How We Fight Fire - The Science of Fire (fire behaviour and fire ecology)

Introduction

Understanding how fire works is important to controlling wildfire and using fire as a management tool. The Ontario Ministry of Natural Resources promotes the study of fire science, including fire behaviour and the effect of fire on the environment. The information gained from this study is used to develop new techniques, computer models and tools that improve forest fire management decision-making, safety, and efficiency.

 

The articles below will provide you with some information that will help you understand a bit about how fires start and act and why resource managers make some of the choices they do concerning forest fires.

 

Fire Behaviour

 

The Fire Triangle showing fuel, oxygen and heatThe manner in which fuels ignite, flames develop and wildfire spreads is collectively referred to as fire behaviour. Fire managers require a working knowledge of certain universal, scientific principles applicable to all spreading fires, which are based in physics and the chemistry of combustion.

 

For a fire to exist and spread it has three fundamental needs -- fuel, oxygen and heat -- which fire managers refer to as the fire triangle. The fire triangle demonstrates that oxygen, fuel and heat in proper proportions are all required to create a fire. In the absence of any one of these factors, fire cannot start or exist. We use this knowledge to prevent and suppress forest fires. By removing or weakening any one of the sides of the fire triangle, we can reduce the intensity of, or eliminate the fire altogether.

 

Fuel

 

In general terms, forest fires require a continuous and dry layer of “fine fuels” on the surface of the ground in order to continue spreading. Fine fuels may be conifer needles, hardwood leaves, Fine fuels on the surface of the groundgrass, lichens, moss or small shrubs. The moisture content of the fuels is a very important factor in determining fire behaviour. A high moisture level will slow the fire because much of the heat energy is used to eliminate this moisture in the fuels. Live trees usually contain a great deal of moisture, dead trees very little. Past and present weather determines the moisture content of the fuels. The effect of fuel size and arrangement is also important; larger fuels require more energy to ignite and burn, than finer fuels.

 

Oxygen

 

Oxygen is the second requirement and must be available in sufficient quantities to sustain combustion. Again, weather plays an important role, as wind can bring a fresh supply of oxygen to the fire and also push the fire toward a new fuel source.

 

Heat

 

The final requirement for fire to burn and spread is a source of heat, or ignition. An external source of heat is usually required to start a fire. In forest fires, this can be a natural source, such as a lightning strike, or it can be caused by people, with sources ranging from campfires, to sparks from machinery. Temperature is an important weather factor, since higher temperatures accelerate the drying of fuels allowing for easier ignition and faster burning.

 

Once all three sides of the fire triangle are present, the combustion process can take place. The combustion process consists of three more or less distinct but overlapping phases.

 

The first phase is preheating, in which the fuels ahead of the fire are heated, dried and partially distilled into ignitable gases.

 

The second phase involves the continual distillation of gases, which is now accompanied by their burning, or oxidation, in which energy is released in the form of light and heat. An ignition source is the link between phases one and two. The flames seen at a forest fire are the burning of these distilled gases. Combustion products are principally invisible -- water and carbon dioxide. If combustion is incomplete, some distilled substances will condense without being burned and remain suspended in very small droplets of liquid or solid over the fire. These condensed substances are the familiar smoke, which accompanies forest fires.

 

The third phase of combustion is the glowing phase. The charcoal left from the second phase is burned with glowing combustion and leaves a small amount of residual ash.

 

For the distillation phases of combustion to take place, heat must be applied to the fuels. There are three primary ways for heat to be transferred from one location to another. These are conduction, radiation and convection.

 

During conduction, heat passes through solid objects such as a branch, the same way in which a spoon absorbs heat sitting in a cup of hot tea. This form of heat transfer is not important to forest fire spread but plays a role in deep burning fires spreading underground.

 

Radiation is the type of heat energy one feels when sitting near a campfire. It travels in straight lines. Most of the pre-heating of fuels in front of a forest fire is done by radiation. The intensity of the radiation drops with distance from the fire. On large fires, there is ample radiant heat in front of the fire to complete the first phase of combustion and begin release of flammable gases, which will ignite with an ignition source.

