by fire for many years. This process is part of ecological succession, where a burned area changes over time as new plants and animals return. Scientists study these changes by observing how plant life changes. After a fire, the first plants to return are those with seeds already in the soil or those that can quickly reach the burned area. These are usually fast-growing plants that need sunlight and cannot grow well in shaded areas. Over time, slower-growing plants that can survive in the shade, like trees and shrubs, begin to replace some of the earlier plants. Conifer trees, such as pines, often appear early after a fire, while broad-leaf trees, like oaks, may take over if fires do not occur again. Because of this, many conifer forests depend on regular fires to survive. Both natural fires, like lightning-caused wildfires, and fires started by humans affect all ecosystems, from wetlands to forests to tropical areas. This changes how ecosystems are organized and how they work. While wildfires have always happened naturally, their frequency has increased quickly in recent years. This is mainly because there is less rain, higher temperatures, and more fires caused by people.
Different types of plants, animals, and tiny living things, like fungi, specialize in living in different stages of this process. By creating these varied areas, fire helps more species live in the same place. The type of soil, the climate, and the shape of the land all influence how an ecosystem adapts to fire. The frequency of fires also affects how ecosystems change over time. If fires happen too often, trees may not have time to regrow, so plants with lighter seeds, like grasses and flowering plants, take over.
Genetic Diversity
Fire patterns affect how genes change over time by influencing how organisms reproduce, survive, and move within populations. Genetic and genomic tools help scientists study how fire shapes the traits of living things. Genetics is the study of how genes control cell functions and body processes by directing the creation of proteins. Genomics, in contrast, examines all the genes in an individual and how these genes interact with the environment and other genes.
Some genetic tools used to study fire's effects include microsatellites (STRs), single-nucleotide polymorphisms (SNPs), and mitochondrial and nuclear genomic data. STRs help track genetic differences between populations by comparing short, repeating DNA sequences. This provides information about how fire affects population structure and genetic diversity over time. SNPs are used to study small DNA changes that show how genetic patterns shift in different regions with varying fire patterns. This helps scientists understand how species adapt to fire.
Mitochondrial and nuclear genomic data help scientists study both short-term and long-term effects of fire on species. Mitochondrial data can reveal recent genetic changes in female lineages. Nuclear data, though more damaged by intense heat, shows how entire populations have adapted through the combined genetic material from both parents.
Fire affects biodiversity in different ways, depending on the ecosystem and type of disturbance. Factors like species death, movement into new areas, fire strength, and species adaptations influence biodiversity after a fire.
Sometimes, fire reduces genetic diversity. If a fire causes many deaths, a population may shrink, leading to fewer genes in the species. This can happen if few new individuals move into the area after the fire. Large, intense fires may also harm biodiversity. Some research suggests human-caused wildfires in tropical or temperate forests can reduce biodiversity. Fires can also allow invasive species to grow, which harms native species by changing nutrients like nitrogen in the soil.
In other cases, fire increases biodiversity. Many ecosystems depend on fire to support life and promote growth. Some species have traits that help them survive fires, such as plants that regrow after a fire. These traits can lead to genetic changes that help species adapt faster. While large fires can harm biodiversity, smaller, less intense fires can create varied habitats. These varied environments provide homes for many species, increasing biodiversity.
Species Richness
Species richness is highest right after a fire but decreases over time. Some species can survive after a fire, allowing many different types of life to exist together. Fires change biodiversity by reducing the numbers of some species while helping others grow. Disturbances from fire can help certain animal populations recover, leading to new populations forming in areas that were affected. Fires often reduce the number of species and can cause some species to disappear completely. The effects of a fire depend on its size, when it happens, and how intense it is. If fires happen more often or are larger, it can affect how populations recover between fires, creating areas where populations are spread out differently. Fires have been used to change which species are present, such as by reducing the numbers of invasive species.
Examples of fire in different ecosystems
Mild to moderate fires burn in the lower parts of forests, removing small trees and plants that grow on the ground. High-severity fires burn into the tops of trees and kill most of the main plants in the forest. Crown fires can burn in the tree tops with help from ground plants (passive crown fires) or without ground plants (active crown fires). High-severity fires create early forest stages with many dead trees and high levels of plant and animal life. When forests burn often, there is less plant material on the ground, so soil temperatures rise only slightly and do not harm deep roots. While other forest features affect fire impact, climate and land shape are important in deciding how severe and widespread fires are. Fires spread most during dry years, are most severe on upper slopes, and depend on the type of plants growing.
