Fire Ecology
The Role of Wildfire in Ecosystems
Fire Ecology
The Role of Wildfire in Ecosystems
Fire Ecology
The Role of Wildfire in Ecosystems
Fire Triangle
The Fire Triangle
Heat can refer to several aspects of wildland fire.
A heat source is responsible for the initial start of a wildland fire, and heat is also needed to maintain the fire and permit it to spread. In addition, heat is constantly being released from the fire, warming the surrounding air and preheating fuel in its path.
Heat transfer is a critical issue in the study of wildland fire. For a fire to grow and spread, heat must be transferred to the initial and surrounding fuel. Heat allows fire to spread by removing (evaporating) the moisture from the nearby fuel, enabling it to travel more easily. The mechanism and the speed of heat transfer play a great role in wildland fire behavior.
Three mechanisms of heat transfer exist: convection, radiation, and conduction. All three contribute in different ways to the combustion process, depending in part on the available fuel distribution, the wind speed at the fire site, and the slope of the terrain.
Convection is the transfer of heat through the flow of liquids or gases, such as when hot air rises through a chimney. Convection currents are often responsible for preheating higher shrub layers and the canopy, carrying the groundfire upwards.
Radiation transmits heat by rays, such as from the sun or a flame. Radiation accounts for most of the preheating of fuels surrounding a fire. The temperature of these fuels can sometimes grow so high that the fuels ignite prior to contact with flames, spreading the fire.
Conduction moves heat from one fuel particle to the next, as when the stove burner heats a pan and its contents. Conduction allows the heat to be transferred inside and throughout the fuel, rather than only heating the surface. Because wood is a poor heat conductor, meaning heat does not pass through it easily, conduction is usually not the primary mechanism of heat transfer in a wildland fire.
Fuel
The fuel side of the fire triangle refers to both the external and internal properties of the fuel. External properties refer to the type and the characteristics of the fuel material. Internal properties of fuel address aspects of fuel chemistry. Types of fuel include living vegetation, dead vegetation, (duff, twigs, needles, standing dead snags, leaves, and moss), organic subsurface material (peat and coal), and human built structures. Fuel can be defined as any combustible material.
The size and shape of fuel in part determines its moisture content. Lighter fuels such as grasses, leaves, and needles quickly expel moisture, and therefore burn rapidly. Heavier fuels, such as tree branches, logs, and trunks, take longer to warm and ignite. In areas of light fuel, the temperature required for ignition is lower than in areas of heavier fuel. The oxygen surrounds lighter fuels and allows the fuel to burn with greater intensity, quickly exhausting the fuel supply.
The quantity of combustible fuel in a given area is known as fuel loading. These fuels may be arranged in a uniform pattern and distributed continuously across the ground, allowing a wildland fire to travel uninterrupted. Or, the fuel may be distributed unevenly in a patchy network, forcing the fire to travel over rocks and other barriers by wind-borne embers.
The vertical arrangement of fuel is also an important factor in wildland fires. Ground fuels are all of the combustible materials found below the ground surface, and include tree roots, duff, and organic material. Surface fuels are found at the ground level, including twigs, grass, needles, wood, and other vegetation. Aerial fuels are standing vegetation including tree crowns, branches, leaves, snags, and hanging moss. Crown fires are able to burn independently of surface fires, moving through the treetops.
Oxygen
The third side of the fire triangle represents oxygen. Air contains about 21% oxygen; most fires require air with at least 16% oxygen content to burn under most conditions. Oxygen supports the chemical processes that occur during a wildland fire. When fuel burns, it reacts with oxygen from the surrounding air, releasing heat and generating combustion products, e.g., gases, smoke, particles. The process is known as oxidation.
Forest Disturbance
Disturbance
Events described as disturbances generally cause major changes in a forest, usually altering the way it functions (an area that has recently experienced a stand-replacement burn, for instance, functions in a different way than an old growth forest). They can be natural or human caused. Natural disturbances include events like fire, insects and disease outbreaks, floods, drought, windthrow, and storm damage. Human-caused disturbances include timber harvesting, heavy grazing, and so on.
The consequences of human disturbances can be similar to those caused by natural forces, or they can be of another magnitude, altering ecosystems in ways natural disturbances seldom do.
