The Old Fire burning in the San Bernardino Mountains (image taken from the International Space Station)
Fire ecology is a scientific discipline concerned with natural processes involving fire in an ecosystem and the ecological
effects, the interactions between fire and the abiotic and biotic
components of an ecosystem, and the role as an ecosystem process. Many
ecosystems, particularly prairie, savanna, chaparral and coniferous forests, have evolved with fire as an essential contributor to habitat vitality and renewal. Many plant species in fire-affected environments require fire to germinate, establish, or to reproduce. Wildfire suppression not only eliminates these species, but also the animals that depend upon them.
Campaigns in the United States have historically molded public opinion to believe that wildfires
are always harmful to nature. This view is based on the outdated belief
that ecosystems progress toward an equilibrium and that any
disturbance, such as fire, disrupts the harmony of nature. More recent
ecological research has shown, however, that fire is an integral
component in the function and biodiversity
of many natural habitats, and that the organisms within these
communities have adapted to withstand, and even to exploit, natural
wildfire. More generally, fire is now regarded as a 'natural
disturbance', similar to flooding, wind-storms, and landslides, that has driven the evolution of species and controls the characteristics of ecosystems.
Fire suppression, in combination with other human-caused
environmental changes, may have resulted in unforeseen consequences for
natural ecosystems. Some large wildfires in the United States have been
blamed on years of fire suppression and the continuing expansion of
people into fire-adapted ecosystems, but climate change is more likely responsible. Land managers are faced with tough questions regarding how to restore a natural fire regime, but allowing wildfires to burn is the least expensive and likely most effective method.
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A
combination of photos taken at a photo point at Florida Panther NWR.
The photos are panoramic and cover a 360 degree view from a monitoring
point. These photos range from pre-burn to 2 year post burn.
Fire components
A fire regime describes the characteristics of fire and how it interacts with a particular ecosystem.
Its "severity" is a term that ecologists use to refer to the impact
that a fire has on an ecosystem. Ecologists can define this in many
ways, but one way is through an estimate of plant mortality. Fire can
burn at three levels. Ground fires will burn through soil that is rich
in organic matter. Surface fires will burn through dead plant material
that is lying on the ground. Crown fires will burn in the tops of shrubs
and trees. Ecosystems generally experience a mix of all three.
Fires will often break out during a dry season, but in some areas
wildfires may also commonly occur during a time of year when lightning
is prevalent. The frequency over a span of years at which fire will
occur at a particular location is a measure of how common wildfires are
in a given ecosystem. It is either defined as the average interval
between fires at a given site, or the average interval between fires in
an equivalent specified area.
Defined as the energy released per unit length of fireline (kW m−1), wildfire intensity can be estimated either as
- the product of
- the linear spread rate (m s−1),
- the low heat of combustion (kJ kg−1),
- and the combusted fuel mass per unit area,
- or it can be estimated from the flame length.
Radiata pine plantation burnt during the 2003 Eastern Victorian alpine bushfires, Australia
Abiotic responses
Fires
can affect soils through heating and combustion processes. Depending on
the temperatures of the soils caused by the combustion processes,
different effects will happen- from evaporation of water at the lower
temperature ranges, to the combustion of soil organic matter and formation of pyrogenic organic matter, otherwise known as charcoal.
Fires can cause changes in soil nutrients through a variety of
mechanisms, which include oxidation, volatilization, erosion, and
leaching by water, but the event must usually be of high temperatures in
order of significant loss of nutrients to occur. However, quantity of
nutrients available in soils are usually increased due to the ash that
is generated, and this is made quickly available, as opposed to the slow
release of nutrients by decomposition. Rock spalling (or thermal exfoliation) accelerates weathering of rock and potentially the release of some nutrients.
Increase in the pH of the soil following a fire is commonly
observed, most likely due to the formation of calcium carbonate, and the
subsequent decomposition of this calcium carbonate to calcium oxide
when temperatures get even higher.
