Once again we are approaching the time of year when many people anticipate the showcase of autumn colors that our forests provide. Observing this annual phenomenon is simple, understanding it is a little more complicated. Scientists still have more answers to find, but what they do know about the annual cycle of deciduous trees is quite fascinating.
Why and how do certain trees become so colorful each fall? The driving scientific factors behind it all are biology, chemistry and meteorology. Leaves appear green due to the chemical pigment of chlorophyll that allows sunlight to be absorbed for the production of sugar and starches. When daylight hours decline, it signals the trees to halt the production of chlorophyll. When this happens, other chemical pigments are revealed which have been in the leaf all season but were masked by the dominant chlorophyll.
When you think about it, fall is more about the loss of color than a changing of color. Newly visible carotenes provide orange coloration as they do in carrots and pumpkins. Another pigment group, the yellow xanthophylls, is also the color source in sunflowers, corn and egg yolks. As autumn advances, the leaves form a wall of cells at the point where the leaf stem joins the branch. This blocks fluids from flowing in and out of the leaf and traps sugars in the leaf. This stimulates the production of anthocyanins, which are red in color and are also found in the skins of many fruits. The browns that appear, like those you see in tea, are tannins that are also produced mainly in the fall.
The ideal conditions leading up to the best fall color is a warm, wet spring, typical summer conditions, and sunny, mild autumn days with cool evenings above 32º. Cooler temperatures tend to slow down the transfer of sugars out of the leaves and increase the production of anthocyanins. A hard frost, however, can kill off the leaves before the gradual color enhancement takes place. Above normal temperatures in the fall for an extended period may lead to less intense colors and the foliage may remain on the trees one or two weeks longer. The severe drought which we have experienced may cause some leaves to turn brown and fall off early, especially on young trees or trees with additional distress. The pattern of weather conditions during the year, combining with the biological and chemical characteristics of each species of tree, produces a unique color display each time the fall season arrives. Whether the colors are vibrant or more subdued, brief or long lasting, we look forward to whatever the forest serves up for our viewing pleasure.