Why Do Leaves Change Color in the Fall?

When the abscission layer of a plant grows, it cuts off the circulation of nutrients, sugar and water to the leaves, which causes the chlorophyll that gives the plant its green color to disintegrate and makes carotene shine through. The result is the yellow-colored leaves that are found on maple and other trees, such as birch and aspen, in the fall.

The abscission layer is a band of cells. Auxins control this band of cells, which is located in the leaf's stem. The cooler weather along with the shorter days of fall puts an end to the creation of auxins. The lack of auxin is what causes the abscission layer to grow in the leaves.

Only some trees are affected by the change in weather. Trees such as spruces, hemlocks, firs and pines are evergreen, and their leaves do not change color. Factors that affect fall color include temperature and light. The bright red leaves of fall are created by the pigment anthocyanin. Frost usually causes the bright red color created by this pigment to fade. However, if the day is rainy or overcast, the colors that people see on the leaves of the trees seem a lot more intense.