by Whit Gibbons

November 14, 2010

What determines the color of tree leaves in the fall, and why do colors vary so much from year to year and place to place? I asked Linda Lee, a botanist at the Savannah River Ecology Laboratory, if recent scientific research has made predicting the timing and regional occurrence of fall colors more reliable. According to her, scientists are a long way from predicting accurately what fall foliage will be like in a given year. Despite what you may read about the best time to view fall colors, no one knows for sure.

We do know that temperate zone forests change color in autumn. And thousands of leaf peepers make an annual pilgrimage to view trees in the Smoky Mountains and elsewhere, despite not knowing exactly when or where fall colors will be the most spectacular or whether colored leaves will even still be on the trees.

Why do some trees have brown leaves, others have brilliant red or yellow ones, while still others stay green all year? Three basic pigments are responsible for most annual color patterns in plants. Chlorophyll, which makes leaves green, is the dominant pigment. As the days get shorter, some leaves display two additional pigments: carotenoid, which results mainly in yellow and orange, and anthocyanin, which results in red. Carotenoids absorb energy and help protect leaves from sun damage. Anthocyanins are less well understood but may also reduce sun damage and deter fungal pathogens. As far as leaves falling, Linda Lee notes that "autumn weather per se is not what kills the leaves; the tree itself does, by pulling nutrients out of leaves and sealing them off. Once the leaf is an empty shell, it falls. In other words, the tree sort of beats winter to the punch."

The pigments that produce the variety of fall colors in tree leaves are themselves at the mercy of three primary environmental factors--temperature, day length and rainfall. All are critical in determining how pigments express themselves. Cool autumn temperatures cause chlorophyll to degrade in many deciduous trees. Thus the carotenoid and anthocyanin, normally masked by the chlorophyll, are accentuated. Colorful displays of reds, yellows and oranges are created. But a sudden, heavy frost may break down the accessory pigments as well as chlorophyll, which means colors are muted.

If autumn cooling is gradual, colors may be dull because chlorophyll remains in the leaves, preventing full expression of the brightest yellows, reds and oranges. To further complicate predictions, pigments respond differently to temperatures based on the timing and amount of rain during previous days or weeks. Predicting exactly when fall colors will appear in a particular year and what they will look like is as complicated as forecasting the weather--and just as unreliable.

Heavy summer rainfall can have a profound effect on fall foliage, as the plants will produce more sugars and more pigments. If chlorophyll is broken down rapidly, the remaining colorful pigments will be at highest density, creating intense autumn colors and delighting the leaf peepers. A wet summer means more leaves, more leaf surface, and therefore more color displayed in the forest.

The geographic region and the types of trees also have a major influence on leaf color. Brown autumn leaves characteristic of many trees in warm regions of the South are often a result of pigments associated with tannin, which mask the red and yellow colors. Tannins are believed to discourage insects from feeding on the leaves. Evergreens, such as pines and magnolias, are another special case. Many evergreens have tough, waxy leaves and less watery sap that allow them to withstand extreme winter cold. The chlorophyll remains visible in such trees and other pigments are seldom expressed.

Many questions about leaf color remain unanswered, and scientists may never be able to predict exactly when and where fall colors will be at their most intense. Nonetheless, each year people will venture out to admire Mother Nature's fall spectacular, a pastime that will always bring its own rewards.

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