by Whit Gibbons

September 27, 2015

When should you make that trip to see this autumn's leaf colors? What determines the intensity of color, and how do you predict the best time to go? Why do colors vary so much from year to year and place to place? Despite what you may read about the best time to view fall colors, no one knows for sure. Botanists are a long way from predicting accurately the timing and regional occurrence of fall colors in a given year or how vibrant the colors will be.

We all 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 and where fall colors will be the most spectacular or whether colorful 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, and chlorophyll subsides, additional pigments become apparent: carotenoid results mainly in yellow and orange; anthocyanin 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, botanist Linda Lee of the Savannah River Ecology Laboratory 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. 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. 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.

The biological purpose, if any, of the colors produced by leaf pigments remains unresolved, and many questions about leaf color remain unanswered. This doesn't prevent us from marveling at the vivid display of trees during autumn, the season of spectacular color. The more we appreciate the complexities of nature, from all perspectives, the more likely we are to want to preserve and maintain the delicate networks that make up our natural world.

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