Abscission, in botanical terms, most commonly describes the process by which a plant drops one or more of its parts. This shedding or dropping process includes spent flowers, secondary twigs, ripe fruits and seeds and, for the sake of this discussion, a leaf.
When leaves fulfill their summer duty of producing food and growth regulators, a process of shutting-down and sealing-off the leaf begins. The leaf is connected to a tree via its petiole and the twig-to-leaf connection is called the abscission zone. The connective tissue cells in this zone specifically grow to be easily broken apart when the sealing process begins and have built-in weak point which allows for proper shedding.
Most deciduous (means 'falling' in Latin) plants (including hardwood trees) drop their leaves by abscission before winter, while evergreen plants (including coniferous trees) continuously abscise their leaves. Fall leaf abscission is thought to be caused by a reduction of chlorophyl due to shortened hours of sunlight. The zone connective layer begins to harden and blocks the transport of nutrients between the tree and leaf. Once the abscission zone has been blocked, a tear line forms and the leaf is blown away or falls off. A protective layer seals the wound, preventing water evaporating and bugs getting in.
Interestingly, abscission is the very last step in the process of cellular senescence of deciduous plant/tree leaves. Senescence is a naturally designed process of the aging of certain cells that takes place in a series of events that prepares a tree for dormancy.
Abscission can also occur in trees outside of autumn shedding and dormancy. Leaves of plants can abscise as a means of plant defense. Some examples of this are: dropping of insect-damaged and diseased leaves for water conservation; leaf fall after biotic and abiotic tree stresses including chemical contact, excessive sunlight and heat; increased contact with plant growth hormones.