Once the leaf is drained of its chlorophyll, the tree begins a process that removes the leaf. During the growing season, trees release two hormones - ethylene, which causes the leaves to fall, and auxin, which counteracts ethylene. As winter approaches, ethylene levels rise as auxin levels drop. This causes the cells in the abscission zone to separate and the leaf to drop to the ground. The tree actively removes that leaf.
Trees shed their leaves each year for a variety of reasons.
Fragile leaves often are damaged by insects, diseases or weather. The thin leaves also are easily damaged by freezing. Replacing the leaves each spring allows the tree to start fresh each year with a set of undamaged light collectors. In the spring the tree will use the nitrogen it took up from the leaves to make amino acids, which are the building blocks of proteins used to make new tissue, such as new leaves. Some of the nitrogen also will be used to create new chlorophyll, so the tree can begin the process over again.