The Role of Fire in Boreal Forests

Several parameters influence the ignition and intensity of a fire in the boreal forest, most of which are dependent on climate and weather (6). Mean daily temperature affects the growth of vegetation, and therefore the amount of biomass available as fuel (6). Temperature, precipitation, and humidity influence the moisture content of organic matter on the forest floor, which regulates the spread of fire on the ground. Lightning strikes are the most common method to ignite forest biofuels; once a fire is established, wind strength and speed will determine the rate and direction of fire spreading (2). The combination of physical fire parameters comprises the fire weather index (FWI), which is a measure of the susceptibility of an area to fire under the prevailing weather conditions (10). When conditions have been generally hot and dry, fire regime is dominated by small, frequent fire outbreaks. Due to the abundance of dry fuel, these fires ignite easily; however, after each fire the amount of biomass remaining to fuel future fires diminishes rapidly (5). Counterintuitively, the most widespread fires often start under relatively wet and cool conditions. In this scenario, an intense fire is established in a dry section of forest, and is able to maintain enough energy to spread rapidly (5). In general, ~1% of all wildland fires account for 95-98% of area burned in the northern hemisphere boreal zone (4). The intensity of a fire depends on the fuel type and abundance, weather conditions, the topography of the region, and the general health of the forest ecosystem (4). Intensity is quantified by the depth to which a fire burns the surface soil organic layer--more intense fires have more energy and are able to burn through more of the soil profile (4). Different types of fire are characterized by specific weather conditions and intensities. Ground fires burn on the surface exclusively, usually at relatively low intensities. Crown fires burn much more intensely and ignite in the forest canopy, although they frequently burn through the entire forest if they are allowed to grow unchecked (6).

As stated above, fires in the boreal forest are the dominant factor controlling mass movements of carbon from the land surface to the atmosphere; fires determine the net carbon balance of boreal ecosystems (2). The conversion of carbon from reduced organic matter to oxidized carbon dioxide occurs either directly, during burning and combustion of living and dead organic matter, or indirectly through interactions with soil carbon during and after burning. A relatively intense fire has the ability to consume not only the organic litter on the forest floor, but also organic carbon buried centimeters below the soil surface (4). As fire strips at least the surface organic layer from the forest floor, deeper soil horizons are exposed to aerobic respiration. If a fire has stripped the overhanging canopy, sunlight reaching the soil will greatly enhance the rates of carbon respiration (6).

A fire cycle describes the average length of time any given patch of forest goes between burns. In the decades to centuries between fires, biomass builds up in the living vegetation, as detritus on the forest floor, and within the soil profile. During a fire event, all this biomass is combusted and the carbon released into the atmosphere in the span of hours or days. In the Canadian boreal forest, the typical fire cycle is between 75 and 100 years, and stand replacing crown fires burn approximately two million hectares every year (6). However, the area of land burned can vary by up to fifteen times from year to year (4). The fire season extends from about April to September--during this period the FWI often reaches and maintains high levels, indicating a strong risk of fire at any time (6).