Like all living things, forests have a life cycle. They are young, grow older and, if left undisturbed, can even die.

But a forest rarely remains undisturbed. Many things can happen to it: fire, insect infestation, wind, flood, drought, fungal attack.

"After the disturbance, the forest clock is set back," says Sue Olson, forest ecologist for the Forest Resources branch of the Department of Indian Affairs and Northern Development. The boreal forest -- the forest that grows in northern latitudes like the Yukon needs disturbance in order to regenerate and restore fertility to the soil.

The earliest phase of a boreal forest doesn't look much like a forest at all. In the beginning, says Olson, the area is dominated by grasses and shrubs, plants that need direct light to grow. They die off each year, a process that puts nutrients back into the soil.

In the next stage of forest life, deciduous trees like willow, poplar, and aspen join the shrubs. They, too, need light and drop their leaves back to the earth each year, but they grow taller and shade the lower plants, changing the composition of plant life on the forest floor.

In the third phase, conifers or evergreen trees gradually take over. Conifers don't need direct light, so they grow under the canopy of the deciduous forest. Eventually they grow tall enough to shade the deciduous trees, which begin to disappear.

In most of the Yukon, the mature boreal forest is largely a spruce forest. Because spruce generally take more nutrients out of the soil than they put in, the forest floor is very different from the grass and shrub land of the early forest, Olson says. Besides losing nutrients, the soil in the shade of the trees gets colder. Shrubs and grasses do not survive, but mosses, fungi and lichens thrive.

The coniferous forest might last two or three centuries if nothing disturbs it. However, most Yukon forests are disturbed regularly by fire. Olson says fire sweeps through most forests every 150 to 250 years, on average. After it passes, the forest returns to its earliest shrub and grassland phase and its life cycle begins again.

In some parts of the Yukon, however, fire is fairly rare. Forests in the Kluane area are in a "lightning shadow" created by the St. Elias Mountains, Olson says. Since most forest fires are started by lightning, the result is fewer forest fires in Kluane and some forests that are 250 to 300 years old.

Going three centuries without a forest fire is not necessarily a good thing, says Olson. Moss grows thick under the old trees and combines with fallen branches, twigs and needles to form a deep layer of organic material. Beneath the insulating organic layer, the soil is too cold for seedlings to develop, so there are no new trees growing up to replace the old.

Olson says scientists are not sure what would happen if no disturbance or renewal took place. One theory is that the forest would eventually turn into a bog.

However, even without fire, forests are usually disturbed. Insects and disease attack the trees, low areas flood, high winds blow trees over, or a single tree falls.

"I think a single tree falling over is as important as the larger events," Olson says. One tree that topples, pulling up its root mass and exposing the mineral-rich soil to the sun, will start a pocket of new forest in the midst of the old.

In the Kluane area, spruce bark beetles are helping regenerate older forests, Olson says. Hidden in the pores of the beetles' wings are fungi spores that rub off and enter the tree's circulation system. A healthy tree can usually fight off the fungi, Olson says, but in weakened trees the fungi block the flow of nutrients up the trunk and the tree dies.

The process is different from regeneration by fire, Olson says. Fire sets the clock of the entire forest back to an early stage, and creates a single-age forest. Attack by insect or disease removes only the older or more stressed trees, opening up pockets of new growth. The result is a multi-age forest, with all the phases of the forest life cycle represented.

For more information about the boreal forest, contact Forest Resources, DIAND, in Whitehorse.