Outwardly, trees present such a stalwart, stately appearance that we often overlook the constant reactions they make to changes in their environment. We take them for granted, in their mute majesty, never suspecting the complexity of the processes taking place in underground root systems, beneath outer layers of bark, using mechanisms buried as deep as heartwood or as exposed as the surface of a leaf. We who live in the eastern United States are fortunate to experience an annual show of fall color put on by our extensive forests of leaf-dropping deciduous trees-though the display is not for our benefit, but is instead a survival mechanism necessary to make it through the cold winter months.

In grade school we all learned that chlorophyll gives leaves their green color. But go a little further, and it soon becomes evident that this green-pigmented substance drives a truly remarkable chemical process-the engine of photosynthesis which converts sunlight into chemical energy. Disc-like structures called chloroplasts absorb light, supplying the energy to turn carbon dioxide and water into oxygen and carbohydrates like sugars and starch. Since chlorophyll takes in red and blue light from the sun, the light is reflected as the color green.

During the spring and summer, leaves serve as food factories, manufacturing all the energy needed for growth, flowering, and producing seed. But to continue synthesizing sunlight, chlorophyll needs long days and warm temperatures. Starting on the summer solstice on June 21st, the days steadily grow shorter, until the autumn equinox on September 22nd, when day and night are roughly equal in length, becoming ever shorter until the winter solstice on December 21st. Deciduous hardwood trees react to these shortening days with a mechanism called photoperiodism which regulates their processes in preparation for winter. This regulation factor, along with cooling temperatures, triggers leaves to cease their food-making process.

As a result, chlorophyll starts to break down and the green color begins to disappear, revealing other pigments hidden during the flush of spring and summer. Carotene (which gives carrots their color) presents itself as orange and yellow. This compound, which is always present in leaves, serves as a sort of accessory to chlorophyll, absorbing sunlight and transferring it to the photosynthetic process. Since it is more stable than chlorophyll, it remains as the green goes away.

Other chemical changes are occurring as well. Red pigments called anthocyanins develop, not as part of the cell membrane like chlorophyll and carotene, but dissolved in the trees’ sap. Anthocyanins are pH-sensitive, so the more acidic the sap, the redder it appears; sap that is less acidic gives off a more purplish color.

Additionally, a cork-like membrane forms at the juncture of the leaf stem and branch, effectively cutting off the flow of nutrients, including chlorophyll. This process, called senescence, causes the leaf’s death, but serves to prepare the tree for its winter dormancy. When the leaf drops, whether through gravity or weather, it leaves a clean, distinctive scar.

The stage is now set for the infinite palette of fall. The varying amount of carotene and anthocyanin present in the leaf determines what color it will turn. Each one of the billions of trees in the Eastern forest is different, but species tend to follow similar color patterns. Some, such as birches, tulip poplars, redbuds, and hickories, are so rich in carotene they invariably turn yellow. Some trees are usually red, scarlet, or purple, including sugar maples, red maples, dogwoods, red oaks, black gums, sassafras, and sourwoods. However, some of these trees, especially maples, can exhibit yellow to orange colors when conditions are right.

Conditions such as temperature and moisture also determine the range and intensity of fall color. A common misconception is that a hard freeze or series of cold frosts causes trees to change color. The best colors actually come after warm sunny days and cool-but not below freezing-nights. Low temperatures promote anthocyanins, as does bright sunshine. Dry weather encourages sugars, which also increase anthocyanins. Droughts can produce brilliant falls, while warm, wet weather in late fall can put a damper on color intensity. On the other hand, a warm, dry summer followed by gentle autumn rains can cause leaves to stay on the trees longer. Gusty wind and rain in late fall can cause millions of leaves to drop almost overnight.

Because of a fortuitous combination of climate and geography, eastern North America enjoys some of the best fall leaf color in the world. And while New England falls are justly famous, the color show, while very intense-in part because of huge stands of single species-is relatively brief. In the southern Appalachians, we are blessed with a longer season and greater variation of colors.

Fall’s brief flare-up of red, orange, and yellow is nature’s last hurrah before the duns, browns, and grays of winter. But even falling leaves serve a purpose. Rich in elements such as calcium and potassium, they cover the forest floor by the billions, returning these same elements to the earth as they decompose. Decaying leaves form a rich layer of water-absorbing humus, which in turn supports the trees, shrubs, and other plants that make up our woodlands. It’s part of a great cycle of life and death that happens each year, in suburban streets and in the deepest forest, and it’s entirely pain-free. If you’re going to go, you might as well go out in a blaze of glory.

Charlottesville’s William Cocke is always turning over rocks and rotten logs to see what’s underneath. As a young Boy Scout, he learned that cedar bark makes good tinder and four-of-a-kind beats a full house. He also remembers the Camp-o-ree where he had to kill a live chicken and cook it for dinner. He was not traumatized in the least and has no plans to sue. He can be reached at wtc4q@virginia.edu.