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Effects of wildfire on forest community structure
< Student Name >
Department of Biology
St. Francis Xavier University
Antigonish, Nova Scotia, Canada
16 March, 20__
Introduction
Many studies have shown that disturbances not only negatively affect ecosystems but also play
an important role in ecosystem formation and restoration (Laughlin et al., 2004). One major
such disturbance is fire. The term ‘wildfire’ refers to a fire that was not planned by humans and
burns uncontrollably (Whelan, 1995). In contrast, prescribed fires are planned and executed in a
specific area and set to a certain intensity to achieve specific goals and are usually conducted by
ecological management teams (Walstad et. al, 1990 in Certini, 2005). Frequent prescribed fires
were performed in response to the discovery that fire suppression had negative effects on
ecosystems, but York (2000) showed that both scenarios (fire suppression as well as frequent
burning) adversely affect communities. Though the effects of fires on communities have been
well studied, their complexity has hindered accurate predictions by humans. The complexity
arises because many factors contribute to the effect of fires on ecosystems. In a forest, there are
different types of fires; each impacts the community structure in a different way. There are also
certain conditions that favor different types of fires. Additionally, the different types of
vegetation and species also affect the type of fires that can occur in a given area. Ultimately, it is
the interaction of all of these that makes each wildfire different and therefore makes the effect of
wildfire on the forest community structure plentiful and variable.
Types of Fire
Ryan (2002), after reviewing numerous reports on wildfires in forests, summarized descriptions
of three main types of fires: ground, surface and crown fires (Figure 1). Ryan noted that usually,
fires were categorized based on their behavior: what they burned and how they burned. Surface
and crown fires combust by flaming while ground fires combust by smoldering. Surface fires
creep along the ground, destroying smaller and younger trees and shrubs while crown fires burn
predominantly the canopy layer in the forests; both involve major flames (Ryan, 2002). Ground
fires on the other hand, burn deep within the ground and displays very little surface flaming.
Transition fires, which some researchers considered a fourth type, occur when a surface fire is on
its way to becoming a crown fire.
Figure 1. The three major types of forest fires as characterized by fire behavior. Adapted from
Ryan (2002)
Additionally, some fires burn slowly and last relatively long, while other fires burn quickly and
sometimes only last for a few minutes. Fires can also be categorized based on their intensity:
low versus high intensity. Fire duration and intensity seem to be closely linked to which of the
three major types of fire is occurring. Ground fires usually burn for hours to weeks and are
characterized by temperatures over 300oC. Surface fires on average last for a few minutes but
may last more if they encounter large amounts of woody debris which will burn up to a couple
hours. Crown fires are typically very brief, lasting for about thirty seconds to eighty seconds but
release the most energy and so tend to be the most intense. Which type of fire occurs in a forest
depends on multiple factors, some of which are explained next. .
Factors that affect fire type
Generally, organic soil depths greater than 4 cm are ideal for ground fires. Loose litter and short
plants increase the likelihood of a surface fire occurrence while lots of foliage, taller plants,
twigs and epiphytes within the canopy above the forest floor encourages crown fires occurrence
(Ryan 2002).
Seasonality, climate and weather also plays an important role in determining what type of fire
occurs in forests. Crown fires are more dominant in the spring when the leaves have reduced
moisture and the ground is still wet with melted snow while ground fires are more common
during the summer months when the leaves are moist and the ground is dry. Also, a rainy day
will make a ground fire less likely while high winds increase drying by the sun, which then
increases the potential severity of the fire. Surface fires tend to transition into crown fires when
winds are high and upper level fuel is available (Finney, 2001, Ryan, 2002) and crown fires
collapse when winds drop (Van Wagner, 1977). It must be noted however, that the presence of
low branches on tall trees provide fuel continuity or a path for surface fires to make that
transition.
The Many Effects of Wildfires
Different plants have different properties that cause them to either be more or less susceptible to
fires. This paper will examine wildfires mainly in coniferous boreal forests and explore how this
vegetation type (and the different species within it) responds to the different types of fires
creating a vast range of possible fire effects on such ecosystems.