The photos at right show different types of natural disturbances. Click on a thumbnail to see a larger version of the picture.
Human-caused Disturbances
In 1928, Frank Jaquette, an early settler in the Bitterroot Valley reminisced about Salish land management practices in the 1800s and described them as superior to those of the white settlers. Referring to the West Fork of the Bitterroot River in 1887, he said: "At this time the creek was thoroughly set with a growth of willows and very completely so on the south side. Since it has become part of the white man’s domain and fires are less general and frequent, the large alder growth has very generally replaced these willows. It might be noted here that the Indians were great foresters, as all old-time prospectors will affirm. They left the forests to the tender mercies of nature… [While the Forest] Service spends millions of dollars battling against nature’s force, the result is a tendency to a scrubby growth of timber and a fire trap."
What the Salish, Kootenai, and Pend d'Oreille took for granted, but few Europeans understood, is that fire is a necessary process of forest renewal. Rather than thinking of natural disturbances (events like fire, floods, drought, windthrow, storm damage, insects and disease, etc.) as enemies of a healthy forest, they viewed them as integral, even beneficial. They understood that the plants and animals they harvested and hunted depended on a variety of habitats, and that disturbances created that variety. That’s why they lit so many fires.
Deer and elk, for example, need densely timbered areas to hide in and large old trees for protection from winter cold and summer heat, but they also need open areas for foraging. Grizzly bears need variety too, open areas to feed in and timbered areas for cover.
Other species rely almost exclusively on a single kind of forest habitat, such as openings or old growth. The point is, periodic disturbances, especially fire, which is our most common type of natural disturbance, ensure the diversity that wildlife needs.
Fire can be beneficial as a disturbance. In addition to increasing the variety of habitats for wildlife, in certain fire regimes it tends to favor tree species that are more fire resistant and less vulnerable to insect attack and disease infection. In addition, fire is an important part of the forest nutrient cycle, especially on drier sites.
Without periodic fires, nutrients become less available to plants and soil organisms. Although fire can be kept out of the forest for long periods, it will eventually return, and when it does, the fuel buildups caused by decades without disturbance usually result in much larger and more catastrophic burns — burns that can consume tens of thousands of acres and do long-term damage to soils and other resources.
Key Concepts: The Take-home message
Ecosystems constantly change in ways that are only partially predictable.
Disturbances are common and important in virtually all ecosystems.
The positive effects of disturbance on biodiversity are now well recognized by land managers, but not necessarily by the public.
Efforts to suppress disturbance, such as lightning-caused fires, floods, erosion, drought, disease, and insects, which have been perceived to be in conflict with economic interests, have resulted in reduced biodiversity and ecosystem health.
The more we attempt to maintain an ecosystem in a static condition, the less likely we are to achieve what we intended. We must be willing to bear both the economic and biologic consequences of such management.
Forest Succession
Forest Succession
One of the most important concepts you can take away from this website is that change is a natural part of healthy forest communities. The term forest succession refers to change, specifically the patterns of change that occur within a forest over time.
In simplified presentations of forest succession, the forest moves toward a final or climax condition (as in the animation above). Following a major disturbance, such as a stand-replacement fire, the site enters a short period in which grasses, forbs, and/or shrubs dominate. This period is typically followed by a time when tree species that require lots of sun dominate the site. These trees are called shade-intolerant or early seral species by foresters. In a final stage, shade tolerant or climax species begin to flourish. Eventually they overtop the seral species, and they come to dominate the site, establishing a climax forest community.
The climax condition is often thought of as the goal to which all forests are progressing. It's easy to envision it as the last and best forest, the forest primeval. However, it is not necessarily the natural state of forests in the Northern Rockies nor is it necessarily the best for all wildlife and other forest resources.
Before European settlement in the Northern Rockies, many forest types rarely reached climax conditions because of frequent fires and outbreaks of insects, disease — all made worse by periodic drought. Depending on the site, the interval between disturbances could range from a decade or less to several centuries. However, disturbance was inevitable and played a key role in maintaining the health and diversity of the forest.
Over the last one hundred years, the patterns and types of disturbance have changed. Fire has played a much smaller part in bringing change to our forests, while human disturbances such as logging and grazing have increased.
Natural fires cannot be eliminated from forested ecosystems, however. In preventing fires we've allowed fuels to build and created the opportunity for larger, catastrophic fires and insect and disease epidemics.
