Fire behaviour

Fire behaviour refers to the manner in which fuel ignites, flame develops and fire spreads. In wildland fires, this behaviour is influenced by how fuels (such as needles, leaves and twigs), weather and topography interact.

Once a fire starts, it will continue burning only if heat, oxygen and more fuel are present. Together, these three elements are said to make up the “fire triangle.”

To put out a fire requires eliminating one or more of the fire triangle’s elements. Firefighters work to do that by:

• cooling fuels below the combustion temperature through the use of water, foam, retardant or dirt
• cutting off the oxygen supply through the use of water, retardant or dirt
• removing fuel by clearing a swath of trees and brush ahead of the advancing fire

Types of wildland fires

There are three basic types of forest fires:

• Crown fires burn trees up their entire length to the top. These are the most intense and dangerous wildland fires.
• Surface fires burn only surface litter and duff. These are the easiest fires to put out and cause the least damage to the forest.
• Ground fires (sometimes called underground or subsurface fires) occur in deep accumulations of humus, peat and similar dead vegetation that become dry enough to burn. These fires move very slowly, but can become difficult to fully put out, or suppress. Occasionally, especially during prolonged drought, such fires can smoulder all winter underground and then emerge at the surface again in spring.

Causes of wildland fires

The fire season generally runs from April to October, with the peak of activity occurring from mid-May to August. Catastrophic fires tend to occur during periods of extended drought or wind storms.

Lightning strikes cause slightly less than half of all wildland fires in Canada, but account for nearly 67% of the land area burned. There are two main reasons for this:

• Lightning-caused fires often occur in remote areas where human life, property and timber values are not threatened. Fire suppression in these areas may therefore be intentionally limited, leaving fire to play its natural role.
• Several lightning fires can be ignited simultaneously, leaving agencies with difficult decisions about where to send available firefighting crews and equipment.

Humans cause slightly more than half of all wildland fires in Canada, typically in populated forest and grassland areas. Because of where these fires occur, they are usually spotted early and can be reached quickly by firefighting crews. Still, the threat they pose to human safety and property makes them a major concern for firefighting crews.

Canada’s main fire management tools

Research into wildland fire as a physical, biological and socio-economic phenomenon continues to increase our understanding of its behaviour, costs and benefits, and of how it can best be managed. Research has also improved our predictive capabilities and led to the development of many practical tools for evaluating fire risks, analyzing fire behaviour and predicting where fire may threaten forest values.

For more than four decades, Canadian Forest Service researchers have been developing and refining several national systems for identifying where and when the risk of wildland fire is greatest. This work has played a crucial role in protecting Canadians, their property and forest resources.

The main national system is the Canadian Forest Fire Danger Rating System (CFFDRS).

Tools developed to support the CFFDRS include:

• Canadian Forest Fire Weather Index System (FWI) — a system used across Canada to assess day-to-day changes in the potential for fires to ignite and spread
• Canadian Forest Fire Behavior Prediction (FBP) System — a system used to estimate potential fire spread rate, fuel consumption and fire intensity for a range of forest fuel types in Canada
• Canadian Fire Effects Model (CanFIRE) — an extension model of the CFFDRS used to analyze the immediate physical effects of fire on stands and the resulting ecological effects on forest vegetation

As well, government and university researchers have developed several fire occurrence prediction models for predicting the number of lightning-caused and human-caused forest fires in a given area.