 

The third heat transfer mechanism is convection, which is the transfer of heat through gas. This is the principal heat transfer method from a ground fire to the crowns of trees. Hot gases rise, drying the forest canopy and raising its temperature to the ignition threshold. It is the primary method of heat transfer in large, high intensity fires.

 

Forest fires can spread in three distinct manners. Sub-surface fires burn in the organic matter beneath the surface litter, and are sustained by glowing combustion.

 

Picture showing tree fuelsSurface fires spread with a flaming front, which burns leaf litter, fallen branches and other fuels on the ground level. Crown fires burn across the tree tops, which results in a very intense fire which is difficult to control. Crown fires also require an intense surface fire to support them.

 

In Ontario, the Canadian Forest Fire Danger rating System is used to determine potential forest fire behaviour. It uses information on the forest fuels, topography, past and present weather information and applies the scientific principles of combustion to predict fire behaviour characteristics. Examples of those characteristics are rates of spread (how fast the fire will spread), intensity (how hot the fire will be), and the type of fire (surface, or crown). Fire managers use this information to determine daily fire suppression strategies, fire prevention needs and potential uses of prescribed fire.    

 

Fire Ecology

 

What is Fire Ecology?

 

Fire Ecology can be defined as the study of the relationship between fire, the physical environment and living organisms.

 

Fire is a natural element in the renewal of Ontario's forest lands. It is as much a part of a healthy ecosystem as sunlight and water, and it has played a major part in the growth and shaping of Ontario's forests for several thousand years.

 

Fire is a natural force that sustains the cycle of growth, death and re-growth in Ontario's forest, grassland and shrub ecosystems. Fire can also be an unnatural force, introduced by humans that may destroy property and threaten human life.

 

The Role of Fire in the Ecosystem

 

Some ecosystems have evolved with fire and depend on it for renewal and maintenance. For example, frequent grassland fires promote the growth of herbs and grasses and prevent trees, shrubs and non-native plants from invading. The plants and animals in these ecosystems are adapted to fire and the conditions it creates. Conifer trees are very susceptible to fire. Deciduous trees, on the other hand, do not readily burn and large stands can act as a natural barrier to an advancing fire.

 

Effect of Fire on Plants

 

Jackpine regenerating after a fire passesFire is so prevalent in the natural forest ecosystem that many species are uniquely adapted to regenerating after a fire. Fire may trigger the release of seeds from serotinus cones, from a tree species such as jackpine, which requires no tree survivors for their regeneration after a fire passes. The intense heat forces the cones to open and seeds drop into the ash-enriched soil. It stimulates the flowering and fruiting of many plants, as a result of the soil being enriched by the ash, which acts as a natural fertilizer. Fire alters seed-beds by burning off litter and humus and baring the mineral soil, making it easier for seeds to get started, which is especially beneficial for species like white pine and yellow birch.

 

Plant species regenerating after a fireIn the southern parts of the province there are a variety of ecosystems such as tall grass prairies and oak savannahs, that depend on regular occurrence of fire for their renewal. Frequent fires prevent tree species from taking over these rare and endangered ecosystems and encourages the production of seed from the prairie species.

 

Removal of the forest canopy by fire will allow in more light and rainfall, stimulating reproduction of many herbs and woody plants. It also acts to temporarily reduce competition for moisture, nutrients, heat and light, favouring whatever species have the means of reproduction readily available.

 

Plant species regenerating after a fireThe frequency of forest fires in a given area determines the stage of succession or age that plant communities can be expected to reach. The fire regime describes the temporal and spatial pattern of fire disturbance in an ecosystem. Fire regimes are described by the fire cycle: the average interval between fires on a site, and the fire's intensity (heat level).

 

Overall, the impact of forest fires is to control the relative abundance and location of plant communities. These determine animal habitat patterns, regulating both population and distribution.

 

Effects of Fire on Animals

 

The effect of fire on animals is hard to judge. It can be both beneficial and detrimental. Most, but not all animals can take protective action during a large forest fire. Large animals such as moose, deer, caribou and bear move out of the way, ahead of the fire. Ground animals burrow underground and are insulated from the fire's heat. While most birds easily escape a fire, young birds may be killed if they are unable to fly. Ground nesting birds and animals are particularly vulnerable, even to a low intensity, surface fire. Smoke and heat from a fire can disorient and kill large numbers of insects.

