Forest ecosystem impacts

Thousands of diseases and species of insects exist in Canada’s forests. Their occurrence varies by forest type across the country.

Their ecology also varies. Some forest insect species, for example, eat only a tree’s leaves. Other species feed only beneath the bark of live trees. Stands attacked by forest diseases may develop any one of a range of unhealthy conditions, from foliar (leaf) blight to root rot and cankers.

Some trees are healthy enough to resist or recover from such attacks with little lasting damage but older or weakened trees may not survive infestation. In general, the more severe the infestation the more likely even healthy trees will be negatively affected.

The ecological impacts of native insects and diseases

Native insects and diseases play a vital role in nature’s food webs and nutrient cycles and in the process affect forest health.

While some insects and diseases reduce forest values, others provide ecological services, including pollination and decomposition. Even those native insects considered damaging help create the conditions that promote forest renewal and growth. For example, infestations of mountain pine beetle typically kill the largest and oldest trees, but younger trees that survive attack subsequently tend to display accelerated growth. Spruce budworm outbreaks are another example. These insects target smaller trees of species such as balsam fir, and so contribute to stand thinning, which in turn enables longer-lived tree species such as spruce to gain stand dominance.

However, when a major outbreak of a native insect or disease occurs in a scarce or vulnerable habitat and is severe beyond the historical norm, the ecological health of that habitat may suffer.

The threat posed by alien insects and diseases

Alien insect and disease pests are a particular concern for forest managers. Because they are “non-native,” it is more difficult to predict how the forest ecosystem they spread to will respond to the new disturbance. Even the usual benefits that native outbreaks can have on the forest ecosystem may not occur.

In most cases, fortunately, invasive species are unable to adapt to their new environment. This means they cannot establish or spread widely and may therefore have few or no perceptible negative impacts.

Sometimes, however, an alien species does take hold, spreads rapidly and leaves a trail of extensive tree damage. This can happen where the invading insect or disease species has no natural enemies to keep it in check. It can also happen where the trees and other organisms lack natural resistance against the invader.

Since the 1880s, the arrival of well over 80 significant alien forest insects and diseases has been documented in Canada. Several of these introduced pests have seriously damaged native forests. For example:

• Dutch elm disease has vastly reduced numbers of American elm, formerly one of the most widely planted shade trees in North America. It was introduced into Canada about 1940.
• The emerald ash borer has killed millions of ash trees in Ontario and Quebec. A hidden impact of this insect’s destruction is the loss of unique gene pools represented by populations of each of the native ash species. It was introduced into North America in the 1990s.
• The gypsy moth, an invasive alien species from Asia, has plagued Ontario, Quebec and the Maritime Province at different times throughout the last century. It is known as one of the most serious defoliators of hardwoods in North America.

A summary of how insects and diseases affect forest ecosystems

Positive impacts

Forest insects and diseases:

• help renew forests by removing old, weakened or otherwise vulnerable trees
• help in soil formation by breaking down dead trees and other plant material and recycling the nutrients
• provide new habitat and food for wildlife
• pollination

Negative impacts

Forest insects and diseases:

• cause radial and height growth loss, volume loss, dieback and deformity
• through that damage, can kill individual trees or entire forests
• through widespread killing of existing forests, can result in the displacement of existing tree species

Changing climate conditions and forest pests

Some forest experts see climate change as a disturbance that may weaken native forests (through drought conditions, for example) and leave them generally more susceptible to pest attacks. Changing climate conditions may also mean that Canada’s forests could become receptive habitats for a wider range of biological invaders.

Researchers at CFS are investigating whether native insects and diseases are causing disturbances beyond their historical levels because of climate and human-caused changes in the forest.

A critical question related to this is whether such a change in disturbance levels could threaten the resilience of forest ecosystems to recover from intense or extensive insect or disease outbreak.