Why Do Autumn Leaves Change Color: Part 1

Why do autumn leaves change color? What controls the change, and why are the colors so different from year to year? Though I am far from an expert, the following is what I have been able to learn about the subject. 

Highland Scenic Highway

Three Prime Factors
The reason autumn leaves change color is far from simple. Some factors are well known, but scientists admittedly do not have all the details. The three prime factors controlling the color change are: 1) Leaf pigments, 2) Amount of light, and 3) Weather conditions. The color palette of leaves consists of three main pigments: chlorophyll, carotenoids, and anthocyanin. 

Babcock State Park

Chlorophyll is the green pigment, so to speak, and is essential for plant growth. Chlorophyll is an exceptional substance that converts sunlight into food for plants in a process called photosynthesis. Using energy from sunlight, chlorophyll combines water and carbon dioxide to produce oxygen and sugars. The oxygen mingles with the air we breathe and helps to sustain human life, which depends upon oxygen. The sugars are sent from the leaf into the tree to help it live, grow, and reproduce.

Leaves remain green as long as chlorophyll is plentiful. But chlorophyll is not a stable compound; bright sunlight causes it to decompose. To maintain chlorophyll in their leaves, plants continuously produce it, which requires sunlight and warm temperatures. Therefore, during the summer, chlorophyll in leaves is continuously broken down and regenerated. The trees of summer owe their lush green leaves to the abundance of chlorophyll in their leaves, which is constantly being destroyed and renewed by the summer sun.

Swinging Bridge at Babcock State Park

Yellow, the Left-Over Color
This leads to the second major factor determining the color change–light. As fall approaches, the days shorten, the amount of sunlight wanes, and the air cools. Like an internal clock, shorter days signal leaves to begin to shut down their food production. The shorter days mean more chlorophyll dies than is replaced. As chlorophyll begins to die off, other leaf pigments, which are masked by the green of the chlorophyll, are revealed. 

Aspen at Summit Lake

Present in the leaves all summer long, shades of yellow from the carotenoids finally begin to show through. Carotenoids produce yellow, orange, and brown colors in such plants as corn, carrots, daffodils, rutabagas, buttercups, and bananas. Striped maple, birch, aspen, hickory, poplar, ash, and other trees glow in shades of deep gold to pale yellow because of the carotenoids in their leaves. Ironically, the beautiful yellows and golds of autumn leaves are colors left over after all the chlorophyll has died out.

Summit Lake near Richwood, WV

In a post to follow I’ll discuss why leaves turn red.
For more on this topic click Why Leaves Change Color