Why do leaves change color in Fall?

by Science Made Simple

We all enjoy the beautiful show of colors as leaves change each autumn. Did you ever wonder how and why this happens? To answer that question, we first have to understand what leaves are and what they do.

Leaves are nature's food factories. Plants take water from the ground through their roots. They take a gas called carbon dioxide from the air. Plants use sunlight to turn water and carbon dioxide into glucose.

Glucose is a kind of sugar. Plants use glucose as food for energy and as a building block for growing.

The way plants turn water and carbon dioxide into glucose is called photosynthesis. That means "putting together with light." A chemical called chlorophyll helps make photosynthesis happen. Chlorophyll is what gives plants their green color.

As summer ends and autumn comes, the days get shorter and shorter. This is how the trees "know" to begin getting ready for winter.

During winter, there is not enough light or water for photosynthesis. The trees will rest, and live off the food they stored during the summer. They begin to shut down their food-making factories. The green chlorophyll disappears from the leaves. As the bright green fades away, we begin to see yellow and orange colors. Small amounts of these colors have been in the leaves all along. We just can't see them in the summer, because they are covered up by the green chlorophyll.

The bright reds and purples we see in leaves are made mostly in the fall. In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn turn this glucose into a red color. The brown color of trees like oaks is made from wastes left in the leaves.

It is the combination of all these things that make the beautiful colors we enjoy in the fall.

Plants make their own food.

They take water from the ground through their roots. They take a gas called carbon dioxide from the air. They turn water and carbon dioxide into food using sunlight and something called"chlorophyll".

Chlorophyll is green. It gives leaves their green color.

Winter days are short and dry. Many plants stop making food in the fall. The chlorophyll goes away. Then we can see orange and yellow colors. These colors were in the leaves all summer, but the green covered them up.

Some leaves turn red. The red color is made in the fall, from food trapped in the leaves.

Brown colors are also made in the fall. They come from wastes left in the leaves.

All summer, with the long hours of sunlight and a good supply of liquid water, plants are busy making and storing food, and growing. But what about wintertime? The days are much shorter, and water is hard to get. Plants have found many different ways to get through the harsh days of winter.

Some plants, including many garden flowers, are called "annuals," which means they complete their life cycle in one growing season. They die when winter comes, but their seeds remain, ready to sprout again in the spring.

"Perennials" live for more than two years. This category includes trees and shrubs, as well as herbaceous plants with soft, fleshy stems. When winter comes, the woody parts of trees and shrubs can survive the cold. The above ground parts of herbaceous plants (leaves, stalks) will die off, but underground parts (roots, bulbs) will remain alive. In the winter, plants rest and live off stored food until spring.

As plants grow, they shed older leaves and grow new ones. This is important because the leaves become damaged over time by insects, disease and weather. The shedding and replacement continues all the time. In addition, deciduous trees, like maples, oaks and elms, shed all their leaves in the fall in preparation for winter.

"Evergreens" keep most of their leaves during the winter. They have special leaves, resistant to cold and moisture loss. Some, like pine and fir trees, have long thin needles. Others, like holly, have broad leaves with tough, waxy surfaces. On very cold, dry days, these leaves sometimes curl up to reduce their exposed surface. Evergreens may continue to photosynthesize during the winter as long as they get enough water, but the reactions occur more slowly at colder temperatures.

During summer days, leaves make more glucose than the plant needs for energy and growth. The excess is turned into starch and stored until needed. As the daylight gets shorter in the autumn, plants begin to shut down their food production.

Many changes occur in the leaves of deciduous trees before they finally fall from the branch. The leaf has actually been preparing for autumn since it started to grow in the spring.

At the base of each leaf is a special layer of cells called the "abscission" or separation layer. All summer, small tubes which pass through this layer carry water into the leaf, and food back to the tree.

In the fall, the cells of the abscission layer begin to swell and form a cork-like material, reducing and finally cutting off flow between leaf and tree. Glucose and waste products are trapped in the leaf. Without fresh water to renew it, chlorophyll begins to disappear.

Other colors, which have been there all along then become visible. The orange colors come from carotene ('kar-uh-teen) and the yellows from xanthophyll ('zan-thuh-fil). They are common pigments, also found in flowers, and foods like carrots, bananas and egg yolks. We do not know their exact role in leaves, but scientists think they may be involved somehow in photosynthesis.

The bright red and purple colors come from anthocyanin (an-thuh-'si-uh-nuhn) pigments. These are also common in plants; for example, beets, red apples, and purple grapes, and flowers like violets and hyacinths. In the leaves, these pigments are formed in the autumn from trapped glucose.

Brown colors come from tannin, a bitter waste product.

Different combinations of these pigments give us a wide range of colors each fall.

As the bottom cells in the separation layer form a seal between leaf and tree, the cells in the top of the separation layer begin to disintegrate. They form a tear-line, and eventually the leaf is blown away or simply falls from the tree.

One more important question remains. What causes the most spectacular displays?

The best place in the world for viewing fall colors is probably the Eastern United States. This is because of the climate there, and the wide variety of deciduous trees. The brightest colors are seen when late summer is dry, and autumn has bright sunny days and cool (low 40's Fahrenheit) nights. Then trees make a lot of anthocyanin pigments. A fall with cloudy days and warm nights brings drab colors. And an early frost quickly ends the colorful display.

PROJECT 1 - Separate Colors in a Green Leaf using Chromatography

PROJECT 2 - Separate Colors in a Fall Leaf using Chromatography

PROJECT 3 - Observe how light affects color development