Why Do Trees Turn Colors in Autumn?


Autumn color, which includes all the reds, yellows, oranges, and purple fall colors, is something many outdoor enthusiast looks forward to each year. All across the country, actually the whole Northern Hemisphere, our daylight hours become shorter and the temperatures begin to fall. This change causes a chemical process in trees to go into action and begins preparing them for the winter season. This article will give you information on the chemical processes that take place in leaves and other valuable information to help you understand the process. A list of fall trees,shrubs, and vines can be found to help you plant your yard with your favorite autumn colors. You will find the fall hotline listed at the bottom of this article.

What Causes Color Changes in Leaves?

The actual color changes are the result of changes in the leafs pigments. There are several different chemicals in plants that cause different colors to appear. There are three major pigments that play an important role in producing autumn colors.

What Do Chloroplasts Do?

The chloroplasts are the place within the plant where the process of photosynthesis takes place.

Activity in a chloroplast

Chloroplast Absorbs & Reflects Light

Remember, photosynthesis is the process in which light energy is converted into chemical energy. The light that is absorbed in the chloroplasts, supplies the energy that plants use to transform carbon dioxide and water into oxygen and carbohydrates. A well known formula of Cx(H2O)y.

In this transformation, the energy of the light that is absorbed by the chlorophyll is then changed and converted into a type of chemical energy that is stored in carbohydrates (sugars and starches). It is this chemical energy that drives the reactions that cause plants to grow, have flowers, and then produce seeds.

Photosynthesis

Photosynthesis During Summer

Chlorophyll

The green pigment, chlorophyll, in leaves absorbs red and blue light from the sunlight. Since these colors are absorbed, they will not reflect these colors, so the leaves appear to be green. *Remember, sunlight is made up of many colors. The molecules of chlorophyll are large. The chlorophyll attaches to membranes known as chloroplasts, and they are not soluble within the plant cells.

Chlorophyll is not a stable compound, as bright sunlight causes it to decompose. To keep the same level of chlorophyll, a plant must continually be synthesizing more chemical energy. To do this, as plant must have a lot of sunlight and warm temperatures to be able to continue these processes. That is why during the summer plants are green and growing well, because the plants have the materials they need.

Carotene      

Carotene is another pigment found in the leaves of many types of plants. This pigment absorbs blue-green and blue light. When the light is reflected from carotene it appears to be yellow. Carotene is also a large molecule like chlorophyll. When carotene and chlorophyll occur in the same leaf, together they both block out red, blue-green, and blue light from the sunlight, so the result is that it still appears green.

The energy of the light that carotene absorbs is combined with the chlorophyll, and the energy produced by both are combined in photosynthesis. Carotene is a more stable compound than chlorophyll so it is able to stay in the leaf, even if chlorophyll disappears from the leaf. When only carotene is left in the leaf, it appears to be yellow in color.

Anthocyanins            

Another class of pigments that are in leaves is the anthocyanins. Anthocyanins absorb blue, blue-green, and green light. The result is that the light reflected by the leaves will now appear to be red. The anthocyanins, unlike the other two pigments, are not attached to cell membranes, but are actually dissolved in the cell sap.

The cell sap has a pH value, and these pigments are very sensitive to that pH. If the sap is very acidic, then the color will be a bright red, but if the sap is less acidic, then the results may be a shade of purple.

*Anthocyanins are also responsible for apples having a red skin, and ripe grapes having a purple skin.

Anthocyanins are formed from proteins and sugars reacting together within the cell sap. This does not happen until the sugar level in the sap gets very high. Light is also needed for the reaction to take place.

The Process Flow

During the warm summer months the leaves on trees produce sugar from carbon dioxide and water by the action of light on chlorophyll. Remember, chlorophyll cause the leaves to appear green.

Next, water and nutrients flow naturally up from the roots, through the branches, and into the leaves. The sugars produced by photosynthesis in the leaves, will then flow to all parts of the tree. Some of this energy is used for growth and some is stored away. The trigger that causes the changes in the trees is the shortening of the days and the cool autumn nights.

lost of chlorophyll

Leaves change colors when chlorophyll levels fall

Changes within the plant

One change is the growth of a cork type membrane that is located between the branch and the leaf stem. This membrane interferes with the natural flow of nutrients into each leaf. When this happens, the amount of chlorophyll is reduced, and causes the green color of the leaf to disappear.

*If the leaf contains carotene, it will will change the leaves from green to bright yellow as the chlorophyll disappears.

In some types of trees, as the sugar level becomes more concentrated in the leaves, the sugar reacts for form anthocyanins.

Remember, these are the pigments that cause the yellowing leaves to turn red. This is seen in many red maples, red oaks, and sumac.

Intensity and Range of Colors- The intensity and range of colors that is produced by a plant is influenced by weather conditions.

  • Low temperatures- destroy chlorophyll, and if the temperature stays above freezing, anthocyanins will form.
  • Bright sunshine- also destroys chlorophyll and produces more anthocyanin.
  • Dry weather- increases sugar concentration in sap, and raised the level of anthocyanins.

*The best conditions for the brightest autumn colors are produced when we have dry, sunny days, followed by cool, dry nights.

Fall Foliage Location & Information

The U.S. Forest Service has a foliage Hotline at (800) 354-4595 that you can access to find information, or, you may choose online info at  USDA Forest Service - Caring for the land and serving people.

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