Fall leaves

USDA Forest Service

We almost always think of trees as being green, but there is one time of year when their leaves turn a myriad orange, red, yellow and brown: the beautiful and chilly days of fall. Those living in the Eastern or Northern United States come to anticipate the change in color starting in September or October every single year. But what causes the leaves to change color, and why?

Just like many animals that hibernate for the winter, trees experience a unique change during the winter months. During summer, for instance, plants use the process of photosynthesis to transform carbon dioxide found in the air into organic compounds like sugars using energy from the sun. During the winter, however, there is less light to go around, and their ability to create food from the photosynthesis process is limited.

What does that have to do with a leaf’s color? The substance that allows trees to turn carbon dioxide into food (chlorophyll) is also the cause for the leaf’s green sheen. As the photosynthesis process wanes in the colder months due to the lack of sun, so does its greenish hue, allowing other elements present in the leaf to show through. Believe it or not, the yellows and oranges that appear in fall have actually been there all year in the form of nutrients like carotene (also found in carrots). The intense green color of the chlorophyll had simply overshadowed them.

But what about the reds and browns? And what causes the leaves to fall away after they change color?
The bright reds and purples in each leaf come from a strong antioxidant that many trees create on their own because of their protective qualities. The antioxidant helps protect the trees from the sun, lower their freezing levels, and protect them from frost. As winter comes, so does the need for the antioxidant (similar to the way a dog gets more fur during winter to stay warmer).

As for the leaves falling, that is another story. At the base of each leaf, there is a layer of cells that carry food and water from the leaf to the tree during the summer months to keep it fed. In the fall, that layer actually starts to harden, preventing the passage of nutrients. Because of this, the nutrients and waste that previously passed from the leaf into the tree become trapped in the leaf with no fresh water to clean it. Not only does this cause the leaf to turn brown, eventually it causes the cells within it to harden so much that the leaf tears and blows away. Thus the pile of leaves you enjoyed jumping in as a child.

Because each tree, and each leaf, contains a unique amount of nutrients depending on how well-nourished it was over the spring and summer, the way each leaf breaks down during the winter months is also quite different. The result is the unique and complex facet of colors we see in each neighborhood or forest each fall.

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