Fire Terms
Fire Triangle
The Fire Triangle
Heat can refer to several aspects of wildland fire.
A heat source is responsible for the initial start of a wildland fire, and heat is also needed to maintain the fire and permit it to spread. In addition, heat is constantly being released from the fire, warming the surrounding air and preheating fuel in its path.
Heat transfer is a critical issue in the study of wildland fire. For a fire to grow and spread, heat must be transferred to the initial and surrounding fuel. Heat allows fire to spread by removing (evaporating) the moisture from the nearby fuel, enabling it to travel more easily. The mechanism and the speed of heat transfer play a great role in wildland fire behavior.
Three mechanisms of heat transfer exist: convection, radiation, and conduction. All three contribute in different ways to the combustion process, depending in part on the available fuel distribution, the wind speed at the fire site, and the slope of the terrain.
Convection is the transfer of heat through the flow of liquids or gases, such as when hot air rises through a chimney. Convection currents are often responsible for preheating higher shrub layers and the canopy, carrying the groundfire upwards.
Radiation transmits heat by rays, such as from the sun or a flame. Radiation accounts for most of the preheating of fuels surrounding a fire. The temperature of these fuels can sometimes grow so high that the fuels ignite prior to contact with flames, spreading the fire.
Conduction moves heat from one fuel particle to the next, as when the stove burner heats a pan and its contents. Conduction allows the heat to be transferred inside and throughout the fuel, rather than only heating the surface. Because wood is a poor heat conductor, meaning heat does not pass through it easily, conduction is usually not the primary mechanism of heat transfer in a wildland fire.
Fuel
The fuel side of the fire triangle refers to both the external and internal properties of the fuel. External properties refer to the type and the characteristics of the fuel material. Internal properties of fuel address aspects of fuel chemistry. Types of fuel include living vegetation, dead vegetation, (duff, twigs, needles, standing dead snags, leaves, and moss), organic subsurface material (peat and coal), and human built structures. Fuel can be defined as any combustible material.
The size and shape of fuel in part determines its moisture content. Lighter fuels such as grasses, leaves, and needles quickly expel moisture, and therefore burn rapidly. Heavier fuels, such as tree branches, logs, and trunks, take longer to warm and ignite. In areas of light fuel, the temperature required for ignition is lower than in areas of heavier fuel. The oxygen surrounds lighter fuels and allows the fuel to burn with greater intensity, quickly exhausting the fuel supply.
The quantity of combustible fuel in a given area is known as fuel loading. These fuels may be arranged in a uniform pattern and distributed continuously across the ground, allowing a wildland fire to travel uninterrupted. Or, the fuel may be distributed unevenly in a patchy network, forcing the fire to travel over rocks and other barriers by wind-borne embers.
The vertical arrangement of fuel is also an important factor in wildland fires. Ground fuels are all of the combustible materials found below the ground surface, and include tree roots, duff, and organic material. Surface fuels are found at the ground level, including twigs, grass, needles, wood, and other vegetation. Aerial fuels are standing vegetation including tree crowns, branches, leaves, snags, and hanging moss. Crown fires are able to burn independently of surface fires, moving through the treetops.
Oxygen
The third side of the fire triangle represents oxygen. Air contains about 21% oxygen; most fires require air with at least 16% oxygen content to burn under most conditions. Oxygen supports the chemical processes that occur during a wildland fire. When fuel burns, it reacts with oxygen from the surrounding air, releasing heat and generating combustion products, e.g., gases, smoke, particles. The process is known as oxidation.
Forest Disturbance
Disturbance
What is a Disturbance?
Events described as disturbances generally cause major changes in a forest, usually altering the way it functions (an area that has recently experienced a stand-replacement burn, for instance, functions in a different way than an old growth forest). They can be natural or human caused. Natural disturbances include events like fire, insects and disease outbreaks, floods, drought, windthrow, and storm damage. Human-caused disturbances include timber harvesting, heavy grazing, and so on.
The consequences of human disturbances can be similar to those caused by natural forces, or they can be of another magnitude, altering ecosystems in ways natural disturbances seldom do.
The photos at right show different types of natural disturbances. Click on a thumbnail to see a larger version of the picture.
