Why Leaves Change Colors In The Autumn, And Why Humans Obsess Over Fall Foliage

By Eliena Mahgerefteh on October 7, 2013 4:27 PM EDT

PS-OV-ART Patty Sue O'Hair-Vicknair
PS-OV-ART Patty Sue O'Hair-Vicknair

Autumn is a time where the distinct smells of home cooked food fill the air and parks are covered with the crunchy leaves of orange sugar maple and yellow black maples trees. What causes this beautiful change in scenery? Why is it that leaves that are normally green in the spring and summer transform to vibrant reds, oranges, and yellows?

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First, it helps to understand leaves. Leaves are important to the health of trees; they are nature's food factories. Plants take in water through their roots and carbon dioxide from the air. Through a process called photosynthesis, plants convert water and sunlight into glucose and oxygen, both of which are fuel for the trees. According to Web Exhibits, chlorophyll is a molecule found inside plants that traps and absorbs energy from sunlight and allows this process to take place. Chlorophyll absorbs all the wavelengths of color except green and therefore reflects green -which is why leaves appear a vivid green in broad daylight.

As the days get shorter and shorter in autumn, plants begin to prepare themselves for the winter. Shorter days mean less sunlight, and less sunlight means less opportunity for photosynthesis - so, as fall approaches, the plants began to shut down their food-making powerhouses, according to Science Made Simple.

Throughout the entire year, chlorophyll decomposes in sunlight, and the plant replenishes itself with the molecule. But when days are short, the veins that bring water and nutrients to the leaves close off and no new chlorophyll reappears. As the green chlorophyll fades away we began to see colors of yellow and orange that were originally masked by the reflected green.

But where do the vivid red colors found in leaves come from?

"As veins close (due to less sunlight and cold weather), sugars get trapped in the leaf and they react with other chemicals to form the red pigments," explains Mark Fischetti in a video for Scientific American (which you can watch below). Intensity of color is connected to temperature, Fischetti says. "If days are bright ad nights are cold, more sugars get trapped and the reds intensify, the drier autumn weather triggers a hormone telling the tree to drop its leaves otherwise the tree would use up water the tree needs during the winter."

Excess sugars cause increased production of a pigment called anthocyanin, which releases red color. Carotenoids can be orange, yellow, or red and are another class of pigments found in some leaves. In the absence of pigment, leaves can form tannins that (not so surprisingly) form a tannish color. The mixture of all these pigments in higher and lower doses set the stage for a beautiful array of earthy palettes of fall color.

Why do we love fall colors so much? Well, our brains - and eyes - are programmed to recognize when fruits are full of sugar and ripe to eat. Fall leaves, also full of sugar, trigger those same sensual desires. As it turns out, humans tuned to sense the beautiful autumn changes in color.

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