Ask a Specialist: What Causes the Fall Leaves to Change Color?
Thursday, Sep. 27, 2012
Utah State University Extension horticulturist
Phone: ( 801) 468-3185
USU Extension writer
Phone: (435) 797-0810
ASK A SPECIALIST: WHAT CAUSES THE FALL LEAVES TO CHANGE COLOR?
LOGAN, UT – The autumn season has transformed the mountains and hillsides into vibrant palettes of color that range from yellow to orange and from hot pink to scarlet red to purple. Colors vary from year to year and are a result of different pigments found within the plants.
• Chlorophyll (the green pigment) is found in nearly all plants and is a key component in photosynthesis, the conversion of light to energy. It breaks down readily in sunlight and is replaced constantly throughout the growing season.
• Carotenoids and xanthophylls (the orange or yellow pigments) also aid in photosynthesis and are produced throughout the season but break down slower. Quaking aspen, ginkgo, Norway maple, ash, birch and honey locust are a few examples of trees containing these pigments.
• Anthocyanin (the pink, red or purple pigment) is produced primarily in the fall and is found in species such as burning bush, sumac, dogwood and certain maples.
• Tannin (the brown pigment) is the last pigment to break down in a leaf before it falls. Oaks are notorious for having leaves containing tannin.
It is commonly believed that cooler temperatures and frost bring about the changing colors in leaves. While it is true that the fall temperatures can influence the intensity of some colors, the main reason for the color change is shortened day length.
During the growing season, leaves are constantly producing sugars and shipping them throughout the plant for growth and storage. When days become shorter and nights become longer, a process within the plant triggers the cells around the base of the leaf, or petiole, to divide rapidly but not elongate. This process forms an abscission layer where the leaf separates. This abscission layer blocks or prevents sugars from escaping the leaves.
As chlorophyll within the leaves breaks down, it gives way to the other pigments. If leaves contain carotenoids or xanthophylls, they will take on hues of yellow and orange while anthocyanins will display reds, pinks and purples. Colors are most brilliant when sunny autumn days are accompanied by cool nighttime temperatures. As the season progresses and freezing temperatures occur, all of these pigments (carotenoids, xanthophylls and anthocyanins) will fade, leaving remnants of the boring browns and reminding us that the winter doldrums are soon to come. Before this happens, go outside and enjoy the beautiful colors while they last.