Every autumn we find ourselves in the beauty of a variety of colors. A mixture of orange, red, yellow, and purple appears in the trees as the seasons change from summer to winter. That is when we all enjoy the colors of the autumn leaves. However, have you ever wondered why and how an autumn leaf changes color? Where do the yellow and orange colors of the leaves come from? Why do maple or acer leaves turn bright red while the leaves of other trees turn yellow? What is behind this wonderful, artistic, and delightful color transformation in autumn?
In order to answer these questions, let’s first take a closer look at the nature of leaves and how they function. Simply put, leaves are nature’s food factories. During spring and summer the leaves serve as factories where most of the food necessary for tree growth is manufactured. The process of transforming water and carbon dioxide into sugar is called photosynthesis, which means literally “putting together with light” (Figure 1). This food-making process is performed by the chlorophyll molecules that are present in the leaf cells. Chlorophyll absorbs energy from sunlight. While the water is sucked in from the soil by the roots and carbon dioxide is inhaled through the pores of the leaves, chlorophyll synthesizes carbohydrates, such as sugars and starch. During winter, when this process cannot continue due to lack of light or water, the trees rest and live off the food they stored during the summer.
What is most astonishing is that it is again chlorophyll that gives the leaves their green color. Along with the chlorophylls (green pigment), spectrums of pigments, such as carotenoid, anthocyanin, and xanthophyll, give different colors to the leaves (Figure 2). When chlorophyll is abundant in the leaf cells, as it is during the growing season, the green color of chlorophyll dominates and masks the colors of any other coloring pigments that may be present in the leaf. Thus, the leaves of summer are characteristically green. When for some reason the number of chlorophylls decreases significantly, the color of other pigments paint the leaves, such as in the fall. For example, maple and acer leaves turn to bright red because of the carotenoid and anthocyanin pigments while sugar maple leaves turn yellow due to xanthophyll pigments.
At the end of summer and the beginning of autumn, the length of days and the average temperature begin to decrease. Then the trees “know” that it is time to get ready for winter, their sleep-time, and the leaves stop making food. Since there is no longer a need for them, the chlorophyll molecules break down, and so the green color begins to fade. As we, the grieving viewers of this process, say farewell to the color of life, we are surprised by the splendor demonstrated by vibrant colors ranging from yellow to orange.
All these colors are due to the mixing of varying amounts of the chlorophyll residue and other pigments in the leaf becoming visible during the fall season. For the realization of this beauty, mixtures of pigments give rise to the reddish and purplish fall colors of trees such as dogwoods and sumacs, while others offer the sugar maple its brilliant orange or yellow. In some trees, like the maple, a red pigment will be formed in the fall if the days are warm and the nights cold. These trees produce sugar in the leaves during the day, but this sugar cannot move out when the nights are cold, and the leaf’s connections to the tree begin to break down. After that the high sugar concentration favors the formation of a class of pigments called anthocyanin, which is red. Thus, the leaves left out in the sunlight turn red through this process. The brown color of trees (like oaks) that appears after chlorophyll breaks down comes from plant wastes left in the leaves. The autumn foliage of some trees is only yellow, so it is the combination of all these things that makes the beautiful colors we enjoy in the fall.
Thanks to scientific research, today we know the details of leaf color change. The approximate size of these pigments is so small that hundreds of millions of them, put edge to edge, could measure only 1 meter. Isn’t it amazing how the beauty of autumn is exhibited through the hands of such small and blind painters?
Abdullah Akpinar is a graduate student in Planning and Landscape Architecture at Clemson University, Southern Carolina.