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Why Trees Turn Color in Autumn

What is it that triggers coloring to take place in certain tree species?

The areas famous for fall coloration because of deciduous trees are well-known. New England, the mountains of New York, Pennsylvania and the Rockies all have their claim to fame for sometimes breathtaking colors. As overwhelming as they may appear when one is taking in these majestic colors, only 14 percent of the world’s forests are temperate deciduous forests with a habit of giving us brilliant fall coloration.

In regions where sugar maples abound, such as New England, the red leaf displays dominate, but are intermingled with the bright yellows of aspen, beech and birch trees. In spite of the striking beauty that we enjoy, Mother Nature doesn’t do it for our benefit because these hues of red, yellow, gold and brown represent more than just a pleasing experience for humans.

“Shortly after peaking in color intensity, the leaves abscise, or fall, to the earth,” says Ron Smith, North Dakota State University Extension Service horticulturist. “This move is designed to help conserve the energy balance in the trees. This reduces and balances the respiration rate of the tree to approximate the lowered rate of photosynthesis that takes place during our winter months.”

What is it that triggers this coloring to take place in certain tree species? Many think it may be initiated by a touch from Jack Frost, but in reality, the colors we enjoy only can be witnessed only on living, senescing trees. If Jack Frost arrives too early in the fall, there will be no fall colors, just dull browns because of the “killing frost.”

“What triggers these events is a specific combination of shorter days and cooler temperatures in autumn in a specific locale that is sensed by plant receptors, which results in hormone production,” Smith says. “This, in turn, initiates leaf senescence. This specificity to a narrow climatic zone is important for us to understand because it is generally effective within 130 miles north or south of the origin. For example, this is why a red maple that looks great in Ames, Iowa, is mediocre in Fargo. Or, at worst, has the leaves nipped by a hard frost before it has a chance to senesce sufficiently.”

Typically we see reds, yellows and oranges in our region of the country. Carotenoid pigments are unmasked during leaf senescence. This gives the viewer the yellow colors we see in the common Ohio buckeye, birch and ash trees.

“The most favorite color for most observers is the red that comes from anthocyanin pigments,” Smith says. “Red and sugar maples, along with sumac shrubs, are the sources in our region for this color. Unlike other pigments, anthocyanins are not commonly present in leaves until autumn coloration begins. There are exceptions we all know about, such as the red leafed chokecherry and the crimson king Norway maple. Trees lacking the genes for red color from anthocyanins instead will develop yellow and brown shades in autumn.”

Fall weather favoring bright red autumn leaf colors are warm, sunny days followed by cool, but not freezing, nights. Rainy or cloudy days, with reduced sunlight near the time of peak coloration, decrease the intensity of the reddish autumn colors by limiting photosynthesis and the sugars available for anthocyanin production.

NDSU Agriculture Communication

Source:Ron Smith, (701) 231-8161, ronald.smith@ndsu.edu
Editor:Rich Mattern, (701) 231-6136, richard.mattern@ndsu.edu
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