By Donnetta Wilhelm, Colorado Master Gardener
The Reason Why Leaves Fall
Each leaf on a tree connects to the tree’s circulatory system. Where the leaf meets the stem is called the abscission zone which consists of tubes that carry water and nutrients from the tree to the leaf. In the autumn, the tubes in the abscission zone swell and secrete a waxy substance that reduces the flow of water and nutrients from the tree to the leaf. No longer able to receive nutrients, the leaf dies, and eventually falls. The closed abscission zone protects the circulatory system while shedding leaves.
In oaks, beeches, and hornbeams, the swelling of the abscission layer doesn’t occur until the spring, so the leaves remain throughout the winter. Most scientists believe genetics are why this occurs on these particular trees.