Leaf color change starts very subtly late in September and early October in temperate North America. Trees respond to such factors as fall drying conditions, temperature change, altered sun position, and light. It takes approximately two weeks to begin and complete the color change.
The major factor influencing this whole process is the lack of water. Not a lack of water to the entire tree, but a purposeful weaning of water from each leaf. Every leaf is affected by colder, drier, and breezy conditions and begins a process which results in its own demise and removal from the tree. The ultimate sacrifice of a leaf-bearing tree is the ultimate in visual pleasure for us.

The broadleaf tree goes through a process of sealing off the leaf from the stem (abscission). This halts the flow of all internal water to the leaf and causes a color change. It also seals the spot of leaf attachment and prevents precious moisture from escaping during winter dormancy.

This lack of water to each leaf causes a very important chemical reaction to stop. Photosynthesis, or the food-producing combination of sunlight, water, and carbon dioxide, is eliminated. Chlorophyll must be renewed (by photosynthesis) or be taken in by the tree along with photosynthetic sugar. Thus chlorophyll disappears from the leaves. Chlorophyll is the green you see in the leaf.

Once the overwhelming chlorophyll color is removed, true leaf colors will dominate over the receding green pigment. True leaf pigments vary with the species of tree and thus the different characteristic leaf colors. And because true leaf colors are water soluble, that makes the color disappear very quickly after drying out.

Carotene (the pigment found in carrots and corn) causes maples, birches, and poplars to turn yellow. The brilliant reds and oranges in this fall landscape are due to anthocyanins. Tannins give the oak a distinctively brown color and is the final persistent color most leaves turn before becoming part of the forest floor.