British Columbia covers about 10% of Canada’s land but is home to 70% of the bird and land mammal species that breed there. Natural fires help keep a wide variety of animal species alive in up to 12 different forest types in British Columbia. Different animals use the changes in forests after fires, such as fallen trees and debris. The size and strength of a fire affect how the habitat changes and how animals use burned areas. Studies from 1600 to today in central British Columbia show that fire patterns have changed since humans began controlling fires.
The northern part of the Siberian Taiga in northern Asia has frozen ground (permafrost) and few people. Larch trees cover about 80% of the forested areas in this region and have the highest fire area among all forest types, with about 1.13% of the area burning each year from 1996 to 2019. Wildfires are natural here and often start from lightning. Fires in this region help larch trees grow by removing moss on the ground. This process increases ground thawing, adds nutrients, and allows seeds to reach the soil.
Shrub fires usually burn in the tops of shrubs and spread quickly if shrubs are close together. Shrublands are often dry and have lots of flammable materials, especially on hillsides. Fires move where there is the least moisture and most dry plant material. Soil temperatures during shrub fires are usually higher than in forests because fires burn closer to the ground. Common plants in shrublands include manzanita, chamise, and coyote brush.
California shrubland, also called chaparral, is a plant community of low-growing species found on dry hills in the California Coast Ranges and western Sierra Nevada foothills. Common plants include salal, toyon, coffeeberry, and Western poison oak. After fires, new plant growth is important for these species to return.
Fynbos shrublands are found in a narrow area of South Africa. The plants here are very diverse, but most need fire to grow. Fire causes their seeds to germinate, starting a new life cycle. These plants may have evolved to rely on fire because the soil has few nutrients. It is more efficient for them to make many seeds and die in the next fire rather than invest energy in roots that would not benefit much from poor soil. Their fast life cycle may have led to more plant diversity in this area.
Grasslands burn more easily than forests or shrubs, with fire moving through grass stems and leaves and heating the soil only slightly, even during strong fires. In most grasslands, fire is the main way plants break down and recycle nutrients. Some grasslands only started using fire for decomposition after large animal herds disappeared. Without these herds and their predators, using fire too much may cause too much carbon loss and turn grasslands into deserts. Some grasslands do not handle fire well.
In North America, invasive grasses like Bromus tectorum increase fire frequency, which harms native plants. This is a problem in Western U.S. grasslands.
Before people settled, fires and grazing together helped keep grasslands healthy, as shown by changes in soil material. In the Flint Hills of Kansas and Oklahoma, tallgrass prairies are doing well with current fire and grazing practices.
In South African savannas, recently burned areas grow new plants that are tasty and nutritious for large animals. These animals move from unburned areas where grass is short and tough. On these unburned areas, only plants that can survive heavy grazing stay. The burned areas give plants that cannot handle grazing a chance to grow back, helping them survive in the ecosystem.
Much of the southeastern U.S. once had open longleaf pine forests with rich grasses, sedges, carnivorous plants, and orchids. These areas had the most frequent fires, about once every 10 years. Without fire, trees that lose leaves in winter grow and shade out pines and grasses. Plants like yellow pitcher plant and rose pogonia depend on fire. Rare animals like gopher tortoises and indigo snakes also rely on these open areas. Restoring fire is important to keep plant and animal diversity.
Many wetlands are affected by fire, especially during droughts. In areas with peat soil, like bogs, fires can burn the peat, creating holes that fill with water. Less intense fires remove plant litter, letting wetland plants regrow from buried seeds or roots. Wetlands influenced by fire include coastal marshes, wet prairies, peat bogs, floodplains, prairie marshes, and flatwoods. Since wetlands store much carbon in peat, the frequency of fires in northern peatlands is linked to carbon dioxide levels.
Fire suppression
Fire has many important roles in ecosystems that are adapted to fire. It helps move nutrients through the environment, keeps plant and animal diversity, and shapes habitats. When fire is prevented, ecosystems can change in ways that harm plants, animals, and people who rely on those habitats. Wildfires that differ from past fire patterns because of fire prevention are called "uncharacteristic fires."
In 2003, large wildfires occurred in the chaparral areas of southern California. Many homes and hundreds of thousands of acres of land were destroyed. Very dry conditions, low fuel moisture, and strong winds, along with a buildup of dead plant material from eight years of drought, caused the fires to spread quickly. Some people believe fire prevention led to more fuel buildup, but studies of past fire records suggest this may not be true. Fire prevention efforts in southern California did not stop fires from happening. Research comparing fire patterns in southern California and Baja California has been used to suggest that larger fires in the north are due to fire prevention, but this idea has been questioned by scientists.
