Why do leaves change color in the fall?

GUEST POST: I’m excited to introduce Madeline, who’s joining us for a special guest post. Madeline is a longtime reader of Today You Should Know and also runs ​Bluestocking​, her own educational newsletter and YouTube channel.

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The short answer

Leaves change color when chlorophyll (which we perceive as green) is replaced by other pigments, such as carotenoid and anthocyanin.

The long answer

t’s hard to imagine autumn without colorful trees and crunchy piles of leaves. But why do leaves change color?

To answer that question, let’s start by understanding why spring leaves are green. The substance that makes leaves green is chlorophyll, a pigment that helps trees absorb sunlight and convert it into energy. This process is known as photosynthesis.

This is a close-up image of plant cells with visible chloroplasts, found in a type of moss. "​Plagiomnium affine laminazellen​​" by ​​Fabelfroh​​ is licensed under ​​CC BY-SA 3.0 DEED​​.

At the beginning of fall, the temperature drops and the nights get longer. This signals to trees that it’s almost winter and resources are becoming scarce. To conserve energy, trees stop producing chlorophyll, and the chlorophyll in their leaves begins to break down.

When the chlorophyll is depleted, other pigments in the leaves take center stage. Carotenoid pigments appear as yellow or orange, while anthocyanin pigments appear as red or purple. Without chlorophyll to mask them, these pigments cause the leaves to appear yellow, orange, or red.

Left: Close-up view of yellow carotenoid pigments from a persimmon rind. Right: Close-up view of purple anthocyanin pigments in an unidentified flower cell.

Left: "​Кожура хурмы​" by ​Andrei Savitsky i​s licensed under ​CC BY 4.0​. Right: "​Microscopic view of unidentified flower cells with anthocyanins (4x magnification)​" by ​Shkyrik​ is licensed under ​CC BY-SA 4.0​.

So why do leaves fall to the ground after they change color? Since chlorophyll produces the energy leaves need to stay alive, leaves slowly weaken without it. Throughout the fall season, they gradually separate from the tree until they fall to the ground.

While this process might seem sad, it’s a completely natural part of a leaf’s life. It’s actually a positive thing for nature as a whole! Rather than using up a tree’s much-needed energy, fallen leaves are able to serve as food for animals or fertilizer for the soil. Losing leaves also helps the tree survive through the chilly winter so it can sprout new leaves the next year.

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