As the leaves start to change color, you might ask yourself about trees' autumn habits. Why do trees lose their leaves? Is it because leaves would freeze in winter? Is there not enough light over the winter months to justify having leaves?
There are a number of reasons deciduous trees shed their leaves in certain months, including diminishing light and, surprisingly, drought--physiological drought, that is.
A tree loses water through its leaves. This transpiration (and the resultant decrease in hydrostatic pressure in the leaves) is what keeps water, a necessary ingredient for photosynthesis, flowing to the tree's extremities. In the winter, the drop in temperature makes the transport of water problematic. The cold, dry winds would suck way too much water out of the leaves, dehydrating and killing the tree. Evergreens don't have this problem because their leaves have a waxy coating that keeps moisture inside. Deciduous trees have to drop their leaves to keep from drying out. (Even deciduous trees in the tropics, which don't have shortened days, lose their leaves during the dry season to conserve water.)
Sunlight plays a role as well. With shorter days, the amount of energy spent on maintaining leaf health would be greater than the amount the leaves would be able to create.
So when fall comes, leaves stop producing chlorophyll, which up to this point has given them their green color. The absence of green pigments allows the yellow and orange pigments that have always been in the leaves to show. Some leaves start producing red pigments in late summer, and these pigments also start to show when chlorophyll production slows down. After the color change, the tree sheds its leaves and goes into a dormant state until spring.