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. 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.