Why Do Leaves Fall in Autumn?

Leaf fall (abscission) is a survival strategy that deciduous trees use to endure cold, dry winters. During spring and summer, leaves are the tree food factories, using chlorophyll to photosynthesise. But in winter, water is often frozen and unavailable, and broad leaves would lose too much water through transpiration and be damaged by frost. As days shorten and temperatures drop in autumn, trees detect the change through light-sensitive pigments and begin preparing for dormancy. They stop producing chlorophyll (the green pigment), revealing the yellow and orange carotenoid pigments that were present all along, while some trees produce red anthocyanins. The tree reabsorbs valuable nutrients like nitrogen from the leaves. Meanwhile, a layer of specialised cells called the abscission zone forms at the base of each leaf stalk, gradually cutting off water and nutrients and weakening the connection until the leaf detaches and falls, often aided by wind. This leaves a protective scar that seals the branch against water loss and infection.