Why Do Leaves Change Color in Autumn?

Every autumn, many people in Japan travel to the mountains or to old temples to enjoy the red and yellow leaves. This custom is called momiji-gari, which literally means hunting for maple leaves. But why do green leaves change color in the first place?

In spring and summer, leaves are full of a green pigment called chlorophyll. Chlorophyll helps the tree use sunlight to make sugar, which is its food. As autumn comes and the nights become longer, the tree stops making new chlorophyll, and the old chlorophyll slowly breaks down.

As the green fades, yellow and orange colors start to appear. These come from pigments called carotenoids, which have been in the leaves all along. They were simply hidden by the strong green color during the summer.

Red is different, however. In many trees, red pigments called anthocyanins are newly made in autumn, using sugars that are trapped in the leaves. Scientists are still studying why trees make these red pigments.

The weather also plays a big role. The brightest colors usually appear when autumn days are sunny and cool, and the nights are cold but not freezing. As a result, the colors can be stronger in some years than in others. So when you go to see the autumn leaves this year, remember that you are also watching science in action.