They drop their leaves each autumn. These aspens, oaks, maples, hickories, ashes, birches, and other trees make our fall forests colorful. Conifers' narrow, tough needles retain moisture and allow the tree to survive in winter when the air is cold and dry. The broad leaves of deciduous trees evaporate a lot of water and use a lot of sap. So to prepare for winter, deciduous trees shut down and drop their leaves.
Color change is triggered by the shifting rhythm of day and night.
As the long, warm days of summer turn into the shorter, cooler days of autumn, a chemical clock inside a deciduous tree urges the tree to begin its trip into dormancy, or winter sleep. Living leaves contain several kinds of chemicals, called pigments, each with a characteristic color. During the summer the most visible pigment is chlorophyll. In the autumn, the tree starts the dormancy process. As a result, it stops producing food.