Windfall

Trees in a forest can fall on their own because they are diseased, old or otherwise in a weakened state. Most trees fall, however, because they are blown over by wind.

The term “windfall” refers both to a tree that has toppled to the ground, and to the disturbance created by its fall.

Small windfalls

A small windfall (affecting one or a few fallen trees) creates gaps in the forest cover. As a result, more light reaches the ground and young shoots take advantage of this input of energy, quickly occupying the new space.

At the same time, the uprooting of trees disturbs the soil, favouring the germination of certain seeds (such as those of yellow birch) that would otherwise remain dormant. Trees that lie dead on the ground also attract decomposers and provide shelter for all kinds of animals, including insects and small mammals.

Windfall therefore makes it possible for mature forests, especially where fire is very rare, to renew themselves.

Major windfalls

A major windfall can sometimes occur during a violent windstorm, affecting tens of thousands of hectares of forest at once.

After a major windfall, ecosystem dynamics are disrupted by:

• the creation of gaps in the forest cover
• the accumulation of debris on the ground
• holes and mounds caused by the uprooting of trees
• injuries to the branches and trunks of trees that remain standing

On the positive side, such an event initiates secondary succession. Because seedlings and saplings are not necessarily destroyed, as happens after a fire, regrowth can be very rapid. Young shoots that are already in place no longer have to compete with large trees.

The period when the windfall occurs can also affect the regeneration and future composition of the forest. For example, at the peak time of seed production, the newly disturbed soil may support germination of the seeds that have just fallen to the ground. As well, the accumulation of branches and trunks on the ground provides shelter for wildlife.

On the negative side, a major windfall can also disturb wildlife by destroying the habitat provided by mature trees. For example, young birds that are still unable to fly will not be able to survive. Furthermore, every injury caused by breakage to branches or bark is an open door to insects and diseases. As a result, some trees may be injured but remain standing, only to die and fall later on because they have come under attack by pests.

Sensitivity to windfall

The risk of windfall can be increased by certain environmental conditions:

• Some areas are subject to strong winds.
• Thin or poorly drained soils may increase a forest’s sensitivity to windfall.
• Some tree species are more susceptible than others to windfall. Pines and other trees that grow taller than other species are more easily overturned than shorter species. Fir trees are particularly sensitive to windfall because of their superficial root systems. In addition, firs that are older than 50 years often suffer from root rot, which increases the risk of windfall in mature fir stands.