How Leaves Fall From the Tree

Leaf Abscission

• The process of leaves falling from a tree is scientifically referred to as leaf abscission. The tree, ready to enter into dormancy for winter, begins to shed those parts of itself that would otherwise drain energy from the tree and threaten its ability to survive a long, cold winter. Leaves are not the only part of the host plant that are shed; twigs, fruit, flowers and bark are also dropped. As the days begin to grow shorter in the fall, the tree is alerted that a change is necessary.

Process

• The shortening days and the accompanying reduction in sunlight prod hormones within the tree to take action. Those hormones send forth a chemical message that prompts a collection of cells to develop at the place where the leaf stem meets the branch. These cells are referred to as abscission cells -- the words "scissors" and "abscission" emanate from the same root meaning, and just like scissors, the purpose of these cells is to cut the leaf away from the branch.

Death

• Over the course of days or weeks, the abscission cells force the leaf away from the branch, reducing the hold the remaining tissue has on the tree. Eventually, gravity or a strong breeze takes over and the leaf flutters to the ground. Throughout the spring, summer and early fall, the leaves served as an effective food factory for the tree, providing energy through the process known as photosynthesis. But once the leaves are unable to continue that work, the tree cuts them loose.

Fun Fact

• Not all trees shed their leaves annually. Evergreens, like pine trees, retain their leaves -- or needles -- for more than one growing season. Some hardwoods, like oak and beech, have leaves that die in the fall but are not shed until the following spring. This normally happens in younger oaks and beeches, with the leaves on the lower portion of the tree hanging on until spring, while older leaves in the upper section of the host are shed in the fall.