It could also be due to the increased cation content in the soil due to
the ash, which temporarily increases soil pH. Microbial activity in the
soil might also increase due to the heating of soil and increased
nutrient content in the soil, though studies have also found complete
loss of microbes on the top layer of soil after a fire. Overall, soils become more basic (higher pH) following fires because of acid combustion. By driving novel chemical reactions at high temperatures, fire can even alter the texture and structure of soils by affecting the clay content and the soil's porosity.
Removal of vegetation following a fire can cause several effects
on the soil, such as increasing the temperatures of the soil during the
day due to increased solar radiation on the soil surface, and greater
cooling due to loss of radiative heat at night. Fewer leaves to
intercept rain will also cause more rain to reach the soil surface, and
with fewer plants to absorb the water, the amount of water content in
the soils might increase. However, it might be seen that ash can be
water repellent when dry, and therefore water content and availability
might not actually increase.
Biotic responses and adaptations
Plants
Lodgepole pine cones
Plants have evolved many adaptations to cope with fire. Of these adaptations, one of the best-known is likely pyriscence,
where maturation and release of seeds is triggered, in whole or in
part, by fire or smoke; this behaviour is often erroneously called serotiny,
although this term truly denotes the much broader category of seed
release activated by any stimulus. All pyriscent plants are serotinous,
but not all serotinous plants are pyriscent (some are necriscent,
hygriscent, xeriscent, soliscent, or some combination thereof). On the
other hand, germination of seed activated by trigger is not to be confused with pyriscence; it is known as physiological dormancy.
In chaparral communities in Southern California, for example, some plants have leaves coated in flammable oils that encourage an intense fire.
This heat causes their fire-activated seeds to germinate (an example of
dormancy) and the young plants can then capitalize on the lack of competition in a burnt landscape. Other plants have smoke-activated seeds, or fire-activated buds. The cones of the Lodgepole pine (Pinus contorta) are, conversely, pyriscent: they are sealed with a resin that a fire melts away, releasing the seeds. Many plant species, including the shade-intolerant giant sequoia (Sequoiadendron giganteum),
require fire to make gaps in the vegetation canopy that will let in
light, allowing their seedlings to compete with the more shade-tolerant
seedlings of other species, and so establish themselves.
Because their stationary nature precludes any fire avoidance, plant
species may only be fire-intolerant, fire-tolerant or fire-resistant.
Fire intolerance
Fire-intolerant
plant species tend to be highly flammable and are destroyed completely
by fire. Some of these plants and their seeds may simply fade from the
community after a fire and not return; others have adapted to ensure
that their offspring survives into the next generation. "Obligate
seeders" are plants with large, fire-activated seed banks that
germinate, grow, and mature rapidly following a fire, in order to
reproduce and renew the seed bank before the next fire.
Seeds may contain the receptor protein KAI2, that is activated by the growth hormones karrikin released by the fire.
Fire tolerance. Typical regrowth after an Australian bushfire
Fire tolerance
Fire-tolerant
species are able to withstand a degree of burning and continue growing
despite damage from fire. These plants are sometimes referred to as "resprouters."
Ecologists have shown that some species of resprouters store extra
energy in their roots to aid recovery and re-growth following a fire. For example, after an Australian bushfire, the Mountain Grey Gum tree (Eucalyptus cypellocarpa)
starts producing a mass of shoots of leaves from the base of the tree
all the way up the trunk towards the top, making it look like a black
stick completely covered with young, green leaves.
Fire resistance
Fire-resistant
plants suffer little damage during a characteristic fire regime. These
include large trees whose flammable parts are high above surface fires.
Mature ponderosa pine (Pinus ponderosa)
is an example of a tree species that suffers virtually no crown damage
under a naturally mild fire regime, because it sheds its lower,
vulnerable branches as it matures.
