Trees shed their leaves to conserve energy during the winter and to prevent damage to the tree.
Deciduous trees lose their leaves each winter, instead of maintaining green foliage year-round like evergreens do. Trees often put on a radiant show of fall color before their leaves fall, and in some regions this fall color is a holiday attraction enjoyed by visitors from around the world. There are various reasons for leaf loss, but essentially, this happens to conserve energy over the winter and prevent damage to the tree. In spring, the tree produces new leaves so that the process begins again.
Some trees have leaves that change color before they fall.
Many deciduous trees are found in regions with cold, dark, harsh winters. Trees shed their leaves for protection during the winter months, as the cold, dry winds in these regions easily draw moisture from the trees through the leaves, which have a large surface area. By losing their leaves, trees can retain their moisture in the trunk and branches, instead of drying out and dying. Additionally, leaf loss puts the tree into a dormant state and greatly reduces the amount of energy the tree needs to stay alive.
During the warmer months, the leaves photosynthesize sunlight and produce chlorophyll.
During spring and summer, leaves photosynthesize the abundant sunlight that falls on them, producing chlorophyll, which turns them green. Photosynthesis provides the tree with energy, and the tree feeds the leaves with the nutrients it takes from the soil to keep them healthy. The bright yellows, oranges, and reds that make fall color distinctive are already there, but chlorophyll masks them. As the days get shorter, the trees have less sunlight to work with, and the efficiency of the leaves begins to decline. The leaves fall because they attract the energy of the tree, otherwise the tree would have to feed on the leaves during the winter.
The leaves photosynthesize sunlight in the spring and summer months.
As the nights grow longer, the trees prepare to shed their leaves, beginning with the secretion of chemicals to cut the leaf from the rest of the tree. As the leaves stop producing chlorophyll, they begin to change color. Ultimately, trees lose their leaves when the chemicals they secrete effectively cut the leaves off the main branch. These chemicals, mainly ethylene and abscisic acid, cause leaf drop by breaking the bond between the leaf and the tree. The leaves fall to the ground, providing a layer of mulch to insulate the roots, and the tree conserves its energy for the next year and a new growing season.