Fire in Canada’s forests varies in its role and importance.
In the moist forests of the west coast, wildland fires are relatively infrequent and generally play a minor ecological role.
In boreal forests, the complete opposite is true. Fires are frequent and their ecological influence at all levels—species, stand and landscape—drives boreal forest vegetation dynamics. This in turn affects the movement of wildlife populations, whose need for food and cover means they must relocate as the forest patterns change.
The boreal: A forest shaped by fire
The Canadian boreal forest is a mosaic of species and stands. It ranges in composition from pure deciduous and mixed deciduous-coniferous to pure coniferous stands.
The diversity of the forest mosaic is largely the result of many fires occurring on the landscape over a long period of time. These fires have varied in frequency, intensity, severity, size, shape and season of burn.
This photo sequence shows: 1) A fire, 2) The regrowth of aspen 1 year after fire, 3) Burned tree with black-backed woodpecker, 1 year after fire, 4) A 50-year stand, 5) A 100-year stand, 6) 150 years of growth, 7) Old-growth forest (with gap dynamics)
How various boreal species respond to fire
Fire strongly influences the structure, growth and renewal of many of Canada’s forest and grassland communities. Different species, however, respond differently to fire.
- After a fire, forest regeneration on burned sites begins with the establishment of pioneer species, notably aspen, white birch, jack pine and lodgepole pine. All of these species require full sunlight to thrive, and all are well adapted to landscapes where fires regularly recur.
Aspen and birch are able to re-establish quickly by sprouting from stumps and roots of burned trees. These species are also able to recolonize burned sites by producing abundant seeds that can be blown by wind over long distances.
Jack pine and lodgepole pine have serotinous cones (protected by a waxy coating) that require the heat of fire to release their seeds. Fire also produces favourable conditions for the seeds of these pines to germinate. Nutrients are released in the soil, mineral soil is exposed, competing species are eliminated and the amount of sunlight on the forest floor is increased. Both jack and lodgepole pine depend on fire to regenerate.
- Black spruce, with its semi-serotinous cones, may also become established in the years following a fire, but this species grows slower in full sunlight than the pioneer species do. If fire does not occur for more than 100 years, the early pioneer trees eventually die and become replaced by the black spruce growing in the understory. Other shade-tolerant species then establish under the shady cover.
- Species such as balsam fir, white spruce and white cedar have no special adaptations to fire. They can colonize burned areas only by coming in from unburned refugia. (This might occur, for example, when the seeds of these species are blown into a burned area by the wind or carried by animals.) As a result, these species take a long time to reappear in burned stands after a fire—in some cases, as long as 150 years.
Because extensive fires place balsam fir and cedar at a disadvantage, these species are rare in areas that are repeatedly severely burned or where fires are large.
The fire management balancing act
Fire is a vital ecological component of Canadian forests and will always be present.
The ongoing challenge for fire management agencies is therefore how to manage fire to protect human values while still allowing fire to play its important ecological role in maintaining healthy forests.
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