One result of the 2003 fires was the growth of more invasive and non-native plant species in burned areas, especially places that had burned in the past 15 years. Shrubs in these areas are used to a specific fire pattern, and changes in fire patterns can create conditions that favor invasive plants that grow better in these new environments.
The Boise National Forest is a U.S. national forest located north and east of Boise, Idaho. After several unusually large wildfires, fish populations were immediately harmed, especially small and isolated fish. Over time, however, fire can improve fish habitats by changing water movement, which increases flooding and removes silt. This creates better conditions for fish to return and grow in these areas.
Fire as a management tool
Restoration ecology is a way to fix or reduce the damage humans have caused to ecosystems. Controlled burning is a method being used to help restore and manage ecosystems. Using fire can create homes for species harmed by stopping fires, or it can help control invasive plants without using chemicals. However, people debate whether ecosystems should be restored to conditions before humans lived there or before Europeans arrived. Native American use of fire, along with natural fires, helped keep the diversity of North American savannas.
Oak savannas and oak woodlands once covered large areas in the Midwest, forming a transition between eastern forests and the Great Plains grasslands. Today, these ecosystems are among the most damaged in the world. Less than 1% of the original savanna areas remain in the United States, making their protection very important. Managing these areas helps preserve native plants and animals by providing homes for wildlife.
These habitats are defined by oak trees, few trees in the middle layer, and a mix of grasses, flowers, and sedges on the ground. Fire helps keep these ecosystems healthy. The Midwest has rolling hills and open plains. Historically, fire would move through these areas, creating a mix of prairie, savanna, and woodland habitats because fire intensity and frequency varied. Woodlands were more common on hills, while savannas connected hills to the Great Plains.
The canopy of oak trees is key to defining these habitats. Oak savannas have a canopy covering about 10-30% of the area, while oak woodlands can have canopies covering up to 80%. A tool called a densitometer measures tree coverage compared to open sky. Due to fire suppression, fewer large animals, and farming, these areas have become damaged with thick canopies, less ground diversity, and more invasive plants. One goal in restoring these ecosystems is to reduce canopy cover by removing middle-layer trees and some canopy trees. After reducing tree density, fire is used to stop woody plants from growing too much, help seeds grow, and remove extra plant litter.
Fire management is important for restoring oak savannas and woodlands, and depends on the goals of land managers. Goals may include reducing canopy cover, middle-layer trees, or small woody plants, helping young oak trees grow, or favoring flowers over grasses or vice versa. Deciding when, how intensely, and how often to burn is a major challenge. In addition to fire, herbicides, grazing animals, and browsing animals can improve biodiversity. Herbicides can control invasive plants and stop regrowth after cutting trees. Choosing herbicides with low soil movement helps protect other organisms and groundwater. Grazing animals like bison and browsing animals like deer can create varied landscapes. Combining these methods is important for managing these ecosystems.
Heavy livestock grazing and fire suppression have changed the structure, makeup, and diversity of shortgrass prairies on the Great Plains, letting woody plants take over and allowing invasive species that do not like fire to grow. In dry areas where woody material breaks down slowly, fire is important for returning nutrients to the soil and keeping grasslands productive.
Fire can happen during growing or dormant seasons, but burning during the dormant season is most effective for increasing grass and flower cover, biodiversity, and plant nutrients in shortgrass prairies. Managers must also consider how invasive species respond to fire to restore native ecosystems. For example, fire can control invasive spotted knapweed in Michigan’s tallgrass prairie only during summer, when the plant’s growth is most active.
Mixed conifer forests in the Sierra Nevada had fire return intervals from 5 to 300 years, depending on location. Lower areas had more frequent fires, while higher, wetter areas had longer intervals. Native Americans often set fires in fall and winter, and higher areas were usually occupied only during summer.
Loss of habitat has caused many species to be listed as endangered by the International Union for Conservation of Nature. A study on Finnish boreal forests showed that improving habitat quality outside protected areas can help endangered beetles that depend on dead trees. These beetles and fungi need deadwood to survive. Old growth forests provide this habitat, but most boreal forests are used for logging and lack protection. Controlled burning and keeping deadwood in forests were studied, and results showed more beetle species and numbers after management. Beetle populations increased further where deadwood was abundant. This shows that fire management can help protect endangered species.
Much of Australia’s old growth eucalypt forests are protected. Managing these forests is important because species like Eucalyptus grandis rely on fire to survive. Some eucalyptus trees lack a lignotuber, a root structure that helps regrow after fire. For these trees, fire management can help by creating fertile soil, killing competitors, and releasing seeds.