Why Are Fall Leaf Colors Different Every Year?

Every fall, tourists, and those lucky enough to live in northern temperate climates, look forward to the brilliant color show promised before the deciduous trees (those that drop their leaves annually) close shop for winter. Deciduous leaves change color due to decreasing day length and falling temperatures. But the specific autumn temperatures and the amount of cloud cover can make a big difference in a tree's fall colors from year to year.

Plant Pigments

When a tree begins its preparations for dormancy, chlorophyll, the green sunlight capturing pigment that gives leaves their summer color, begins to break down. But chlorophyll is not the only pigment that a plant has at its disposal. There are also other colored pigments present but masked by the intense green of chlorophyll.

These other pigments include carotenoids and anthocyanins. Carotinids create the bright yellows and oranges that we see in some fruits and vegetables. Anthocyanins impart a red color to plants, such as that seen in cranberries, red peppers, cherries, strawberries and the leaves of red maples.

The Reds, Yellows, Golds and Oranges of Autumn

When the autumn days are sunny and cool, but nighttime temperatures do not freeze, these conditions foster a color show with more red pigments. This happens because the cool nighttime temperatures prevent the glucose (a sugar that feeds the plant) from flowing down from the leaves through the branches and trunk to be stored. Anthocyanin pigments come to the rescue to help the tree recover these nutrients before the leaves fall off, and in the process, make the leaves appear more red in color.

The yellow, gold and orange colors in leaves, created by carotenoid pigment, remain fairly constant and do not change in response to weather conditions. So these fall colors don't vary as much from year to year.

Effect of Rain and Frost on Fall Colors

The amount of rain in a growing season can affect the autumn leaf colors, and severe droughts can delay the arrival of the fall color show for weeks. Warm, wet autumns tend to lower the intensity of fall colors. And severe frost will kill the leaves, causing the leaves to turn brown and drop. The best autumn colors result from weather patterns such as:

• a spring that was warm and wet
• a summer that was not too hot or dry
• a fall that had a series of warm sunny days and chilly (but not freezing) nights

Using Fall Color to Help Identify Trees

Although you can't identify the species of tree by its fall colors alone, the color of the leaves is one factor that can aid in identification (in addition to leaf shape, bark texture, tree shape and type of fruit produced). Here is a list of the fall leaf colors commonly produced by different types of trees:

• Oaks: brown, sometimes with some yellow or red
• Hickories: golden bronze
• Dogwood: deep purplish-red
• Birch: bright yellow
• Poplar: golden yellow

Maples show a wide range of colors that can vary in intensity from year to year. Sugar Maples are typically orange-red. Black Maples produce glowing yellow leaves, and Red Maple become brilliant scarlet.

More Information on Trees' Fall Color Change

To learn more about autumn leaf colors and what creates them, see Why Leaves Change Color and the Wisconsin DNRs page on Fall Color.

Sources

Starr, C. & Taggart, R. (1992). Biology: The Unity and Diversity of Life. Wadsworth Publishing.

Campbell, N & Reece J. (2002). Biology, 6th edition. Benjamin Cummings.