Human-caused Disturbances
In 1928, Frank Jaquette, an early settler in the Bitterroot Valley reminisced about Salish land management practices in the 1800s and described them as superior to those of the white settlers. Referring to the West Fork of the Bitterroot River in 1887, he said: "At this time the creek was thoroughly set with a growth of willows and very completely so on the south side. Since it has become part of the white man’s domain and fires are less general and frequent, the large alder growth has very generally replaced these willows. It might be noted here that the Indians were great foresters, as all old-time prospectors will affirm. They left the forests to the tender mercies of nature… [While the Forest] Service spends millions of dollars battling against nature’s force, the result is a tendency to a scrubby growth of timber and a fire trap."
What the Salish, Kootenai, and Pend d'Oreille took for granted, but few Europeans understood, is that fire is a necessary process of forest renewal. Rather than thinking of natural disturbances (events like fire, floods, drought, windthrow, storm damage, insects and disease, etc.) as enemies of a healthy forest, they viewed them as integral, even beneficial. They understood that the plants and animals they harvested and hunted depended on a variety of habitats, and that disturbances created that variety. That’s why they lit so many fires.
Deer and elk, for example, need densely timbered areas to hide in and large old trees for protection from winter cold and summer heat, but they also need open areas for foraging. Grizzly bears need variety too, open areas to feed in and timbered areas for cover.
Other species rely almost exclusively on a single kind of forest habitat, such as openings or old growth. The point is, periodic disturbances, especially fire, which is our most common type of natural disturbance, ensure the diversity that wildlife needs.
Fire can be beneficial as a disturbance. In addition to increasing the variety of habitats for wildlife, in certain fire regimes it tends to favor tree species that are more fire resistant and less vulnerable to insect attack and disease infection. In addition, fire is an important part of the forest nutrient cycle, especially on drier sites.
Without periodic fires, nutrients become less available to plants and soil organisms. Although fire can be kept out of the forest for long periods, it will eventually return, and when it does, the fuel buildups caused by decades without disturbance usually result in much larger and more catastrophic burns — burns that can consume tens of thousands of acres and do long-term damage to soils and other resources.
Key Concepts: The Take-home message
Ecosystems constantly change in ways that are only partially predictable.
Disturbances are common and important in virtually all ecosystems.
The positive effects of disturbance on biodiversity are now well recognized by land managers, but not necessarily by the public.
Efforts to suppress disturbance, such as lightning-caused fires, floods, erosion, drought, disease, and insects, which have been perceived to be in conflict with economic interests, have resulted in reduced biodiversity and ecosystem health.
The more we attempt to maintain an ecosystem in a static condition, the less likely we are to achieve what we intended. We must be willing to bear both the economic and biologic consequences of such management.
Forest Succession
Forest Succession
One of the most important concepts you can take away from this website is that change is a natural part of healthy forest communities. The term forest succession refers to change, specifically the patterns of change that occur within a forest over time.
In simplified presentations of forest succession, the forest moves toward a final or climax condition (as in the animation above). Following a major disturbance, such as a stand-replacement fire, the site enters a short period in which grasses, forbs, and/or shrubs dominate. This period is typically followed by a time when tree species that require lots of sun dominate the site. These trees are called shade-intolerant or early seral species by foresters. In a final stage, shade tolerant or climax species begin to flourish. Eventually they overtop the seral species, and they come to dominate the site, establishing a climax forest community.
The climax condition is often thought of as the goal to which all forests are progressing. It's easy to envision it as the last and best forest, the forest primeval. However, it is not necessarily the natural state of forests in the Northern Rockies nor is it necessarily the best for all wildlife and other forest resources.
Before European settlement in the Northern Rockies, many forest types rarely reached climax conditions because of frequent fires and outbreaks of insects, disease — all made worse by periodic drought. Depending on the site, the interval between disturbances could range from a decade or less to several centuries. However, disturbance was inevitable and played a key role in maintaining the health and diversity of the forest.
Over the last one hundred years, the patterns and types of disturbance have changed. Fire has played a much smaller part in bringing change to our forests, while human disturbances such as logging and grazing have increased.
Natural fires cannot be eliminated from forested ecosystems, however. In preventing fires we've allowed fuels to build and created the opportunity for larger, catastrophic fires and insect and disease epidemics.
