How disturbances shape Canada's forests

Canada’s forests are influenced by a range of natural disturbances that vary in severity, extent and frequency. Natural disturbances have occurred in Canada’s forests at least since the retreat of glaciers, more than 10,000 years ago, often renewing whole forest landscapes and shaping forest composition, structure and habitat diversity. Fire, insects, disease, drought and wind storms all affect the forest on an ongoing basis, with their relative importance varying regionally. Climate change is affecting all these disturbances and, through them, may change future forest landscapes, with impacts on society.

Climate change and fires

The area affected by forest fire across Canada is highly variable among regions and between years because climate is different across regions, and weather varies from year to year. Climate change is gradually imposing an increasing trend on forest fires, a trend that is partially masked by the large variability of this disturbance. An increasing annual area burned will gradually make Canada’s forests younger and possibly enhance the slow process of change in species dominance and composition. An increase in area burned could also impact society – directly by increasing fire risks to communities and infrastructure and indirectly by reducing the area available for harvest.

Climate change and insects

Insect populations also respond to climate change, but through a more complex web of biological interactions that may enhance or decrease outbreak extent and severity. A succession of warm winters in central British Columbia made the mountain pine beetle outbreak possible, affecting tens of millions of hectares of forest. The current spruce budworm outbreak in eastern Canada is taking place in northern regions that used to be too cold for the insect to cause significant damage; climate change may also gradually push it yet farther north, out of the range of balsam fir, its currently preferred tree host.

Climate change and invasive species

Always more uncertain are the risks posed by invasive insects and diseases. The cold climate that is typical for much of Canada’s forested land has served as a barrier to many invasive species, but the warming climate is lowering this barrier. Some invasive species already established in North America are slowly moving north, while new arrivals may have a better chance of surviving. Expanding global trade increases the risk of introducing new invasive species, although ever-improving phytosanitary measures help reduce this risk.

Assisted migration is an adaptation option through which the generally northward or upslope movement of warmer growth conditions is matched when regenerating a site after harvesting, by planting seeds and/or seedlings of species or populations that are better adapted to the warmer conditions. With more than 400,000 hectares planted in Canada annually, forest managers have an opportunity to apply this relatively low-cost and low-risk approach. Some provinces are implementing assisted migration. For example, in Alberta, guidelines have now extended the seed zone northward by up to 2° latitude and upslope by up to 200 metres.

Adapting our forest and forest sector

Knowledge of past forest growth and dynamics will still inform forest management decisions, but climate change must be recognized as an agent of increasing uncertainty and change. Because of the relatively long lifespan of trees, forests tend to change slowly in response to the changing climate. Single or cumulative disturbances can nevertheless create rapid local changes, such as a change in the tree species that dominate or even a reduction in how many trees grow back. Identification and monitoring of areas prone to such events may help inform forest management decisions. Capturing the increased growth potential resulting from a warming climate is also being explored in many regions of Canada as an adaptation option.

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