For those of us living where winters are severe, it's easy to think that trees must lose their leaves during the fall, else those leaves will freeze and die when the first cold weather arrives. However, as you might suspect, there's more to the story than that...
In plants such as trees there are five chemical substances known as plant hormones. Three of these plant hormones promote plant growth and two inhibit it. The three growth promoters are auxins, gibberellins and cytokinins. The two growth inhibitors are ethylene and abscisic acid.
Hormone levels in trees depend on the day length (photoperiod) along with the temperatures being experienced by the tree. As you might guess, spring's lengthening days and warming temperatures cause trees to produce lots of growth-promoting hormones, while fall's shortening days and cooling temperatures produce growth-inhibiting hormones.
In the fall, growth-inhibiting hormones build up at the base of tree leaves (the petiole's base), causing the cells there to become so soft that eventually the leaves snap off. The cell zone where this weakening in the leaf's attachment to the stem takes place is called the abscission layer ("abscise" just means "to cut off"), and abscisic acid takes its name from this process. In the picture at the right, the black arrow points to the abscission layer forming at the base of a leaf of a Sweetgum tree outside my door. This picture was made in October just before the Sweetgum lost its leaves. By the way, the conical item at the top of the stem is the twig's terminal bud, from which next spring's new stem growth will arise.
In review: Leaves fall when fall's shorter and cooler days cause trees to produce growth-inhibiting hormones, which cause abscission layers to form at the base of the tree's leaves.
Of course, not all trees lose their leaves...
Even where winters are bad, some trees do not drop their leaves, and therefore are known as evergreen trees (trees losing their leaves are deciduous trees). Among the conifers known as pines, spruce and fir, leaves appear as needles and remain on the twig during even the coldest winters. In much of North America, magnolias bear thick, leathery, green leaves during winters, though during really bad cold snaps those leaves can be damaged. Where I live, in southern Mississippi in the US's Deep South, some of the local trees lose their leaves during the winter and some do not. Magnolias grow wild in the forest here and some of the oaks, especially the very common Water Oak (Quercus nigra), hold most of their leaves year round. On the other hand, many tropical trees drop their leaves even where there is no winter, and often even when there is no dry season. Therefore, there must be certain advantages to dropping leaves other than to escape injury caused by cold.
For one thing, when deciduous trees drop their leaves they are getting rid of leaf-eating insects, and those insects' eggs and larvae. By dropping leaves, a tree interrupts the process of generation after generation of insects producing more and more of their leaf-chewing numbers. A corollary of this fact is that trees with evergreen leaves have special problems with insect populations building up season after season.
For this reason, evergreen leaves are often not only tough, making it hard for insects to chew, but also frequently filled with chemicals that insects don't like. Maybe the chemicals are bitter, or scramble the insect's nerve impulses, or simply kill them. A magnolia leaf is a good example. It is tough and leathery, and if you crush it you can smell a strong odor. The odor is not unpleasant, but if you taste the leaf you'll find that it's very bitter. Bitter chemicals are often poisonous. In fact, among plants with evergreen leaves the main defense against leaf-eating critters is chemical warfare.