Why Do Trees Shed Their Leaves?

In mild climates like Western Europe, leaves fall from trees in autumn. This happens because trees have adapted to the combined effects of colder weather and reduced daylight hours. If a tree is kept in a hot greenhouse, it keeps its leaves for longer.

When a tree is exposed to the cold, photosynthesis stops. The chlorophyll in the leaves breaks down, which allows other pigments to appear. Some are red (anthocyanins), some are orange (carotenes), and others, yellow (xanthophylls).

The carbon and nitrogen in chlorophyll are recycled in the form of nutrients that are transferred to the rest of the tree. In the same way, different parts of the fallen leaves are broken down by the microorganisms that live in the earth and are reabsorbed by the tree.

Shedding leaves also acts as protection against the cold. It allows trees to avoid the winter embolism phenomenon: if leaves freeze, then air pockets can form in their vessels. When the leaves defrost, these pockets block the vessels, cutting off sap circulation.

Conifers are well-protected against winter embolism because their needles contain very narrow vessels that allow only tiny air pockets to be trapped.

Trees may lose their leaves in the summer. This is an act of self-defense against drought.

Trees can sacrifice their leaves by closing their pores to reduce evaporation. Gas embolism is a major cause of tree death in the context of climate change, which has led to a major increase in the number of droughts worldwide.