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. Both deciduous trees and evergreens also undergo a less visible process of hardening to prepare for the cold.