Succession is a natural process of ongoing change in the forest. One community of trees and shrubs gradually replaces another, and then another, over time. Each plant community affects the soil, amount of light and shade, and other factors to create conditions that allow the new community to take over.
In and around Edmonton, trembling aspen, balsam poplar trees and white birch are typically considered “pioneer” deciduous species. They will grow where there are few other trees.
White spruce require a canopy of aspen and poplar to get started. Over time, the spruce will grow taller than the aspen and poplar. Once they dominate the upper canopy, less light enters the lower areas of the forest. This changes what grows in the under storey as well as close to the ground.
Eventually spruce age and die. When they fall, they leave an open space in the forest canopy. Light shines in and “shade intolerant” trees like aspen and poplar establish again.
This succession takes place naturally. Or, it can be triggered by disturbances like fire, insect infestations, strong winds, landslides, or logging. Some degree of disturbance is desirable. After all, northern boreal forests evolved with cycles of fire, disease, and pests.
If not exposed to dramatic disturbances, a forest will eventually progress to a climax stage. In our region the climax stage is a mixed-wood forest consisting of white spruce and trembling aspen. You can observe this climax stage in Larch Sanctuary: old spruce trees mixed with younger trembling aspen.
True climax forests are becoming increasingly rare due to human activity. The climax forest in the Whitemud Creek ravine is there because disturbances like fire, flooding and logging have been limited for some time.
Looking to the future, a warmer climate in northern latitudes could have many effects on this and other forests, including increased insect outbreaks, diseases, droughts and/or floods.
Genetic diversity is nature’s insurance policy: the more genetic variation in a population of trees, the better it will withstand these stresses. Similarly, the varying ages of the trees in the Whitemud Creek ravine strengthens the forest’s ability to survive.
Finally, damage to forest soils can result from seemingly harmless activities like short-cutting or braiding a trail or removing organic matter (See the importance of dead trees). Keep this in mind as you enjoy the Sanctuary’s forest.