Deciduous trees lose their leaves every winter, rather than maintaining green foliage throughout the year like evergreens do. Often, the trees provide a radiant show of fall color before they lose their leaves, and in some regions, this fall color is a vacation attraction that visitors from all over enjoy. There are a number of reasons for leaf loss, but essentially, it happens to conserve energy over the winter and prevent damage to the tree. In the spring, the tree puts out new leaves so that the process can begin anew.
Many deciduous trees are found in regions which have cold, dark, harsh winters. Trees lose their leaves to protect themselves during the winter months, as the cold dry winds in these regions will readily strip moisture from the trees through the leaves, which have a large surface area. By losing their leaves, trees can conserve their moisture in the trunk and branches, rather than drying out and dying. In addition, the leaf loss puts the tree into a state of dormancy, and greatly reduces the amount of energy that the tree needs to stay alive.
During the spring and summer, leaves photosynthesize the plentiful sunlight that falls on them, producing chlorophyll, which turns them green. The photosynthesis provides energy for the tree, and the tree feeds the leaves with nutrients it takes up from the ground to keep them healthy. The bright yellows, oranges, and reds that make fall color distinctive are actually already there all the time, but the chlorophyll masks them. As the days get shorter, the trees have less sunlight to work with, and the efficiency of the leaves starts to decrease. The leaves fall because they become a draw on the energy of the tree, as the tree would otherwise have to feed the leaves through the winter.
As the nights get longer, trees make preparations to lose their leaves, starting with the secretion of chemicals to cut the leaf off from the rest of the tree. As the leaves stop producing chlorophyll, they start to change color. Ultimately, trees lose their leaves once the chemicals they secrete have effectively cut the leaves off from the parent branch. These chemicals, primarily ethylene and abscisic acid, make the leaves fall by severing the link between the leaf and the tree. The leaves fall to the ground, providing a layer of protective mulch to insulate the roots, and the tree conserves its energy for the next year and a new growing season.