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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