Why Do Leaves Change Color in Autumn?

In autumn, many leaves turn yellow, orange and red. Why? In spring and summer, leaves are green. They have a green color inside called chlorophyll. It helps the tree make food from sunlight. In autumn, the days get shorter and the air gets colder. The tree stops making chlorophyll, and the green goes away. Then we can see the yellow and orange colors. They were in the leaf all summer!

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Every autumn, people in Japan go out to enjoy koyo, the changing colors of the leaves. But why do leaves change color? In spring and summer, leaves contain lots of chlorophyll, a green pigment that helps trees make food from sunlight. There is so much chlorophyll that it hides the other colors in the leaf. As the days get shorter and the temperature drops, trees stop making food and the chlorophyll breaks down. When the green disappears, yellow and orange pigments called carotenoids become visible. They were there all summer, just hidden. Red colors are different. Some trees, such as many maples, produce new red pigments called anthocyanins in autumn. Finally, the tree forms a special layer of cells where the leaf joins the branch, and the leaf falls.

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The bright reds and golds of autumn draw crowds to parks and mountains across Japan, but the colors are really the result of a tree preparing for winter. During the growing season, leaves are packed with chlorophyll, the green pigment that captures sunlight for photosynthesis, the process plants use to make food. Leaves also contain yellow and orange pigments called carotenoids, the same pigments that give carrots and corn their color. In summer, however, there is so much chlorophyll that these colors are masked. According to the State University of New York College of Environmental Science and Forestry, changes in the length of daylight and in temperature signal trees to stop producing food as autumn arrives. The chlorophyll breaks down, and the hidden carotenoids appear, turning leaves yellow and orange. This is why ginkgo trees turn such a clear yellow. Red and purple shades come from a different source. In some species, including many maples, chemical reactions in autumn produce new pigments called anthocyanins, which form when sugars combine with other compounds in the leaf. Weather therefore plays a large role in how spectacular a season will be. Carleton College's arboretum notes that the best reds appear when warm, sunny days are followed by cool nights, while an early frost can weaken the colors. At the same time, the tree grows a special layer of cells where the leaf stem joins the branch. This layer gradually cuts the leaf off, and the tree seals the spot, leaving a small scar. Not every tree follows this pattern: most conifers, such as pines, keep their needles all year.

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