You probably know that autumn is a time for the trees to change colors and drop their leaves. Now that we’ve had our first cold snap on campus and a nice big windstorm to follow, it really is looking like fall around here. But sometimes we need to stop and think about why these changes are happening and what we can learn from them.
Most everyone knows that the changing of color and dropping leaves are a result of the tree shutting down photosynthesis in preparation for the winter. But there’s more to it than that. Three different pigments create the color of tree’s leaves at any given time: chlorophyll, carotene, and anthocyanin. Chlorophyll is green and specializes in photosynthesis. Carotenoids are yellow or golden and help absorb light energy into the chloroplast, which is the cell that produces the energy. Anthocyanin is responsible for red and purple colors and is produced as a result of high sugar concentration in the sap of the leaves.
When the days begin to get shorter, chlorophyll production drops off. This is because there is not enough sunlight in the day to make the energy loss of that production worthwhile. This explains the loss of green pigment and the increase of yellow pigment, as the carotene remains after the chlorophyll is gone. As the tree continues to shut down, it separates off each leaf from the body of the plant, and in doing so, stops providing new water to the leaves. Since the sap in the leaves is less dilute, the concentration of sugar increases. The amount of sugar in the sap varies between species and even between individual plants, which is quite visible in the variety of colors you can see on a beautiful autumn day in the Arb! Every tree is unique, giving fall its gorgeous variety of colors and hues.