In September the fall colours are generally at their best in the Yukon, from the dark red buckbrush high on the mountain slopes to the glowing yellow of poplars in the valleys.

However, long before their leaves changed colour, the trees were already preparing for winter, says Sue Olsen, forest ecologist for the Forest Resources branch of the Department of Indian Affairs and Northern Development.

"It starts in early August as the days get shorter. The change in day length triggers a chemical change in the trees, and two things happen. They stop growing, and they start moving all their resources down to the root system."

"Although growth stops above the ground, growth in the roots just starts at this point," she says.

At first glance, trees don't seem to have as many options as animals have for coping with winter. They can't migrate south to warmer weather, and they can't nourish themselves over the winter. But they have many ways of protecting themselves, says Olsen.

Deciduous trees and shrubs get rid of their leaves in fall. In summer, water moves out through the leaves and is replenished from water in the ground, but in winter the water in the ground is frozen and unavailable to replace lost moisture. So the plants shut down most of their above-ground operations and move all available nutrients down to their well-protected root systems.

There are a few exceptions, says Olsen. The bark of aspen trees in the north often has a greenish tint. The green is created by a small amount of chlorophyll, the chemical that allows trees to create energy from the sun's light through photosynthesis. The chlorophyll means a low level of photosynthesis can occur even when the leaves are gone.

Chlorophyll is also what makes leaves green. As the trees shift their operations underground, the chlorophyll disappears from the dying leaves. The bright reds, oranges, and yellows of fall leaves are created by the materials that are left in the leaves after the chlorophyll retreats.

If you look at a stand of aspen from a distance, says Olsen, you can often see that some clumps of trees change colour earlier than neighbouring clumps. The reason, she says, is that aspen trees can reproduce by creating genetically identical clones, and all the genetically identical trees change colour at the same time.

Not every tree, of course, drops its leaves. Evergreen trees have hard, needle-shaped leaves that stay through the winter. However, the needles adapt to the cold of winter in ways that are not easy to see, Olsen says.

Evergreen needles are thick and often waxy to reduce water loss. In summer tiny openings on the bottoms of the needles allow water to evaporate, but in winter those openings, called stomata, close up to prevent water loss.

Another sign that evergreens are preparing for winter is the appearance of buds in August, says Olsen.

"That signifies that growth has stopped for the year," she explains.

Both deciduous trees and evergreens also undergo a less visible process of hardening to prepare for the cold. The liquid in cells becomes more concentrated so that it freezes at a lower temperature and in a spear-like shape that keeps cells from bursting with the pressure. Some water moves outside individual cells and cell walls become more elastic in the winter, Olsen says, so that frozen water crystals are less likely to break them.

Hardening is a slow process, triggered by gradually decreasing temperatures. If the temperatures drop too quickly, trees can be damaged or even killed.

"It's kind of like the way people respond," says Olsen. "If the temperature goes down slowly, minus 40 doesn't seem so bad. But a drop straight from minus 15 to minus 40 feels brutal."

For more information about trees and how they prepare for winter, contact Sue Olsen at Forest Resources, Department of Indian Affairs and Northern Development, Whitehorse.