The Canadian boreal forest began to form after the retreat of the glaciers approximately 10,000 years ago. Natural disturbances such as fire and insects have played, and continue to play, a major role in the boreal forest's development and renewal.

While parts of Canada’s boreal region contain significant amounts of relatively undisturbed forest, most stands are replaced naturally by fire, insects, diseases and wind or ice storms every 100 years or so. The oldest trees and stands are those that have escaped the natural catastrophic disturbances that renew forests in the region.

Trees in the boreal forest generally live 100 years, with the oldest trees living up to 300 years. Balsam fir stands often show old-growth characteristics at 70 to 80 years, while the old-growth onset for black spruce is closer to 110 to 160 years. Averages change depending on site conditions.

The actual age, quantity of dead wood, and other old-growth attributes vary depending on the species composition and environmental conditions of a particular forest. Although the range of species and general soil conditions across Canada’s vast boreal region do not change dramatically, there are substantial differences in the landscape patterns and successional trends.