By Dylan Stuntz, American Forests
It’s officially been fall for a little more than a week, and for anyone who lives near a deciduous tree — one that sheds its leaves in the fall — this means some beautiful sights are about to occur. But why do trees go through these changes? There’s a complex chemical process that goes on inside every deciduous tree, and maybe understanding it can give you even more appreciation for such a stunning sight.
To understand why leaves are the color they are, you first need to become familiar with the inside of a leaf. Leaves get their green color from a chemical called chlorophyll, which helps the tree take in sunlight. The tree uses the sunlight in a process called photosynthesis, which is how the tree eats, so to speak. It uses the sunlight to break down carbon dioxide (CO2) and water (H20) it absorbs, turning the CO2 and H20 into oxygen, which gets expelled, and glucose, which the tree consumes for energy.
If you imagine a tree as a factory, then the leaves are seasonal workers. They do their job when resources are coming into the factory (sunlight, water, carbon dioxide), but when resources stop coming in, there’s not much for the workers to do, so the tree sends them a pink slip. Leaves require energy from the tree, so like any good factory, the tree engages in a cost-benefit analysis. When the days become shorter, the tree no longer wants to waste energy on leaves. This starts the internal chemical process that creates fall foliage.
The change in leaf coloration is dependent on the amount of sunlight that the tree takes in. As the seasons change, the days get shorter and the night get longer. Eventually, when the nights reach a certain length, chemical processes in the tree will start to block off the connection between the tree and the individual leaves, by creating a corky layer of cells known as the abscission layer. This layer is to protect the branch when it inevitably becomes exposed to the open air, once the leaf has fallen. The abscission layer protects the tree, but it also disrupts the flow of nutrients and chemicals that move from the branch to the leaf and back. Chlorophyll breaks down when exposed to sunlight, so as a result it needs to be constantly replaced. The abscission layer interrupts this renewal process, so as a result once the chlorophyll starts to fade, other colors start to emerge.