Missouri's Season of Splendor

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Published on: Sep. 2, 1999

Last revision: Nov. 3, 2010

predictable but varies from year to year. Much depends on the weather.

A warm, moist growing season followed by a mild late-season drought sets the stage for a brilliant fall. During September and October, chilly, but not freezing, nights and sunny days are needed to bring out the best colors. These exacting conditions are not met every year, so some years are better than others.

Long periods of rainy and cloudy fall weather result in drab colors because of the low light intensity. On the other hand, trees under drought stress will often drop their leaves without showing much color.

Temperature also affects fall color. Warm temperatures encourage late season chlorophyll production that keeps the leaves green. A belowfreezing cold snap will kill the leaves before they turn to their fall colors. Even though the colors may be bright, heavy rain or high winds during the peak of color will cause the leaves to drop early.

The usefulness of the leaves does not end when their colors fade. Fallen leaves contain large amounts of minerals, such as nitrogen and phosphorus. Decomposition of the leaves returns these nutrients to the soil while adding a water-absorbing layer of humus to the forest floor. Shedding its leaves is another way a tree prepares for winter. Without leaves, broadleaf trees are better able to hold the weight of ice and snow on their branches.

Will October's colors be better or worse than last year's? Will we have a brilliant display of reds, yellows and oranges or one of faded browns? Sorry-but only nature and time will tell.

Why Leaves Change Color

Throughout spring and summer, leaves serve as factories manufacturing the food necessary for a tree's growth. The food-making process takes place inside leaf cells, in tiny structures called chloroplasts. The chloroplasts contain chlorophyll that gives the leaf its green color. The chlorophyll absorbs energy from sunlight and uses it to transform carbon dioxide and water to carbohydrates, such as sugars and starches.

Chlorophyll is unstable and must continually be replaced. Light is essential for chlorophyll manufacture. The tree's nutrition also affects the amount of chlorophyll in the leaf. If nitrogen, iron, magnesium or some other element is scarce, the rate of chlorophyll formation will decline. Restoring these nutrients will increase chlorophyll production. This is why shade trees often "green up" after being fertilized.

Other structures in the leaf called chromoplasts contain nonphotosynthetic pigments. These pigments are the carotenoids, the carotenes and xanthophylls that color carrots, corn, squash and bananas. Most of the year the carotenoids are masked by a greater amount of green chlorophyll. But in the fall the food making process stops. The chlorophyll breaks down, the green color disappears, and the yellow colors become visible.

Carotenoids tint the leaves of hardwood species, such as hickory, ash, maple, tuliptree, birch, cottonwood and sassafras.

A third group of pigments found in leaves are called anthocyanins. These pigments produce the reds and purples we see in fruits, such as apples, grapes, blueberries, cherries and plums. Anthocyanins are not present in the leaf during the growing season like the carotenoids. They develop in late summer in the sap of the cells, formed by chemical reactions between accumulating sugars and organic compounds called anthocyanidins.

Light is necessary for the formation of anthocyanins. Warm, sunny days followed by cool nights are ideal for their production. Sugar is made in the leaves during the day, but cool nights prevent movement of the sugar from the leaves. The brighter the light during this period, the greater the production of anthocyanins, and the more intense the colors. In trees with acid sap, reds will predominate; in alkaline sap, blues and purples prevail.

Anthocyanins temporarily color the edges of some young leaves as they unfold in the early spring. In our autumn forests, they show up vividly in maples, oaks, sweetgum, dogwood and blackgum. These same pigments often combine with carotenoids to give the deeper oranges, fiery reds and bronzes typical of many hardwood species.

Tell Trees by Their Fall Color
Tree Fall Color
Green Ash yellow
WhiteAsh reddish-purple
River Birch yellow
Blackgum scarlet
Cottonwood yellow
Flowering Dogwood scarlet
American Elm yellow
Hackberry yellow
Hawthorn scarlet
Hickory yellow
Red Maple red to yellow
Silver Maple yellow
Sugar Maple yellow, orange, red
Northern red Oak russet to red
Scarlet Oak russet to red
White Oak reddish-purple
Pin Oak bronze or red
Osage-orange yellow
Redbud yellow
Sassafras orange to scarlet
Serviceberry red-orange
Sumac red
Sweetgum yellow to red
Tuliptree yellow
Black Walnut yellow

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