Why do trees drop their leaves?

Trees drop their leaves when it gets too dry, cold, or dark for them to photosynthesize, at which point it’s not worth keeping them around.

Keep in mind that not all trees drop their leaves. Trees that don’t drop their leaves are called evergreens. Trees that drop all their leaves at once are called deciduous (from the Latin for “falling down”). Some evergreens, such as trees in tropical rain forests, eventually drop their leaves, but not all at once.

Keeping a leaf alive costs energy. But so does letting one fall, since whatever nutrients are left in the leaf when it falls will be lost to the tree. A tree drops a leaf when it will cost more to keep it alive than to let it fall. This is a bit like having a car; usually, it will be cheaper to keep repairing the car you have rather than buy a new one. But when the car gets old and worn down, it may be a more cost-efficient alternative to scrap it altogether and get a new one.

During dry seasons or cold, dark winters, the cost of keeping a leaf alive goes up, while the amount of photosynthesis it can undergo decreases. Photosynthesis depends on water and light, and as the temperature drops, so does the rate of photosynthesis. For many trees living in the north or south, by the time autumn arrives, it is simply not worth holding on to their leaves. This is especially true for broadleaf trees, which have big, fragile leaves that can be damaged by frost. So deciduous trees decide to cut and run, or, in other words, drop all their leaves at once. A full-grown tree can have more than 200,000 leaves, and over the course of about 60 years, will drop over 1.65 tons of leaves. Worldwide, there are more than 400 billion trees (about 60 for every human being on the planet), so that adds up to a lot of leaves.

When a tree decides that it will be cheaper in the long run to do away with one of its leaves, it drains as many nutrients as possible from the leaf before cutting it off at the base. The order in which these nutrients are drained is the process that causes leaves to change color. Leaves are green because they have a lot of chlorophyll to help them harvest blue and red light, but they also have other types of pigment to help them harvest other colors of light. These pigments include xanthophylls (which are yellow) and carotenoids (which are yellow, orange, and red). Because chlorophyll is the most valuable of these pigments, it gets drained out of the leaf first, leaving the other colors behind, which makes the leaves red, orange, and yellow. Eventually even these colors are drained, leaving only tannins, which are brown.

A tree drops a leaf when it will cost more to keep it alive than to let it fall.