Hidden Colors: Why Leaves Change in Autumn

Hidden Colors: Why Leaves Change in Autumn

1Every autumn, in many parts of the world, hillsides that were green all summer burst into shades of gold, orange, and red. A walk through the woods can feel like a walk through a painting. Yet the trees are not adding paint to their leaves. Most of these colors were inside the leaves all along, waiting to be seen.

2To understand why, scientists study pigments. A pigment is a substance that gives something its color. The most important pigment in a leaf is chlorophyll, which makes leaves green. Chlorophyll has a big job. It captures energy from sunlight, and the leaf uses that energy to turn water and carbon dioxide, a gas in the air, into sugar. This process is called photosynthesis. The sugar is food that helps the tree live and grow.

3Leaves also hold yellow and orange pigments. Carrots get their orange color from a pigment of the same kind. During spring and summer, however, a leaf is packed with so much chlorophyll that the green hides the other colors. It works much like a thick coat of green paint covering a yellow wall underneath.

4As summer turns to autumn, the days grow shorter, and the nights grow longer and cooler. Trees respond to these changes. Little by little, the leaves stop making new chlorophyll, and the chlorophyll that is left begins to break down. As the green fades, the yellows and oranges that were hidden all summer finally appear.

5Red and purple leaves are a different story. In some trees, such as many maples, the leaves make brand-new red pigments in autumn. These reds are often brightest after a stretch of sunny days and cool nights. The leaves of some other trees, such as certain oaks, turn brown. The brown comes from materials left behind in the leaf after the other pigments are gone.

6Soon after the colors change, the leaves fall. Dropping leaves helps a tree survive the winter. Broad, flat leaves let a great deal of water escape into the air. In winter, the ground may freeze, and roots cannot easily take in water from frozen soil. By shedding its leaves, the tree saves water. Before a leaf drops, a special layer of cells grows where the leaf stem joins the twig. This layer slowly blocks water from reaching the leaf and loosens its grip on the twig. Soon a light breeze can knock the leaf free. The layer also seals the spot where the leaf once hung, leaving a tiny scar on the twig.

7Not every tree loses its leaves in autumn. Evergreens, such as pines and spruces, keep most of their needles all winter. Their thin needles have a waxy coating that helps them hold on to water. Meanwhile, the fallen leaves of other trees do not go to waste. They slowly break down and return nutrients to the soil, which helps plants grow in the spring. So the next time you crunch through a pile of autumn leaves, remember that many of their bright colors were hiding inside them all summer long.