The Science behind Fall Leaves Changing Colors

The changing colors of fall leaves offer spectacular views and Instagram-worthy snapshots. But did you know the beauty of fall foliage is also a striking example of nature taking extreme measures to protect itself?

If you’re not already familiar with the process of photosynthesis, it’s the process that plants use to turn carbon dioxide, water, and light energy into sugars and oxygen. Photosynthesis depends on a green pigment called chlorophyll as well as lots of other pigments. Some of the most important pigments in tree leaves are the carotenoids, flavonoids, and the anthocyanins. The carotenoids are the yellow, orange, and brown pigments that give color to vegetables like corn, carrots, pumpkins, and sweet potatoes. The flavonoids are the yellow and green pigments that are responsible for the vivid colors in foods like kale, citrus fruit, and parsley. The anthocyanins are the red and purple pigments that give color to fruits like cherries, berries, pomegranates, and red apples. All of these pigments play an important role in the plant’s functions, but there’s usually far more chlorophyll in a plant than anything else because photosynthesis is a plant’s number one job.

Many trees are less active in the winter because they grow at northern and southern latitudes that receive less sunlight during those months. These trees are called deciduous, the Latin word that means “to fall off.” Since deciduous trees don’t do much photosynthesis in the winter, it doesn’t make sense to use energy to maintain big, green leaves. So when the days get shorter and the temperature gets cooler, they send less of their limited resources to the leaves and start using what water and nutrients they have to keep the rest of the tree alive.

The chlorophyll in the leaves breaks down, and the green color gradually goes away. When that happens, the other pigments, that were there the whole time, are better able to show off their colors before the leaves die entirely and fall off the tree. The leaves aren’t actually changing colors, they’re just losing their strong, green pigment to reveal the other colors in the tissue. After the tree stops supplying food and water to the leaves, all that’s left is for the tree to cut them off.

The tree forms a special layer of cells near the base of the leaf’s stalk, and another layer of cells at the very bottom of the stalk expands to push the leaf away. Eventually, the leaf can be knocked off easily—even by a light wind, and then it’s your job to rake them up!

Now that you know the science behind fall leaves changing colors, why don’t you try capturing the process throughout the season? Spend some time and look at the variety of colors on a particular tree (or even a particular leaf), and see if you can visualize photosynthesis in motion. If you’re able to capture an image, share it with us in the comments below.

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