In autumn, the leaves of many trees turn yellow, orange, and red. During the summer, leaves are green they contain large amounts of chlorophyll, the that captures energy from . As days become and cooler, trees stop chlorophyll. The green fades, and yellow and orange pigments that were all along become . Red colors, , are newly produced in the .

Deciduous trees, which lose all their leaves each year, follow a careful plan. In autumn, before the leaves fall, the tree breaks down valuable materials in them, especially nitrogen, and moves them into its branches and roots. These nutrients will be reused to build new leaves in spring.

Dropping leaves helps the tree survive winter. Broad, thin leaves lose a great deal of water, and when the ground is frozen, roots cannot take in water to replace it. Leaves can also collect heavy snow, which might break branches. By shedding them, the tree enters a resting state in which it uses very little energy.

The tree itself controls the process. At the base of each leaf stem, a special layer of cells forms. This layer gradually separates the leaf from the branch, and a protective seal forms over the wound so that disease cannot enter.

Not all trees follow this strategy. Pines and other evergreens have narrow needles with a thick, waxy coating that loses little water, so they can keep their leaves through winter. Each approach has advantages: evergreens can begin photosynthesis as soon as conditions allow, while deciduous trees avoid winter damage and grow a fresh set of leaves each year.