Above ground effects of fire damage:
Coniferous boreal forests are typically relatively homogeneous because of its low plant species
diversity. The two dominant plants are pine and spruce trees. Drier soils are dominated by pine
which generally are found in areas of high fire frequency while spruce dominates on mesic-moist
soils with low fire frequency (Esseen et al., 1997).
Apfelbaum and Haney (1981) found that after a wildfire in a pine forest, ecosystem structure
changed considerably. They reported that tree cover decreased form 98% to 48% while herbs
and small woody plants on the ground increased from 28% to 51% replacing much of the moss
that was once present. In addition, community composition also changed. By spring of the year
following the fire, five territorial bird species had been lost while six additional species
frequently visited the study area. These results support Caswell’s (1976) conclusion that
disturbances eliminate dominant species thereby allowing more and different species to enter and
thrive in the community thus increasing diversity. Apfelbaum and Haney specifically noted that
the number of ground-brush foraging species doubled from three to six after the fire and
attributed that change to the increase in herbs and small woody plants on the forest floor. They
concluded that the change in physical structure of the ecosystem brought about by the wildfire
had a direct impact on species composition.
Laughlin et al. (2004) found that greater plant species richness was characteristic of areas that
experienced occasional low intensity surface fires as opposed to fire excluded areas. They found
that the fire excluded areas had high levels of duff: litter ratios. When those areas were subjected
to wildfire, duff depths and duff: litter ratios were reduced and a consequent increase in species
richness occurred.
Fire size also seemed to contribute to the ecosystem structure post fire. In Yellowstone National
Park, larger patches resulted in more opportunistic species colonizing but less chances of
survival of seedlings of forest herbs and shrubs (Turner et al., 1997). They noted that bigger
patches were a result of fires of a higher intensity than the fires that caused the smaller patches.
Turner et al. therefore also concluded that patch size was an effect of fire intensity and that it was
really fire intensity that had an influence on the structure and composition of communities. Also,
variability in fire severity caused heterogeneity in the plant ecosystem once post fire succession
occurred. They suggested that original species were less likely to survive in higher intensity fires
which would increase the chances for different species to colonize the area.
Bark thickness is a major contributor to the mortality or survival of plants exposed to fires. Bark
refers to the dead outer layer of plants that serve to protect the live inner layers; the thinner the
tree bark, the more susceptible the plant is to fires. For example, North American boreal forests
have suffered tremendous tree deaths because the majority of trees there are thin barked; only red
pine is thick barked and fire resistant (Engstrom and Mann, 1991). Eurasian boreal forests on the
other hand, are dominated by fire resistant pines and survive most active surface fires (Ryan,
2002).
Survival of a plant after disturbance often depends on its mode of reproduction and regeneration
in the burned area. This helps determine whether or not the ecosystem composition remains the
same. Rowe (1983) in Ryan (2002) categorized boreal species on their ability to regenerate and
reproduce after a fire as well as their ability to colonize a site after a fire. Species that rely on
vegetative reproduction as well as those that have underground seed banks are most affected by
deep ground fires and usually unaffected by surface and crown fires – the plants are able to re-
sprout, continue growing and the seeds germinate after surface fires or shallow ground fires.
Species that have seeds stored in their canopies are the opposite: they survive ground fires better
than they do surface fires. Some species can establish themselves immediately after a fire and
thrive for a long time, whereas some species establish immediately, but do not live very long.
Others are unable to establish immediately and must wait until other species create a conducive
environment (like shade) for them to colonize. That said, the effect of a specific fire type on an
ecosystem is partly dependent on how the species in that ecosystem are able to regenerate,
reproduce and colonize.
The immediate effect of severe ground fires is generally a shift of the forest ecosystem
composition to one that is made up primarily of species that have seeds in their canopies or have
high seed dispersal. Alternately, the immediate effect of severe surface and crown fires is
generally shift of forest ecosystem composition to one that is made up primarily of species that
undergo vegetative reproduction or stores seeds underground.
