Autumnal leaf drop is a form of self-protection. While evergreen plants in cold climates have thick waxes and resins to protect their leaves from freezing and fracturing, deciduous species generally have thin leaves that are susceptible to cold temperatures. Since water expands when frozen, the tender leaf cells would rupture during the winter, making them useless for photosynthesis. Without dropping these leaves, such a tree would be stuck with thousands of unproductive appendages and no way to make food! The surface area of all those leaves would also pose a threat to the plant’s physical integrity. Winter months are often windier than other seasons, and the wind against the broad leaves on a cold, brittle tree could cause major breakage. The same goes for the weight of snow collecting on all those leaves. Finally, by the end of summer, many leaves are insect-eaten, diseased, or otherwise damaged. Dropping them gives the plant a fresh start in the spring, and the nutrients from the decaying leaves are recycled to help grow the next leafy generation.
Autumn leaves are not simply blown off trees but are separated from the plants in a highly controlled process. As day length shortens and temperatures cool, hormones within the plant are activated to begin the abscission process. Chlorophyll production stops and the pigment starts to degrade, often revealing showy reds and yellows that were masked by green. The vessels that carry water to the leaf and sugars to the rest of the plant are closed off, and a layer of cells starts to grow between the leaf stalk and the twig holding it. These cells serve to slowly cut the leaf from the plant without leaving an open wound. As the leaves fall, the plant enters dormancy, saving its energy for the great bud burst of spring. (Encyclopaedia Britannica)