Why do trees drop their leaves in the fall?

Posted on Jan 12, 2021      217

If trees didn’t shed their leaves for the winter, they would die. There are several reasons for this.

The first reason. A tree’s leaves collectively have an extensive area, and water evaporates intensely from this entire area. In summer, the tree can compensate for losing moisture by extracting water from the soil. But when it gets colder, cold water extraction from the soil decreases; in winter, it is difficult to extract moisture from frozen soil at all. Trees with deciduous cover would die in winter from lack of moisture, i.e. they would dry out.

For the same reason, when the dry season arrives in the tropics and subtropics, trees in those climatic zones drop their leaves and stand bare until the rainy season arrives.

Late autumn. The first snow.

The second reason. Have you ever noticed that after a heavy snowfall, the branches lean heavily to the ground under the weight of the snow? Some branches even break from it. If leaves were left on the trees during the winter, much more snow would be trapped on the branches because the leaf surface, as we said above, is large. Thus, by dropping leaves in the fall, trees protect themselves from mechanical damage under the pressure of snow.

Reason three. During leaf fall, the tree gets rid of excess mineral salts that accumulate in the leaves throughout the summer. As we have repeatedly noticed, the leaf evaporates water at an increased rate. This evaporated water is constantly being replaced by fresh water, which is sucked in by the roots from the soil. But the water that the roots get from the soil has various salts dissolved in it. Thus, the leaves do not receive pure water, but salt solutions. Some salts are used by the plant for nutrition, and the remaining salts are deposited in the cells of the leaves. The more moisture the leaf evaporates, the more it mineralized by fall. As a result, by fall, the leaves accumulate a lot of salts and become a sort of mineralized. An excess of mineral salts disrupts normal leaf function. Therefore, the shedding of old leaves is essential to maintain normal plant life.

By the way...

So how does conifers manage not to lose moisture in winter while remaining green? The secret is that conifers evaporate much less moisture than leaves. First, the surface of a needle is many times smaller than a leaf; second, needles have a thick skin; third, they are covered with a wax coat which also reduces water evaporation. Finally, stoma in needles are in special hollows - this reduces the intensity of water evaporation through them.

Conifers also shed their coat, but not at once, as do deciduous species, but gradually: the needles of conifers live an average of 3-4 years. That is why the change of conifer “clothes” proceeds imperceptibly.