One of the great symbols of the fall season is several weeks of vibrant leaf color in the areas of the country with large deciduous forest. What causes this color change and why are certain species different colors? American Forest Management wants to help you understand some of the biology behind this seasonal beauty.

Leaves are the energy capturing factory of the plant; like solar panels, they absorb the sun’s energy and produce food for the tree or plant during the growing season. During the summer months leaves are many different shades of green, this is because they are full of chlorophyll, a green substance that absorbs light which creates energy and allows the plant to grow and survive. The leaves also contain carotenoids, anthocyanins, and tannins which are pigments. The presence or lack of each of these substances determines the color of the leaf in the fall. In general, certain species display the same color pattern or color progression each fall season, while the vibrancy of the color may depend on local conditions, you can recognize species in part by what color they are in the fall.

Leaf color changes are triggered as part of the process to prepare the plant for winter. There is no need for the plant to expend energy to maintain leaves if conditions are not favorable for that leaf to produce energy to help the plant grow. Leaf abscission, or the dropping of leaves, is caused primarily by the shortening of day length as we enter the fall season. As the days begin to get shorter (and that varies by location, north to south), the chlorophyll production stops and the leaf breaks the chlorophyll down and absorbs it to store all the energy it has created during that growing season. As the chlorophyll is depleted some of the other underlying pigments remain as they break down much slower, producing that vibrant fall color we all enjoy. The carotenoids produce the yellows and oranges, the anthocyanins produce the reds, pinks, and purple, and the tannins produce brown colors.

Good examples of trees with a strong presence carotenoids and few anthocyanins include yellow poplar, hickory, and ginko trees – they all have strong yellow fall color. Anthocyanins can be found in sumacs, black gum, red maple, dogwood, and sweetgum displaying reds, pinks, and purples. Many oak species have strong tannins and can quickly change from red to brown in color. Sugar maples are often planted because of their vibrant orange colors indicating a strong carotenoid presence as well.

The quality of the fall color season depends heavily on the weather conditions in the late summer and early fall. In general, the most vibrant colors occur after a growing season with adequate moisture, followed by a dry period in early fall, with cool nights. A severe drought in fall can cause the leaves to drop early, as well as an early frost, or severe wind and rain.

Take time this season to get outside and enjoy the 2-4 weeks of color provided by our forests. And if you are enjoying this with others, take time to explain to them what is occurring in the plant that creates this beautiful display of color!