Fire Triangle
The Fire Triangle
Heat can refer to several aspects of wildland fire.
A heat source is responsible for the initial start of a wildland fire, and heat is also needed to maintain the fire and permit it to spread. In addition, heat is constantly being released from the fire, warming the surrounding air and preheating fuel in its path.
Heat transfer is a critical issue in the study of wildland fire. For a fire to grow and spread, heat must be transferred to the initial and surrounding fuel. Heat allows fire to spread by removing (evaporating) the moisture from the nearby fuel, enabling it to travel more easily. The mechanism and the speed of heat transfer play a great role in wildland fire behavior.
Three mechanisms of heat transfer exist: convection, radiation, and conduction. All three contribute in different ways to the combustion process, depending in part on the available fuel distribution, the wind speed at the fire site, and the slope of the terrain.
Convection is the transfer of heat through the flow of liquids or gases, such as when hot air rises through a chimney. Convection currents are often responsible for preheating higher shrub layers and the canopy, carrying the groundfire upwards.
Radiation transmits heat by rays, such as from the sun or a flame. Radiation accounts for most of the preheating of fuels surrounding a fire. The temperature of these fuels can sometimes grow so high that the fuels ignite prior to contact with flames, spreading the fire.
Conduction moves heat from one fuel particle to the next, as when the stove burner heats a pan and its contents. Conduction allows the heat to be transferred inside and throughout the fuel, rather than only heating the surface. Because wood is a poor heat conductor, meaning heat does not pass through it easily, conduction is usually not the primary mechanism of heat transfer in a wildland fire.
Fuel
The fuel side of the fire triangle refers to both the external and internal properties of the fuel. External properties refer to the type and the characteristics of the fuel material. Internal properties of fuel address aspects of fuel chemistry. Types of fuel include living vegetation, dead vegetation, (duff, twigs, needles, standing dead snags, leaves, and moss), organic subsurface material (peat and coal), and human built structures. Fuel can be defined as any combustible material.
The size and shape of fuel in part determines its moisture content. Lighter fuels such as grasses, leaves, and needles quickly expel moisture, and therefore burn rapidly. Heavier fuels, such as tree branches, logs, and trunks, take longer to warm and ignite. In areas of light fuel, the temperature required for ignition is lower than in areas of heavier fuel. The oxygen surrounds lighter fuels and allows the fuel to burn with greater intensity, quickly exhausting the fuel supply.
The quantity of combustible fuel in a given area is known as fuel loading. These fuels may be arranged in a uniform pattern and distributed continuously across the ground, allowing a wildland fire to travel uninterrupted. Or, the fuel may be distributed unevenly in a patchy network, forcing the fire to travel over rocks and other barriers by wind-borne embers.
The vertical arrangement of fuel is also an important factor in wildland fires. Ground fuels are all of the combustible materials found below the ground surface, and include tree roots, duff, and organic material. Surface fuels are found at the ground level, including twigs, grass, needles, wood, and other vegetation. Aerial fuels are standing vegetation including tree crowns, branches, leaves, snags, and hanging moss. Crown fires are able to burn independently of surface fires, moving through the treetops.
Oxygen
The third side of the fire triangle represents oxygen. Air contains about 21% oxygen; most fires require air with at least 16% oxygen content to burn under most conditions. Oxygen supports the chemical processes that occur during a wildland fire. When fuel burns, it reacts with oxygen from the surrounding air, releasing heat and generating combustion products, e.g., gases, smoke, particles. The process is known as oxidation.
Forest Disturbance
Disturbance
Events described as disturbances generally cause major changes in a forest, usually altering the way it functions (an area that has recently experienced a stand-replacement burn, for instance, functions in a different way than an old growth forest). They can be natural or human caused. Natural disturbances include events like fire, insects and disease outbreaks, floods, drought, windthrow, and storm damage. Human-caused disturbances include timber harvesting, heavy grazing, and so on.
The consequences of human disturbances can be similar to those caused by natural forces, or they can be of another magnitude, altering ecosystems in ways natural disturbances seldom do.
The photos at right show different types of natural disturbances. Click on a thumbnail to see a larger version of the picture.