The combined effect of an active surface, crown and ground fire is extremely severe and usually
eliminates most of the original species. When this occurs, the fire acts as an equalizer on the
ecosystem, completely clearing the land both above and below, thus affording the opportunity of
colonization to only species that are specifically able to thrive in a recently, completely burned
site. Such sites are likely to generate dense forests with high canopy cover and slow growing
trees, leaving little opportunity for later successors (Johnstone and Chapin, 2006). This results in
the homogenization of the ecosystem at least for some time after the fire.
It is more common however, for fires to burn at different levels of intensity in different areas
within an ecosystem. Climate as well as micro-climates determine the overall productivity of an
area which in turn determines the available fuel load; the higher production, the more fuel
available, the more intense the fire (Whelan, 1995). This leads to areas with low intensity burns
being a refuge for plant species, their seeds and propagules. At the end of the fire, there will be
areas already colonized and areas that are bare. Such sites are likely to generate open-canopied
forests – this allows new species to colonize later and form new interactions with the original
species (Arseneault, 2001).
Interestingly, the heterogeneity of a fire is highly dependent on the heterogeneity of the
ecosystem it interacts with (Turner et al., 1994). A diverse, complex forest, because of its
different spatial arrangements and differential resistance to fire, when burned by fire forms small
patches of different burn intensities. Simple, homogeneous ecosystems tend to burn uniformly
over large areas of land.
Nutrient cycling is an important aspect of ecosystem functioning and is greatly affected by
wildfires. In forests, surface fires burn understory trees and in pine forests, they scorch needles
as opposed to burning them in crown fires – this affects subsequent nutrient cycling as the
scorched needles retain their nutrients and fall to the ground (Ryan, 2002). Surface and crown
fires are also the cause of loss of nutrients that are volatile when subjected to high temperatures.
Of special importance is nitrogen which, in conjunction with being the limiting nutrient on land,
is also most prone to such loss. According to Neary et al. (1999), generally surface fires that
burn large woody debris and litter at temperatures above 500oC cause nitrogen to evaporate.
Temperatures above 760oC cause the release of phosphorus from these sources as well.
Below ground effects of fire damage:
Ecosystems that store most of their organic matter pools below ground seem to be less
susceptible to nutrient loss through fire damage than ecosystems that store more of their organic
matter above ground (Neary et al., 1999). While nitrogen is lost from plant material above
ground, most researchers have found that wildfires often make nitrogen in the soil more available
to plants by converting organic nitrogen (amino compounds) to inorganic nitrogen such as
ammonia and nitrates. These increases, however, are short lived and may be insignificant to
plants if not used right away. This is because ammonia can also evaporate out of the soil at high
temperatures and the nitrates formed are usually quickly lost through denitrification and
leaching. Both the loss of a limiting nutrient and its increased availability to plants have a direct
effect on the primary production of the post-fire ecosystem: a loss greatly diminishes production,
while increased availability enhances production.
Since nitrogen is the limiting nutrient in terrestrial ecosystems, it is important to consider the
effect of fire on the organisms that make nitrogen available to the rest of the ecosystem
community: microbes. Ground fires are by far the most detrimental fire type to microorganisms.
Their inability to move affords them little to no possibility of taking refuge and so most microbes
are consumed by ground fires. Like all organisms, different microbial species display varying
characteristics including varying levels of heat tolerance and so, unless the ground fire is
extremely severe causing sterilization, some heat tolerant species can form spores and survive
and thermophiles will thrive.
In a study of a mixed conifer forest of North America, Yeager at al., (2005) found that the
abundance and composition of the soil microbial community – more specifically the nitrogen
fixers and the ammonia oxidizers - were altered. Yeager et al. reported that burned soils had
reduced microbial biomass. They also noted an increase in diversity in the nitrogen fixers and a
change in the microbial species that was the dominant ammonia oxidizer. The shift from one
dominant ammonia-oxidizing species to another was attributed to the increase in ammonia in the
soil – a fire effect. Yeager et al. concluded that the initially dominant species was better suited
for low levels of ammonia in the soil (before fire) while the second dominant species was better
adapted to thrive in high ammonia environments (after fire).