Animals, birds and microbes
A mixed flock of hawks hunting in and around a bushfire
Like plants, animals display a range of abilities to cope with fire,
but they differ from most plants in that they must avoid the actual fire
to survive. Although birds
are vulnerable when nesting, they are generally able to escape a fire;
indeed they often profit from being able to take prey fleeing from a
fire and to recolonize burned areas quickly afterwards. Some
anthropological and ethno-ornithological evidence suggests that certain
species of fire-foraging raptors may engage in intentional fire
propagation to flush out prey. Mammals are often capable of fleeing a fire, or seeking cover if they can burrow. Amphibians and reptiles
may avoid flames by burrowing into the ground or using the burrows of
other animals. Amphibians in particular are able to take refuge in water
or very wet mud.
Some arthropods also take shelter during a fire, although the heat and smoke may actually attract some of them, to their peril. Microbial
organisms in the soil vary in their heat tolerance but are more likely
to be able to survive a fire the deeper they are in the soil. A low fire
intensity, a quick passing of the flames and a dry soil will also help.
An increase in available nutrients after the fire has passed may result
in larger microbial communities than before the fire.
The generally greater heat tolerance of bacteria relative to fungi
makes it possible for soil microbial population diversity to change
following a fire, depending on the severity of the fire, the depth of
the microbes in the soil, and the presence of plant cover. Certain species of fungi, such as Cylindrocarpon destructans
appear to be unaffected by combustion contaminants, which can inhibit
re-population of burnt soil by other microorganisms, and therefore have a
higher chance of surviving fire disturbance and then recolonizing and
out-competing other fungal species afterwards.
Fire and ecological succession
Fire
behavior is different in every ecosystem and the organisms in those
ecosystems have adapted accordingly. One sweeping generality is that in
all ecosystems, fire creates a mosaic of different habitat
patches, with areas ranging from those having just been burned to those
that have been untouched by fire for many years. This is a form of ecological succession
in which a freshly burned site will progress through continuous and
directional phases of colonization following the destruction caused by
the fire.
Ecologists usually characterize succession through the changes in
vegetation that successively arise. After a fire, the first species to
re-colonize will be those with seeds are already present in the soil, or
those with seeds are able to travel into the burned area quickly. These
are generally fast-growing herbaceous
plants that require light and are intolerant of shading. As time
passes, more slowly growing, shade-tolerant woody species will suppress
some of the herbaceous plants.
Conifers are often early successional species, while broad leaf trees
frequently replace them in the absence of fire. Hence, many conifer
forests are themselves dependent upon recurring fire.
Different species of plants, animals, and microbes specialize in
exploiting different stages in this process of succession, and by
creating these different types of patches, fire allows a greater number
of species to exist within a landscape. Soil characteristics will be a
factor in determining the specific nature of a fire-adapted ecosystem,
as will climate and topography.
Some examples of fire in different ecosystems
Forests
Mild to moderate fires burn in the forest understory, removing small trees and herbaceous groundcover.
High-severity fires will burn into the crowns of the trees and kill
most of the dominant vegetation. Crown fires may require support from
ground fuels to maintain the fire in the forest canopy (passive crown
fires), or the fire may burn in the canopy independently of any ground
fuel support (an active crown fire). High-severity fire creates complex early seral forest habitat, or snag forest
with high levels of biodiversity. When a forest burns frequently and
thus has less plant litter build-up, below-ground soil temperatures rise
only slightly and will not be lethal to roots that lie deep in the
soil. Although other characteristics of a forest will influence the impact of fire upon it, factors such as climate and topography play an important role in determining fire severity and fire extent. Fires spread most widely during drought years, are most severe on upper
slopes and are influenced by the type of vegetation that is growing.
Forests in British Columbia
In Canada,
forests cover about 10% of the land area and yet harbor 70% of the
country’s bird and terrestrial mammal species. Natural fire regimes are
important in maintaining a diverse assemblage of vertebrate species in up to twelve different forest types in British Columbia.
