The Colors of Autumn

With the yearly color change, it is important to understand why trees go through this process.  Throughout the summer months, when trees are actively growing, the majority of the plant’s nutrients are stored within the leaf tissue.  Towards the end of summer, plants will begin to store energy instead of using it for new growth and reproduction.  Photosynthesis is the process by which the plant converts light, water, and carbon dioxide to make the energy the plant needs to grow and survive the winter.  The green color of leaves is produced by the chlorophyll component of photosynthesis that collects the sun light.  These compounds are located near the surface of the leaf for the greatest light absorption and therefore hide the color of other leaf components.

As the days grow shorter and the temperature begins to drop, plants respond by disassembling the leaves photosynthetic components.  The nutrients within these components are moved to the roots and stems where they are stored and will be used the following spring.  This disassembly of leaf components is highly complex, however most of it will occur before we see any leaf color change. It is only during the very last stages of this breakdown of photosynthetic components that the showy leaf colors appear.

Carotenoids are responsible for the bright yellow leaf color.  They are present in the leaves throughout the growing season, but are only visible when the chlorophyll begins to break down.

These pigments protect the leaves from the harmful by-products of photosynthesis.  Without them photosynthetic components would destroy themselves.

Anthocyanin are responsible for the reds, oranges, pinks, purples, and blues in fruits and flowers, and also the brilliant red and orange colors of fall leaves.  These compounds are made midway through autumn and are used to block the photosynthesis systems from bright sunlight as the chlorophyll breaks down.  Plants that do not turn red in fall are usually more resistant to bright sunlight, and therefore don’t need to produce the red pigments.  Due to this protective property of red pigments, many times the outer most leaves of a tree will be more red than the inner leaves that are shaded by the outer leaves will be more yellow in color.

The most important factor in the amount of color that will be seen within a year is the weather during the color change.  Cool, sunny weather will result in very bright fall colors because more pigments will have to be produced to protect the leaves from sun damage.  While cool weather is good, a hard frost may destroy the leaf tissue before all of the colors are able to come out.  Warm weather will increase the speed at which the components are broken down and decrease the amount of red pigments that are produced in the leaves.

Christine Regester

KNC Project Manager

Excerpt from Fall/Winter 2007 KNC Newsletter