Trees lose their leaves in the winter as they enter their dormant period in order to protect cells from freezing temperatures. In this stage, they no longer need the food that the leaves produce. Trees respond to changes in available light and begin to form a layer of dead cells in the stem of their leaves, so each leaf breaks from the limb and falls to the ground.
During the tree's growing season, it constantly produces chlorophyll to supply to the leaves, giving them their green color. Chlorophyll is essential to the tree's production of glucose from carbon dioxide and sunlight. However, in preparation for dormancy, the tree stops supplying chlorophyll, allowing the other colors in the leaves to show. As the preparations continue, the tree also stops producing the dyes that cause these colors as well. As the fallen leaves decompose, they add vital nutrients to the soil, enhancing growth after dormancy.
According to Michigan State University, trees prepare for cold temperatures in several other ways. As ice inside the cell causes death, the tree increases the water supply outside the cell. As this water freezes, it gives off heat, protecting the cells to several degrees below the freezing point of water.