Different species have adapted to exploit the different stages of
succession, regrowth and habitat change that occurs following an episode
of burning, such as downed trees and debris. The characteristics of the
initial fire, such as its size and intensity, cause the habitat to
evolve differentially afterwards and influence how vertebrate species
are able to use the burned areas.
Shrublands
Lightning-sparked wildfires are frequent occurrences on shrublands and grasslands in Nevada.
Shrub fires typically concentrate in the canopy and spread continuously if the shrubs are close enough together. Shrublands
are typically dry and are prone to accumulations of highly volatile
fuels, especially on hillsides. Fires will follow the path of least
moisture and the greatest amount of dead fuel material. Surface and
below-ground soil temperatures during a burn are generally higher than
those of forest fires because the centers of combustion lie closer to
the ground, although this can vary greatly. Common plants in shrubland or chaparral include manzanita, chamise and Coyote Brush.
California shrublands
California shrubland, commonly known as chaparral, is a widespread plant community of low growing species, typically on arid sloping areas of the California Coast Ranges or western foothills of the Sierra Nevada. There are a number of common shrubs and tree shrub forms in this association, including salal, toyon, coffeeberry and Western poison oak. Regeneration following a fire is usually a major factor in the association of these species.
South African Fynbos shrublands
Fynbos shrublands occur in a small belt across South Africa.
The plant species in this ecosystem are highly diverse, yet the
majority of these species are obligate seeders, that is, a fire will
cause germination of the seeds and the plants will begin a new
life-cycle because of it. These plants may have coevolved into obligate seeders as a response to fire and nutrient-poor soils.
Because fire is common in this ecosystem and the soil has limited
nutrients, it is most efficient for plants to produce many seeds and
then die in the next fire. Investing a lot of energy in roots to survive
the next fire when those roots will be able to extract little extra
benefit from the nutrient-poor soil would be less efficient. It is
possible that the rapid generation time that these obligate seeders
display has led to more rapid evolution and speciation in this ecosystem, resulting in its highly diverse plant community.
Grasslands
Grasslands
burn more readily than forest and shrub ecosystems, with the fire
moving through the stems and leaves of herbaceous plants and only
lightly heating the underlying soil, even in cases of high intensity. In
most grassland ecosystems, fire is the primary mode of decomposition, making it crucial in the recycling of nutrients.
In some grassland systems, fire only became the primary mode of
decomposition after the disappearance of large migratory herds of
browsing or grazing megafauna driven by predator pressure. In the
absence of functional communities of large migratory herds of
herbivorous megafauna and attendant predators, overuse of fire to
maintain grassland ecosystems may lead to excessive oxidation, loss of
carbon, and desertification in susceptible climates. Some grassland ecosystems respond poorly to fire.
North American grasslands
In North America fire-adapted invasive grasses such as Bromus tectorum
contribute to increased fire frequency which exerts selective pressure
against native species. This is a concern for grasslands in the Western United States.
In less arid grassland presettlement fires worked in concert with grazing to create a healthy grassland ecosystem as indicated by the accumulation of soil organic matter significantly altered by fire.
The tallgrass prairie ecosystem in the Flint Hills of eastern Kansas and Oklahoma is responding positively to the current use of fire in combination with grazing.
South African savanna
In the savanna of South Africa,
recently burned areas have new growth that provides palatable and
nutritious forage compared to older, tougher grasses. This new forage
attracts large herbivores
from areas of unburned and grazed grassland that has been kept short by
constant grazing. On these unburned "lawns", only those plant species
adapted to heavy grazing are able to persist; but the distraction
provided by the newly burned areas allows grazing-intolerant grasses to
grow back into the lawns that have been temporarily abandoned, so
allowing these species to persist within that ecosystem.
Longleaf pine savannas
Yellow pitcher plant is dependent upon recurring fire in coastal plain savannas and flatwoods.
