On September 15th, 2018

Science of Changing Leaves

An explosion of brilliant colors is a hallmark of the fall season. Summer’s lush greens are replaced with mellow yellows and oranges as well as vivid reds and purples. It seems some trees turn overnight when taking on their glorious fall colors, while others gradually assume their fall color palette.

Eventually, with the colder weather, shorter days, and less daylight, all deciduous trees shed their leaves in colorful heaps that pile up under the tree trunk and stirred with the cold, fall winds. The intense color of autumn leaves even became a part of human culture, as our Thanksgiving and Halloween decorations are primarily orange and red.

At the same time, tree leaves are not just a decoration, rather, they are an important part of the tree’s nutritional system. Both the leaves of deciduous trees and the needles of conifer trees, which stay green even during wintertime, create sugars that feed the tree through the process of photosynthesis.

How does Earth’s rotation make leaves orange?
During photosynthesis, the chemical chlorophyll absorbs energy from sunlight to produce nourishment for the tree. When Earth rotates and receives less sunlight in the fall and winter in the Northern Hemisphere, photosynthesis in deciduous trees slow down then completely stops. With shorter days and cooler temperatures, chlorophyll brakes down and the leaves can no longer feed the tree. This causes the leaves to lose their green hue and reveal the yellows and oranges hiding below.

Over time, the tree builds a barrier between its leaves and its branches, as it no longer can use the leaves for the purposes of nutrition. When the sealing process is complete, the leaves separate from the branches and fall to the ground in colorful piles.

The green mask of summer trees
The same chemical, chlorophyll, that is responsible for photosynthesis, also gives leaves their green color. Without chlorophyll, all leaves would be yellow or orange. This means that during the spring and summer chlorophyll masks the actual color of leaves with its abundant greenery. With the arrival of fall and the accompanying changes in the environment that causes chlorophyll to disappear, the colors we associate with fall become visible in leaves.

Some trees can also turn red or purple, if they have sugars trapped in the leaves. Not all trees will have reds, but those that do produce red leaves go through a specific process that involves the synthesis of anthocyanin pigments. Some species such as oaks and maples are more likely to produce red and purple leaves. Even the same species of trees may produce different color palettes at different locations. Some areas of the world, such as the Northeastern part of the United States, are famous for their fall foliage.

Now You Know!

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