How do shorter days trigger leaf shedding in temperate deciduous trees?

In temperate deciduous trees, the shortening of day length acts as a critical environmental signal that triggers the transition toward winter dormancy and the subsequent shedding of leaves.12

As days grow shorter and temperatures cool, the tree initiates a controlled process known as abscission. This process is regulated by changes in the plant's hormone metabolism. Specifically, declining day length influences the balance of plant hormones, such as auxins and gibberellins, which normally help preserve leaf greenness and retard leaf fall. As these levels shift, other signaling hormones, including ethylene, play a more active role in promoting the separation of the leaf from the tree.132

During this time, chlorophyll production ceases, and the tree begins to form an abscission layer—a specialized zone of cells at the base of the leaf stalk. This layer gradually softens, effectively cutting the leaf from the plant without creating an open wound, allowing the tree to conserve energy and protect itself from winter damage.13