Much of the southeastern United States was once open longleaf pine
forest with a rich understory of grasses, sedges, carnivorous plants
and orchids. The above maps shows that these ecosystems (coded as pale
blue) had the highest fire frequency of any habitat, once per decade or
less. Without fire, deciduous forest trees invade, and their shade
eliminates both the pines and the understory. Some of the typical
plants associated with fire include Yellow Pitcher Plant and Rose pogonia. The abundance and diversity of such plants is closely related to fire frequency. Rare animals such as gopher tortoises and indigo snakes also depend upon these open grasslands and flatwoods. Hence, the restoration of fire is a priority to maintain species composition and biological diversity.
Fire in wetlands
Although
it may seem strange, many kinds of wetlands are also influenced by
fire.
This usually occurs during periods of drought.
In landscapes with peat soils, such as bogs, the peat substrate itself
may burn, leaving holes that refill with water as new ponds.
Fires that are less intense will remove accumulated litter and allow
other wetland plants to regenerate from buried seeds, or from rhizomes.
Wetlands that are influenced by fire include coastal marshes, wet prairies, peat bogs, floodplains, prairie marshes and flatwoods.
Since wetlands can store large amounts of carbon in peat, the fire
frequency of vast northern peatlands is linked to processes controlling
the carbon dioxide levels of the atmosphere, and to the phenomenon of
global warming.
Dissolved organic carbon (DOC) is abundant in wetlands and plays a critical role in their ecology. In the Florida Everglades, a significant portion of the DOC is "dissolved charcoal" indicating that fire can play a critical role in wetland ecosystems.
Fire suppression
Fire serves many important functions within fire-adapted ecosystems.
Fire plays an important role in nutrient cycling, diversity maintenance
and habitat structure. The suppression of fire can lead to unforeseen
changes in ecosystems that often adversely affect the plants, animals
and humans that depend upon that habitat. Wildfires that deviate from a
historical fire regime because of fire suppression are called
"uncharacteristic fires".
Chaparral communities
In 2003, southern California witnessed powerful chaparral
wildfires. Hundreds of homes and hundreds of thousands of acres of land
went up in flames. Extreme fire weather (low humidity, low fuel
moisture and high winds) and the accumulation of dead plant material
from 8 years of drought, contributed to a catastrophic outcome. Although
some have maintained that fire suppression contributed to an unnatural
buildup of fuel loads, a detailed analysis of historical fire data has showed that this may not have been the case.
Fire suppression activities had failed to exclude fire from the
southern California chaparral. Research showing differences in fire size
and frequency between southern California and Baja has been used to
imply that the larger fires north of the border are the result of fire
suppression, but this opinion has been challenged by numerous
investigators and is no longer supported by the majority of fire
ecologists.
One consequence of the fires in 2003 has been the increased density of invasive and non-native
plant species that have quickly colonized burned areas, especially
those that had already been burned in the previous 15 years. Because
shrubs in these communities are adapted to a particular historical fire
regime, altered fire regimes may change the selective pressures on plants and favor invasive and non-native species that are better able to exploit the novel post-fire conditions.
Fish impacts
The Boise National Forest is a US national forest located north and east of the city of Boise, Idaho.
Following several uncharacteristically large wildfires, an immediately
negative impact on fish populations was observed, posing particular
danger to small and isolated fish populations. In the long term, however, fire appears to rejuvenate fish habitats by causing hydraulic changes that increase flooding and lead to silt
removal and the deposition of a favorable habitat substrate. This leads
to larger post-fire populations of the fish that are able to recolonize
these improved areas.
But although fire generally appears favorable for fish populations in
these ecosystems, the more intense effects of uncharacteristic
wildfires, in combination with the fragmentation of populations by human
barriers to dispersal such as weirs and dams, will pose a threat to fish populations.
