The Science Behind Fall Foliage

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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.

Two chemicals are responsible for the fall coloration of leaves. Carotenoids create orange and yellow pigments, and anthocyanins create shades of red and purple. The carotenoids are present in the leaf all summer long, but they’re masked by the green of the chlorophyll. As soon as the chlorophyll renewal is halted, the green begins to fade and the vibrant fall colors appear. The second chemical, anthocyanin, forms as a result of the glucose formed by the remaining, faded chlorophyll. The glucose then becomes trapped in the leaf by the abscission layer, resulting in the formation of anthocyanin.

The colors of a particular tree are a result of the carotenoids and the anthocyanins reacting to each other in different amounts, in combination with any chlorophyll left. The formation of these chemicals and the amount of each of them are dependent on temperature, moisture and sunlight, so every foliage season is unique, because every season the chemical balance found inside the leaf changes.

The anthocyanin formed in the right-hand part of this leaf because it was exposed to sunlight, but the left side was probably in shade, so the carotenoids caused it to stay yellow.

Carotenoids and anthocyanins also break down after being exposed to sunlight. If a leaf manages to stay on the branch after the chemical processes have broken down, you would see the bright colors fade until it would finally be brown, a result of a final chemical, tannin. Tannins are found in the membranes of the cells that make up the leaves, so they never fade, which is why brown is the final color present in late autumn.

In these leaves, the tannins are interacting with the carotenoids and anthocyanins to create subtle, stunning combinations.

So when you go to snap a picture of copper-colored leaves for your Instagram, take a minute to appreciate the intricate chemical interactions going on!

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