Leaves fall from trees because the tree begins to prepare itself for the winter season. Trees seal the spots where the leaves are attached, cutting off water to the leaves. The leaves then change color and fall from the tree.
During winter, less sunlight reaches the Earth, and the atmosphere becomes colder. As winter begins to approach, trees begin to prepare for the change. Tiny holes in leaves cause the tree to lose lots of water. During the winter, there is not enough water available to replace what the tree loses through the leaves. By sealing the holes where the leaves grow and shedding the leaves, the tree is able to use available water to survive the winter. If the leaves did not fall and the holes were not sealed during the winter, the tree would not be able to survive the winter season. By retaining water in the trunk of the tree, the dry air cannot reach the water and evaporate it. When freezing temperatures approach, photosynthesis of the tree is used to help the tree survive as opposed to feeding the leaves of the tree. As spring time approaches, new leaves form on the tree, and the tree uses photosynthesis to feed the new leaves and to grow.