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Why Do Leaves Change Color?

Have the leaves on the trees in your neighborhood started to turn yellow? Or orange or red or brown or purple?

Photo courtesy of Jim Kloss

If you haven’t seen the green leaves start to give way to these new colors, you soon will. It happens every year. But why?

The leaves change color each autumn because the days get shorter. When there is less sunlight, the leaves cannot make as much chlorophyll, the pigment that makes leaves green.

Without the chlorophyll to make them green, other pigments in the leaves are expressed, creating the vivid display of fall color we look forward to each year.

Different pigments create different colors. Carotene makes orange leaves, and xanthophyll makes yellow leaves. Pigments called anthocyanins create leaves in shades of red and purple.

The colorful landscape looks different each year because of the weather. A lot of bright, sunny days in late summer and early fall create more vividly colored leaves. More cloudy weather will create a more muted landscape, with more shades of gold and yellow.

Eventually, these leaves will fall to the ground. Trees shed their leaves to help them survive the cold winter. As the days get shorter, the spot where the leaf stem attaches to the branch starts to close off, cutting the leaf off from water and food. It then begins to dry out and become brittle, eventually falling from the tree.

Not all trees have leaves that change color in the fall. The trees that do are called deciduous trees. Trees that don’t have leaves that change color are called conifers.

Conifers have needles instead of leaves – think Christmas trees and pine trees. We call these trees evergreens because they are always green. And while that’s mostly true, it’s not the whole story.

Conifers have needles that remain green for many years, but they do eventually fall off. The needles on a conifer are covered in a waxy substance that protects them from the cold winter temperatures. And the fluid in the needles is resistant to freezing, further protecting them from the cold.


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