Autumn is welcomed by many people. It marks the end of hot summer days and it is the time of year when tree leaves begin to show their beautiful colors. One of the biggest tourist attractions in New England is the fall leaf colors. The nice thing about living in East Texas is that vivid fall colors can be enjoyed without traveling half way across the US. A trip to the pine and hardwood forests of East Texas during October and November is sure to delight the eye. But why do the tree leaves develop the beautiful red, orange, and yellow colors in the fall of the year?
During most of the growing season, plants (including trees) produce a material called chlorophyll. Chlorophyll enables plants to convert water and carbon dioxide in the presence of sunlight into carbohydrates, which plants use for food. This process is called photosynthesis. Chlorophyll is what gives plants their green color. Tree leaves have other colors present most of the time, but the green chlorophyll is so dominant that it masks the other colors. Near the end of the growing season (autumn), chlorophyll production is reduced or stops and chlorophyll that is present in the leaves begins to break down. When this happens the yellow and orange pigments, called carotenes and xanthophylls, are unmasked and their colors become visible. Red, pink, and purple pigments may also develop.
Chlorophyll formation is sensitive to any factor that disturbs metabolic processes in the plant. Unfavorable levels of light, temperature, water, oxygen, and essential minerals will have an impact on how much chlorophyll a plant will produce. For instance, when a plant is lacking in nitrogen or iron (mineral deficiencies), or is grown in little or no light, it will develop a yellow or pale green color. It is said to be chlorotic, or lacking in chlorophyll. Water and heat stress (hot, dry weather) and cool temperatures (autumn) will destroy chlorophyll and allow carotenes and xanthophylls to be exposed. In most plants there is about three times as much chlorophyll as carotenes and xanthophylls.
Sometimes a genetic mutation in the plant will cause leaves to be variegated (part yellow and part green). The part of the leaf that is yellow lacks chlorophyll. Occasionally a mutation will occur where no chlorophyll is produced and a short-lived albino plant results. Occasionally bud mutations occur resulting in albino or variegated branches in otherwise normal (green) trees and shrubs. These color variations are not associated with fall leaf colors, but involve green and yellow pigments in the leaves.
The discussion above explains why green and yellow colors are seen in the leaves of trees. A pigment called anthocyanin is responsible for the red, pink, and purple colors in leaves that are so attractive. This pigment forms in the sap of leaf cells, usually late in the growing season (autumn). Its formation is dependent on an accumulation of carbohydrates in the plant, on the genetic properties of the tree, and environmental factors. The most important environmental factors controlling autumn leaf coloration are temperature, light, and water. Cool temperatures (above freezing), drought, and bright sunny days favor the production of anthocyanin. Because they are exposed to the most sunlight, leaves in the upper crown usually develop the brightest red colors. On the other hand, several days of rainy or cloudy weather often decrease the intensity of fall colors. The bright red and orange fall colors for which maple trees are famous are due to the production of anthocyanin in the leaves. Also, sassafras, sweetgum, and sumac produce large amounts of anthocyanin, which accounts for their deep red colors. Trees of the same species growing close together often show much color variation. This is due to the tree’s response to environmental factors and different mixtures of chlorophyll, carotenes, xanthophylls, and anthocyanin pigments that produce the red, orange, yellow, crimson, purple, and related colors in the leaves.
Some trees do not form anthocyanin pigments. When the amount of chlorophyll present in leaves begins to decline in the fall, the yellow carotenes and orange xanthophylls become visible. Hickory and elm leaves are an example of this since they often display bright yellow autumn colors. Oak leaves often turn brown with very little yellow or orange coloration. This is because they contain large amounts of tannin (brown color) and relatively few carotenes.
In summary, declining autumn temperatures result in reduced chlorophyll formation and the disintegration of chlorophyll already present. In other words, the green color begins to fade. At the same time stored carbohydrates aid in the production of anthocyanin (red, pink, and purple colors). As the levels of chlorophyll decline, the yellow carotenes and orange xanthophylls along with anthocyanin become visible. In short, the best autumn colors occur under conditions of clear, dry, and cool (but not freezing) weather.
Adapted from: Kramer, P. J. and Kozlowski, T. T. 1979. Physiology of Woody Plants. Academic Press. New York, NY. 811 pp.