Fire as a management tool
Restoration ecology is the name given to an attempt to reverse or mitigate some of the changes that humans have caused to an ecosystem. Controlled burning
is one tool that is currently receiving considerable attention as a
means of restoration and management. Applying fire to an ecosystem may
create habitats for species that have been negatively impacted by fire
suppression, or fire may be used as a way of controlling invasive
species without resorting to herbicides or pesticides. However, there is
debate as to what state managers should aim to restore their ecosystems
to, especially as to whether "natural" means pre-human or pre-European.
Native American use of fire, not natural fires, historically maintained the diversity of the savannas of North America. When, how, and where managers should use fire as a management tool is a subject of debate.
The Great Plains shortgrass prairie
A combination of heavy livestock grazing and fire-suppression has
drastically altered the structure, composition, and diversity of the
shortgrass prairie ecosystem on the Great Plains,
allowing woody species to dominate many areas and promoting
fire-intolerant invasive species. In semi-arid ecosystems where the
decomposition of woody material is slow, fire is crucial for returning
nutrients to the soil and allowing the grasslands to maintain their high
productivity.
Although fire can occur during the growing or the dormant
seasons, managed fire during the dormant season is most effective at
increasing the grass and forb cover, biodiversity and plant nutrient uptake in shortgrass prairies.
Managers must also take into account, however, how invasive and
non-native species respond to fire if they want to restore the integrity
of a native ecosystem. For example, fire can only control the invasive spotted knapweed (Centaurea maculosa)
on the Michigan tallgrass prairie in the summer, because this is the
time in the knapweed's life cycle that is most important to its
reproductive growth.
Mixed conifer forests in the US Sierra Nevada
Mixed conifer forests in the United States Sierra Nevada
used to have fire return intervals that ranged from 5 years up to 300
years, depending on the local climate. Lower elevations had more
frequent fire return intervals, whilst higher and wetter elevations saw
much longer intervals between fires. Native Americans tended to set
fires during fall and winter, and land at a higher elevation was
generally occupied by Native Americans only during the summer.
Finnish boreal forests
The
decline of habitat area and quality has caused many species populations
to be red-listed by the International Union for Conservation of Nature.
According to a study on forest management of Finnish boreal forests,
improving the habitat quality of areas outside reserves can help in
conservation efforts of endangered deadwood-dependent beetles. These
beetles and various types of fungi both need dead trees in order to
survive. Old growth forests can provide this particular habitat.
However, most Fennoscandian boreal forested areas are used for timber
and therefore are unprotected. The use of controlled burning and tree
retention of a forested area with deadwood was studied and its effect on
the endangered beetles. The study found that after the first year of
management the number of species increased in abundance and richness
compared to pre-fire treatment. The abundance of beetles continued to
increase the following year in sites where tree retention was high and
deadwood was abundant. The correlation between forest fire management
and increased beetle populations shows a key to conserving these
red-listed species.
Australian eucalypt forests
Much
of the old growth eucalypt forest in Australia is designated for
conservation. Management of these forests is important because species
like Eucalyptus grandis rely on fire to survive. There are a few eucalypt species that do not have a lignotuber,
a root swelling structure that contains buds where new shoots can then
sprout. During a fire a lignotuber is helpful in the reestablishment of
the plant. Because some eucalypts do not have this particular mechanism,
forest fire management can be helpful by creating rich soil, killing
competitors, and allowing seeds to be released.
Management policies
United States
Fire policy
in the United States involves the federal government, individual state
governments, tribal governments, interest groups, and the general
public. The new federal outlook on fire policy parallels advances in
ecology and is moving towards the view that many ecosystems depend on
disturbance for their diversity and for the proper maintenance of their
natural processes. Although human safety is still the number one
priority in fire management, new US government objectives include a
long-term view of ecosystems. The newest policy allows managers to gauge
the relative values of private property and resources in particular
situations and to set their priorities accordingly.
One of the primary goals in fire management is to improve public education in order to suppress the "Smokey Bear" fire-suppression mentality and introduce the public to the benefits of regular natural fires.
