Fire is a frequent disturbance in the boreal forest because of the combustible nature of the trees and a warm, dry climate that facilitates severe fire weather. Fires in the boreal typically kill most trees, and as a result the boreal forest is a patchwork of forest stands that reflect the time since the last fire. The species composition of plants and animals changes over time through a process called succession.
During a typical year there are over 9,000 forest fires in Canada, burning an average of 2.5 million hectares (ha) or 25,000 square kilometres. The number of fires and area burned can vary dramatically from year to year.
The percentage of annual area burned within a region is often used to estimate the frequency of fires. If an average of 1% of an area burned each year for 100 years, the fire frequency would be estimated at 100 years. Fire frequency varies widely and depends on the location and the period over which it is calculated. For Canadian forests, historic fire frequencies can be as short as 10 years or as long as 1000 years. The boreal forest typically has a fire frequency of 50 to 200 years, fire being more frequent in the west and less in the east.
Species that need fire in order to replace themselves are fire-dependent. For example, jack pine need the heat from fire to open their cones or else seeds will not be released and the stand will not be renewed. If enough time passes between fires, other species may invade and eventually eliminate jack pine from the landscape.
Prescribed burning refers to the use of planned and controlled fires to reduce the built-up fuels in forested areas. This improves the health of an ecosystem and helps to protect the people living and working in forested areas as well as their homes and businesses.
Prescribed burns can be used to reduce the build-up of surface fuels and reduce the risk of large fires starting later or to promote a natural landscape and habitat for wildlife. In some situations, agencies in Canada ignite fires to allow some fires to burn freely in order to promote beneficial ecological impacts or to meet other land management objectives.
Needles, leaves, and branches are highly flammable fuels and can increase the likelihood that a fire will be able to spread through an area if they have accumulated on the forest floor. If an area is fire-free for a long time, the build-up of these forest fuels could possibly increase the intensity of the next fire that occurs.
Some drier forest types, such as in the interior of British Columbia, had more frequent fire cycles in the past. Fire suppression along with logging and grazing practices have resulted in increased stand densities and greater potential for crown fires (fires that move through the upper parts of the trees). For many northern ecosystems there is sufficient fuel available for fire, and build-up is not an issue. A more important consideration is whether there are breaks between fuel sources or if they are continuous, which can determine the area over which a fire will grow.