Human-caused Disturbances
In 1928, Frank Jaquette, an early settler in the Bitterroot Valley reminisced about Salish land management practices in the 1800s and described them as superior to those of the white settlers. Referring to the West Fork of the Bitterroot River in 1887, he said: "At this time the creek was thoroughly set with a growth of willows and very completely so on the south side. Since it has become part of the white man’s domain and fires are less general and frequent, the large alder growth has very generally replaced these willows. It might be noted here that the Indians were great foresters, as all old-time prospectors will affirm. They left the forests to the tender mercies of nature… [While the Forest] Service spends millions of dollars battling against nature’s force, the result is a tendency to a scrubby growth of timber and a fire trap."
What the Salish, Kootenai, and Pend d'Oreille took for granted, but few Europeans understood, is that fire is a necessary process of forest renewal. Rather than thinking of natural disturbances (events like fire, floods, drought, windthrow, storm damage, insects and disease, etc.) as enemies of a healthy forest, they viewed them as integral, even beneficial. They understood that the plants and animals they harvested and hunted depended on a variety of habitats, and that disturbances created that variety. That’s why they lit so many fires.
Deer and elk, for example, need densely timbered areas to hide in and large old trees for protection from winter cold and summer heat, but they also need open areas for foraging. Grizzly bears need variety too, open areas to feed in and timbered areas for cover.
Other species rely almost exclusively on a single kind of forest habitat, such as openings or old growth. The point is, periodic disturbances, especially fire, which is our most common type of natural disturbance, ensure the diversity that wildlife needs.
Fire can be beneficial as a disturbance. In addition to increasing the variety of habitats for wildlife, in certain fire regimes it tends to favor tree species that are more fire resistant and less vulnerable to insect attack and disease infection. In addition, fire is an important part of the forest nutrient cycle, especially on drier sites.
Without periodic fires, nutrients become less available to plants and soil organisms. Although fire can be kept out of the forest for long periods, it will eventually return, and when it does, the fuel buildups caused by decades without disturbance usually result in much larger and more catastrophic burns — burns that can consume tens of thousands of acres and do long-term damage to soils and other resources.
Key Concepts: The Take-home message
Ecosystems constantly change in ways that are only partially predictable.
Disturbances are common and important in virtually all ecosystems.
The positive effects of disturbance on biodiversity are now well recognized by land managers, but not necessarily by the public.
Efforts to suppress disturbance, such as lightning-caused fires, floods, erosion, drought, disease, and insects, which have been perceived to be in conflict with economic interests, have resulted in reduced biodiversity and ecosystem health.
The more we attempt to maintain an ecosystem in a static condition, the less likely we are to achieve what we intended. We must be willing to bear both the economic and biologic consequences of such management.
Forest Succession
Forest Succession
One of the most important concepts you can take away from this website is that change is a natural part of healthy forest communities. The term forest succession refers to change, specifically the patterns of change that occur within a forest over time.
In simplified presentations of forest succession, the forest moves toward a final or climax condition (as in the animation above). Following a major disturbance, such as a stand-replacement fire, the site enters a short period in which grasses, forbs, and/or shrubs dominate. This period is typically followed by a time when tree species that require lots of sun dominate the site. These trees are called shade-intolerant or early seral species by foresters. In a final stage, shade tolerant or climax species begin to flourish. Eventually they overtop the seral species, and they come to dominate the site, establishing a climax forest community.
The climax condition is often thought of as the goal to which all forests are progressing. It's easy to envision it as the last and best forest, the forest primeval. However, it is not necessarily the natural state of forests in the Northern Rockies nor is it necessarily the best for all wildlife and other forest resources.
Before European settlement in the Northern Rockies, many forest types rarely reached climax conditions because of frequent fires and outbreaks of insects, disease — all made worse by periodic drought. Depending on the site, the interval between disturbances could range from a decade or less to several centuries. However, disturbance was inevitable and played a key role in maintaining the health and diversity of the forest.
Over the last one hundred years, the patterns and types of disturbance have changed. Fire has played a much smaller part in bringing change to our forests, while human disturbances such as logging and grazing have increased.
Natural fires cannot be eliminated from forested ecosystems, however. In preventing fires we've allowed fuels to build and created the opportunity for larger, catastrophic fires and insect and disease epidemics.
Fire Terms
© 2021 Confederated Salish and Kootenai Tribes | Contact Us
© 2021 Confederated Salish and Kootenai Tribes | Contact Us