This autumn has been dry and warm so it’s not a great combo for viewing autumn colors. I am sure with more moisture and cooler nighttime temperatures the display would be so much more than what we have now. So why do these leaves change color and put on such an eye-popping show?

First of all, leaves are green because of a green pigment known as chlorophyll that is responsible for the absorption of sunlight to provide energy for photosynthesis. When the chlorophyll is healthy and happy it dominates any other pigment found in the leaf and we see green leaves. But chlorophyll doesn’t just give leaves their color; they are vital in the life of the tree. Chlorophyll cells capture 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 byproduct is oxygen and of course we breathe oxygen. So we are the direct beneficiaries of what the trees do naturally.

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 trees react to the reduced daylight and start to get ready for winter by shutting down the flow of nutrients 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 and from the leaves. As this happens, the food manufacturing chlorophyll starts to die off making it no longer the dominant compound in the leaves. Other pigments such as carotenoids give off a yellow or orange color, and all the shades in between start to become visible.

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 break 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, like we are having in my area this year, or has poor health from a fungal or viral infections the colors won’t be as bright. Also, if autumn is dark and cloudy or dry and warm 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 breakdown of the leaves on the forest floor feeds nutrients back into the soil where the tree’s roots can reabsorb them and help to produce the new leaves the following spring.

While in my area we may be experiencing a drought, and the autumn leaves may not look their absolute best, it is still enjoyable to see the fall colors and understanding exactly why it happens makes it even better for me and I hope it helps you also. Until next time…

Stan Tekiela is an author / naturalist and wildlife photographer who travels the U.S. to study and capture images of wildlife. He can be followed on Instagram, Facebook, or Twitter. He can be contacted via his website at www.NatureSmart.com.