If you live in an area that has four seasons, you probably love the fall. It is a time when the weather turns cooler, the days get shorter, and in Virginia, the leaves of deciduous trees turn brilliant colors in preparation for the winter. Many people speculate why leaves change color, but the real reason is because the nights get longer and the amount of daylight decreases. 

In order to understand why leaves change colors, I have to explain a little bit about leave biology. During the growing season, leaves of deciduous trees are filled with chlorophyll. Photosynthesis is the process where chlorophyll absorbs energy from the sun to transform carbon dioxide and water into carbohydrates such as starches and sugars, that the tree uses for food. The chlorophyll is constantly replenished during the growing season because it fades or breaks down from exposure to the sunlight. 

Maple leaves filled with chlorophyll showing their bright green color.

Leaves also have or make pigments. This is what gives the leaves colors other than green.

 Carotenoids – oranges

Anthophylls – yellows

Anthocyanins – reds, purples, and blues

Tannins – browns  

When the nights reach a certain length trees start to prepare for winter. At the place where the stem and leaf meet, called the abscission layer, the cells start to divide quickly and they block the transport of food (carbohydrates) and minerals to the leaf. When this happens, the production of chlorophyll stops and the green color disappears. This is when xanthophylls and carotenoids (yellows and oranges) that are always present in leaves begin to be seen. Anthocyanins are created from the sugars trapped in the leaves as the chlorophyll disappears. 

Sassafras leaves turn a variety of colors. They aren’t as prevalent as maples, but where they are present, they add excellent color. 

An example of the characteristic purples and reds of dogwood trees.

Poplar trees usually only turn from green to yellow then brown due to the pigments present. 

Oak leaves change into a variety of browns, tans, and yellows.

Great places to see fall leaves!

Shenandoah National Park – Try hiking Jeremy’s Run

Great Falls National Park – beautiful trees along the Potomac River in a spectacular setting!

Mason Neck National Wildlife Refuge

Sky Meadows State Park

References:

USDA Forest Service: Northeastern Area

The Old Farmer’s Almanac

The Chemistry of Autumn Colors

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