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American Forest Management wants to help you understand the biology behind the beauty of fall

2020/09/01
Fall tnnc1061

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!

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Frequently Asked Questions

  • Leaves change color as part of the process trees use to prepare for winter. As days grow shorter in the fall, the tree stops producing chlorophyll, the green substance that absorbs sunlight and fuels growth during the growing season. As the chlorophyll breaks down and is reabsorbed by the plant, other pigments that were present all along become visible, producing the vibrant yellows, oranges, reds, and browns we associate with autumn.

  • Three main pigments determine fall leaf color. Carotenoids produce yellows and oranges, anthocyanins produce reds, pinks, and purples, and tannins produce brown colors. The presence or relative amount of each pigment in a given tree species determines what color that species displays each fall.

  • Each species has its own characteristic mix of pigments, so certain species tend to display the same color or color progression every fall season. For example, yellow poplar, hickory, and ginkgo trees have strong carotenoids and turn yellow, while sumac, red maple, black gum, dogwood, and sweetgum display reds, pinks, and purples due to anthocyanins. Many oak species are high in tannins and shift quickly from red to brown, while sugar maples are known for their vibrant orange color.

  • The most vivid fall colors typically follow a growing season with adequate moisture, succeeded by a dry period in early fall and cool nights. Poor conditions can diminish the display. A severe drought, an early frost, or heavy wind and rain can all cause leaves to drop prematurely rather than progress through their full color change.

  • Leaf drop, known as leaf abscission, is triggered primarily by the shortening of day length as the fall season arrives. When days shorten, the tree stops producing chlorophyll and begins breaking it down to recover the energy stored during the growing season. Once the chlorophyll is depleted, the remaining pigments create the fall color display before the leaves eventually fall.

  • The fall color season generally lasts about two to four weeks. The timing and duration can vary by location, with areas farther north experiencing the change earlier than those farther south, since day length shortens at different rates depending on latitude.