Deciduous trees and plants, the ones who lose their leaves once a year have different cycles depending on where they are in the world. In temperate climates, the leaves generally drop in the fall in preparation for winter and new growth comes in the spring. In tropical regions, the cycle follows the cycles of wet and dry seasons. Evergreen trees do shed their leaves, however not all at once or even annually. Abscission is the process in which leaves shed; whether deciduous or evergreen.
At the base of the petiole, stalk, of each leaf, there is an abscission zone. Changes that take place in this region ultimately result in the drop of leaves. Hormonal changes take place as the leaf ages and two layers of cells become differentiated. (In young leaves hormones prevent these cells from differentiating.) The cell layer closest to the stem becomes the protective layer which is usually several cells deep and suberized, or coated with a fatty suberin substance. The other layer, the separation layer, forms on the leaf side of things. The cells swell and become like jelly. The pectins in the middle lamella of the cells in the separation layer are broken down by enzymes until an external event causes the leaf to fall: this could include the force of gravity overcoming the strength of the strands of xylem holding the leaf to the petiole, thus breaking it off at the gelatinous zone, wind, rain, animals, etc. The pectin breakdown begins in response to environmental conditions such as dropping temperature, lack of adequate water, decreasing day lengths, changing light intensities, or damage to the leaf.