Autumn Colors

by Stan Tekiela
© NatureSmart
October 12, 2007

Who doesn’t love it when the trees turn from their uniform green to the many shades of autumn? I know I do. The only thing that bothers me is how short the autumn colors can be. One good windy day or a heavy rain storm and all the blaze and glory are gone. And you’ll have to wait another year before it’s back again.

Leaves are green because of a pigment known as chlorophyll. When the chlorophyll is healthy and happy it dominates any other pigment found in the leaf. But chlorophyll doesn’t just give leaves their color; they are vital in the life of the tree. Chlorophyll cells capture some of the sun’s energy and combines it with water that is taken up by the tree roots and carbon dioxide which it takes from the air, to produce the trees food—simple sugars. The bi-produce is oxygen.

During summer, chlorophyll is constantly breaking down and is replaced by the tree so the leaves remain green all season. But as autumn approaches the tree reacts to the reduce daylight and starts to get ready for winter by shutting down the flow to the leaves. In addition, the leaves are starting to wear out from a season of hard work producing food for the plant so the tree is ready to shed the old leaves.

At the base of the leaf stalk (where the leaf attaches to the tree) a thin layer called an abscission layer starts to close off, reducing the flow to the leaves. As this happens the food manufacturing chlorophyll start to die off making it no longer the dominant compound in the leaves. Other pigments such as carotenoids, which give off the color yellow, brown, orange and all the shades in between start to show.

Another group of pigment cells called anthocyanins, which are not found in the leaf during summer start to develop. These pigments, common in maple trees, give us the reds and purples of autumn. Unlike the carotenoids, the anthocyanins develop in late summer in the sap and have a complex reaction inside the leaf when in the presents of bright sunlight and a chemical phosphate.

During summer, phosphates breaks down the sugars manufactured by the chlorophyll, but in autumn the amount of phosphates decrease and start to move out of the leaves and into the tree. When this happens the sugar breakdown decreases. The brighter the light during this period the greater the production of anthocyanins and the brighter red the leaves become.

If the tree is under drought stress or has poor health from a fungal or viral infections the colors won’t be as bright. Also if the autumn is dark and cloudy the brightest colors won’t develop. When conditions of temperatures, moisture and sunlight are all in the right amounts we will have a bright and colorful autumn.

Soon the abscission layer will completely close off and the leaf will die and fall from the tree. Since leaves are expendable it really isn’t a big deal for the trees to drop their leaves. Besides the break down of the leaves on the forest floor feeds nutrients back into the soil where the trees roots can reabsorb them and help to produce the new leaves the following spring.

Think of all these chemical reactions that are taking place before your eyes the next time you stop and admire the blaze of autumn. Until next time...

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