 

Choke cherry treesFire increases the food supply for moose, deer, beaver and hare, which depend largely on forage that proliferates within accessible heights immediately following a fire. By contrast, fire eliminates forage plants found in old forests, notably the tree and ground lichens consumed by woodland caribou.

 

Ruffed grouse standing on side of roadAnimal species are adapted to different stages of forest successions and fire regimes. Fire increases the yield of many berry-producing plants to the benefit of bears and many birds.  Game birds such as ruffed grouse benefit from the openings created by fire in bush areas. Fire regulates many insect populations, some of which are important food sources for warblers, woodpeckers and other birds. Following a fire, bark beetles and other insects colonize newly burned trees, providing a rich food source for these birds.

 

Large fires may produce subtle ecological effects by altering the relationship of organisms to their environment. When a fire removes plant cover, soil erosion can result, causing a leaching of nutrients. Water quality may be temporarily affected by this short-term nutrient flush and by suspended silt and ash.

 

Burned trees adjacent to waterRemoval of the forest cover can cause increases in water temperatures. This can have a detrimental effect on cold water fish habitat and populations.

While forest fires contribute to some loss of wildlife, research has found that such mortality is not significant to the entire wildlife community.

 

The immediate impact of fire may seem harsh. The young or weaker animals of any species are most at risk due to their inability to flee and may not survive a fire. Animals that have escaped a fire are forced to seek food and shelter elsewhere, their natural food supplies and habitat having been altered. This temporary setback soon changes as vegetation in the burned over areas grow back quickly. In addition, many wildlife species that are prone to experience loss due to fire have built in reproductive strategies that rapidly replenish their populations.

 

Biodiversity

 

Burned area showing different ages and types of treesOntario's forests and wildfire are linked in irregular sequence of alternating fire disturbance and re-growth that periodically rejuvenates the forest. Fire is critical in maintaining the mix of young and old forest types across the landscape. Fires burn at different times and intensities and under varying weather conditions, producing a patchwork of vegetation of different ages and types, including trees, brush and grasses. Within the boundaries of a wildfire, green islands of residual, unburned forest may remain. The rich variety of created habitat supports many species of animals, insects and birds. The biodiversity or variety of life of an ecosystem is an indicator of health and stability. The periodic return of fire generally keeps a high level of biodiversity on the landscape by creating conditions favourable to the renewal and maintenance of several large types of ecosystems of various ages. Additionally, the landscape mosaic produced by fire is better protected against disease and insect epidemics. Fire has a sanitising effect that reduces the incidence of insects and disease.

 

Fire as a Management Tool

 

Aerial photo showing different pattern of landscapeToday, resource managers use fire as a tool in vegetation and ecosystem management in areas licensed for sustainable forestry and in some large parks. Over the last century fire suppression activities resulted in large areas of even aged, over mature, blown down, or pest killed forests that normally would have burned periodically and been renewed over time.

 

A fire ecology study completed in Quetico Park suggests than the average period before a forest fire re-occurs in a park area previously burned, given no fire suppression efforts, is approximately 78 years. In that park, forest mosaic is changing because of people causing fires and the introduction of modern fire suppression techniques. This adds to the complexity of the fire regime and extends the fire return interval to more than 800 years.

 

Aerial photo showing mix of burned and unburned areasFire ecology principles are applied in developing prescriptions for setting fire or allowing natural fire to take its course to help meet ecosystem management goals. Under a range of weather and fuels types and conditions, fire has a predictable range of immediate and long-term effects on fuel removal, vegetation and soils. By selectively returning fire to our forested landscapes, resource managers help restore and maintain the forest's ecological integrity.

 

Jackpine regeneration on a burned areaAfter a fire, the burned area may seem dead, but many forms of life survive. The rapid, 'green up' of the forest floor is living proof that a new forest cycle has begun. Fire creates ideal growing conditions for new regeneration in several ways. Nutrient cycling is accelerated by the combustion process, which reduces slowly decaying woody material to ash, containing minerals available for plant growth. The blackened and exposed forest floor draws heat, encouraging the germination of dormant seeds and the sprouting of new growth. Fire reduces forest litter, which exposes mineral soil, lowers the pH balance of the soil and creates conditions, which generally favour new growth of tree and shrub species.