Why Do Leaves Change Color in Fall?

 Leaves Change Color in Fall

Each fall, leaves go through dramatic color changes that bring glee to nature lovers, but why do leaves do that? Every year as fall arrives; it amazes the visual senses by showing a stunning display of colors. There are rich reds, fiery oranges and bright yellows that shine from the trees and crunch beneath boot covered feet as the leaves line yards, sidewalks and streets.

As winter changes to spring and the growing season returns, leaves are a green color due to the fact they are full of a pigment known as chlorophyll. This pigment is located in small structures called chloroplasts that are all through the cells of plants. The molecules of chlorophyll take in both blue and red light wavelengths that come from the Sun but barely any of the green. That, in turn, means the green wavelengths are reflected back to anyone looking at the trees. Therefore in the summertime, leaves appear to be this color.

Trees need photosynthesis so they can change light energy over to chemical energy and chlorophyll is vital for this. During the process, chlorophyll grabs light from the Sun which is used for energy to help break down water and carbon dioxide and turn these into oxygen and carbohydrates. These are used as food for the trees. Chlorophyll is basically unstable and subsequently always being broken down in the cells. To keep an adequate amount of pigment inside leaves, it must continuously be produced. This is a process that needs both warm temperatures and sunlight.

Leaves also have other pigments but chlorophyll is the richest during growing season. Therefore leaves are green in both spring and summer. When late summer or early fall arrives, the days begin to get shorter and nights become longer. Trees have a sensitivity to the amount of darkness each day and when nights start to reach a certain threshold that brings a stop to the production of the chemical auxin. With a decline in this, there is a cut-off of nutrients and water to leaves. It also halts the movement of carbohydrates from the leaves to the rest of the tree. When this occurs, leaves stop replacing chlorophyll and it vanishes from leaves fairly quickly.

This is when leaves began to change color in autumn. Chlorophyll typically hides any other pigments in leaves which reflect the distinctive fall scenery. The colors seen are also influenced by the type of weather and how moist the soil is. Cooler temperatures and lots of sunlight help to brighten the colors, but unfortunately freezing weather can come along too early, bringing with it morning frosts and that generates a sometimes premature end to the beautiful tree color.

As autumn progresses, leaf cells dry out even more. Sooner or later, all connections between the leaves and the trees they cling to are so weak that the leaves can no longer hang on, so they fall. While numerous trees become bare comparatively quickly, some are able to hold on to quite a few of their leaves, all through winter, but the gorgeous colors are gone. This is due to the fact that freezing temperatures and sunlight have caused all pigment to leave the tree except for a brown type called tannins. Because of these reasons, listed above, are why those that live in certain parts of the world get to experience leaves going through vivid color changes every fall.

By Kimberly Ruble

The Old Farmer’s Almanac
U.S.D.A. Forest Service
Science Made Simple

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