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Why Do Leaves Change Color?

Each autumn as the weather cools down and the days become shorter, a fiery color palette surrounds the world around us. Vibrant red, orange, and yellow leaves create a crunchy, colorful blanket on the ground, and the foliage on the trees above produces perfect autumnal photo-ops worthy of multiple social media posts. As you take it all in, you may have wondered how the leaves — which were green just months ago — get their annual fall hues. Here’s a look at the science behind it.

Why Leaves Change Color in the Fall

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The same substance that makes leaves grow is also partially responsible for their autumnal transformation: chlorophyll. This green pigment absorbs light and converts it into chemical energy through photosynthesis. This process helps plants generate energy to turn carbon dioxide and water into carbohydrates, which helps plants grow and thrive.

Evergreen trees photosynthesize all year round, which is why they remain green. But the process discontinues in deciduous trees — as the sunlight wanes at the end of the year, their chlorophyll diminishes.

When a tree reabsorbs the energy from its leaves to prepare for winter, chlorophyll is the first to go, and other colorful chemicals become more obvious to the eye. Carotene, the same substance that makes carrots and yams orange, is among them, along with Xanthophyll, a collection of yellow-orange pigments closely related to carotene. A set of red pigments, called anthocyanins, also emerge. Anthocyanins can also contribute to leaves having a purple hue.

The color of the leaves is weather-dependent, too. Rainy days can make autumn foliage brighter. Early frosts can hold red pigments back. Other factors include light and water supply.

Why Leaves Fall

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Leaves don’t just fall off trees because they’re dead. Many deciduous trees actually push their leaves off on purpose to conserve energy.

While photosynthesis is already low with the leaves’ chlorophyll dissipating, by the time winter hits, the weather is likely to render the leaves pretty useless — attached, anyway. Having a tree’s most fragile parts exposed to cold, wind, and snow can lead to damage, so when the trees are done storing the nutrients from their leaves, they grow specialized cells that cut off the leaves and create a protective layer that helps protect them from the elements.

By shedding all its leaves, the tree is also getting rid of leaves that may have already been damaged in the summer, encouraging fresh, healthy growth come springtime.

Evergreen trees have a different process for protecting themselves during the winter. Their needles have a protective layer of resin that’s somewhat weather resistant. The resin is packed with terpenes, compounds that make plants smell — and why your home is infused with a warm scent when you bring in a fresh Christmas tree.