Fungi also play an important role in soil ecosystems and have been shown to be susceptible to
damage by fire. The symbiosis between vascular plants and mycorrhizae is particularly fragile.
Klopatek (1991) recorded temperatures of 50 – 60oC causing a reduction in mycorrhizae by more
than half and temperatures over 90oC causing reductions of up to 95%. Lack of mycorrhizae
significantly reduces the availability of water and nutrients for the plant, thus adversely affecting
general health and production of the plant community.
Another group of organisms important in ecosystem function and drastically affected by ground
fires is the soil invertebrates. Unlike microbes, invertebrates’ ability to move affords them a
chance to escape fires, primarily by moving deeper into the soil – thus increasing their likelihood
of survival. Recovery time of invertebrate species adversely affected by fires is variable and
largely depends on the individual species and the conditions of the soil such as soil moisture and
nutrient availability. For example, Wanner and Xylander (2003) found that amoebae in a
German pine forest were reduced but recovered in a year after a fire, while Collett et al. (1993)
recorded a decrease in earthworms following wildfire that took several years to recover to
original levels.
The abiotic components and the properties of soils that are key for biotic sustenance are also
altered during wildfires. Organic matter combustion degrades the structural units of top soil that
allows normal hydrologic function and smooth movement of roots and invertebrates; combustion
of clay particles results in the same for deeper soil layers (Neary et al., 1990). Degradation of
soil structural units reduces pore size, preventing the soil from storing water. At temperatures
above 176oC, hydrophobic organic compounds cling to soil particles at the surface, forming a
layer that is impenetrable by water. Both of these situations cause an overall decrease in soil
moisture, an increase in surface run-off and susceptibility to erosion on steep terrain, all of which
result in reduced conduciveness to plant growth due to lack of stability and loss of nutrients. It is
expected, therefore, that heavy rains immediately after a forest fire should drastically enhance
these effects. Loss of soil is detrimental to the ecosystem as it leaves many organisms – both
plants and animals - without a habitat. Another factor that enhances fire effect is a prior major
disturbance. Such disturbances can increase the likelihood of a fire occurring and may also
increase the intensity of the fire. A prime example of this is a drought. Droughts cause
substantial drying of all material that once contained moisture, leaving forest fuel and soils dry
and organisms weak with decreased resistance, a recipe for a severe fire (Westerling et al.,
2006).
Conclusion
Wildfires naturally occur in many forest ecosystems. Boreal forests affected by wildfires have
been extensively studied by ecologists to assess the effects of such fires on the ecosystems. Each
of the three main types of fires – crown, surface and ground fires – is more likely to occur under
certain conditions characteristic of that fire type. Such conditions include the moisture content
of the ground and foliage, climate, weather, soil depth and amount of litter. Once established, a
wildfire interacts with many aspects of the ecosystem to create the effects that we see. Though
each individual fire type affects different parts of the forest community, often the fire types occur
simultaneously, producing a large pool of variable effects.
Some major components of an ecosystem that are considered when discussing effects of wildfire
include the impact of weather and climate on the intensity of the fire, the ability of different
species to survive the high temperatures, the ability of different species to reproduce, regenerate
or to colonize soon after a fire and the heterogeneity or homogeneity of the forest structure and
composition. The interaction of these ecosystem components with the different fire types brings
about both positive and negative effects including, but not limited to, increased diversity,
reduction of susceptibility to future severe fires and change in nutrient availability. The
information pooled together in this paper suggests that wildfires when not too severe or too
frequent, bring about positive change to community structure and composition, thus following
the Intermediate Disturbance Hypothesis.
Though many have explored the option of prescribed fires to rectify ecosystem imbalances using
models developed for this purpose, lots of studies still need to be done to further assess how an
individual ecosystem may respond to fire treatment in both the short and long term. Precise and
accurate predictions may seem impossible, but is imperative when attempting to manipulate
something with such diverse effects as fires in forests.
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