Fires are an expected step in a forest's cycle
Whether a forest fire burns in the morning or afternoon can have effects decades later in the boreal forest.
 |
|
Fireweed flourishes on the site of a recent fire.
(photo: DIAND)
|
The kind of forest that grows up after a forest fire depends on a variety of factors, including such apparently minor things as the time of day the fire began to burn, says Al Beaver, Supervisor of Fire Management Planning for the Department of Indian Affairs and Northern Development's Forest Resources Branch.
"You can set out the stages of regeneration in general terms, but in detail the process is highly variable."
Two of many factors that determine how a forest recovers are the intensity and severity of the fire. And both those factors depend, to some extent, on the time of day when the fire starts burning, Beaver says.
Both terms, intensity and severity, have special meaning for forest managers. Intensity is the amount of heat energy released by the fire. Severity is a measure of the area burnt and the depth of the burn.
Fires that start in the morning are generally less intense than fires that start in the afternoon because the air and fuel temperatures are lower in the morning. Fires in the spring, when the moisture level in the duff layer (organic material on the forest floor) is still high, are generally far less severe than fires in mid-season, when the duff is warm and dry.
A fire that burns with high intensity, Beaver says, kills most trees in its path and burns off the duff layer, exposing the mineral soil beneath. A less intense fire might pass through an area quickly, scorching some trees without killing them and leaving much of the duff layer intact.
A severe fire that burns a large area might leave parts of the burnt-out forest exposed to full sunlight and many kilometres away from the nearest living plants. A less severe fire might only burn a pocket in the forest, leaving a partly-shaded opening surrounded by live, seed-dispersing plants.
Each of those situations offers a different habitat for forest regeneration, encouraging different kinds of seeds to germinate and grow in the burn, Beaver explains.
For example, the blackened ground in the middle of a large burn is likely to be repopulated first by plants like fireweed and poplar, whose seeds are carried long distances by the wind. Spruce and pine seeds are not carried as far, so they are more likely to take root around the edges of the burnt area.
New growth can also come from seeds held in the duff layer and exposed to heat and sunlight by the fire. Rhizomes, or underground stems, often survive the fire and throw up new plants. Plants can also grow as suckers from the roots of trees damaged by the fire.
How quickly regeneration starts depends on a range of variables, says Beaver.
"The weather conditions before and after the fire affect the type of germination," he says. "With an early spring fire, seeds have all summer to germinate and grow. But if the fire is in late fall, they might germinate and then freeze."
Topography has an effect as well, Beaver adds. A warm southerly slope offers a very different habitat from a cool north-facing slope. Elevation, moisture, and soil type are more factors that affect which plants will grow and how quickly.
Despite all the variables, there is a general pattern to the regrowth of boreal forests after a fire, Beaver says. First to appear are the shade-intolerant, rapid-growth species like fireweed. The burnt-out area will be home to grasses, then shrubs, then a mixed-wood forest, and finally a mature boreal forest dominated by spruce and fir. Eventually the fuel of duff and old growth builds up to a point where the forest is again ripe for fire.
Of course, not every forest achieves maturity. Fires hit some areas more often than others, so frequency is another factor that affects the character of forest regeneration. If a plant species requires 30 years to produce viable seed, the species could disappear from areas where the average frequency of fire is less than 30 years.
Fire, and other disturbances like disease, are part of the cycle of the boreal forest, Beaver says. No matter how bad the apparent damage, fire releases the nutrients locked up in the forest's organic matter, returns them to the soil, and triggers the process of rebirth.
"We've had some pretty severe fires and they've all come back," says Beaver.
For more information about forest fires and forest fire ecology, contact the Forest Resources Branch of the Department of Indian Affairs and Northern Development, Whitehorse.
