How Deciduous Trees Shed Leaves
Autumn leaf drop is a biological process called abscission.
In autumn, deciduous trees drop their leaves to conserve water and energy during the cold winter months. Broad leaves have a large surface area that would lose water through transpiration and risk damage from freezing temperatures and snow loads. Shedding them is a controlled physiological process called abscission.
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As days shorten and temperatures drop, the tree reduces its production of chlorophyll, revealing yellow and orange pigments that were previously hidden. At the same time, the tree forms a specialized band of cells at the base of the leaf stem, known as the abscission zone.
This zone consists of two distinct layers. The layer closest to the leaf releases enzymes that break down cell walls, weakening the connection between the leaf and the branch. Meanwhile, the layer closest to the stem deposits a cork-like seal made of suberin across the exposed vascular tissue.
When wind breaks the leaf away, the protective seal is already in place. This prevents water loss and protects the tree from fungal infection through the open wound throughout the winter season.
