Fall is here.  There is a bit of a chill in the air this week, and, here in the mid-Ohio valley, the leaves are starting to change.  There are a few factors that cause the leaves of deciduous trees to change in the fall, and the science behind that change is pretty … cool.

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 produces energy to live. 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 is not much for the workers to do. 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 biochemical 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 nights 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 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 present start to be revealed.

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 are 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 because 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. That is also why colors can vary by region, elevation, etc.

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 will 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.

Over the next few weeks take some time to get outdoors and enjoy the splendor of autumn.  Maybe take a road trip to one of West Virginia’s majestic parks or visit Ohio’s Amish country.  Just keep in mind that those beautiful fall colors were there all summer obscured by chlorophyll waiting to be revealed.