Why Do Leaves Change Color in the Fall?

Why Do Leaves Change Color in the Fall?

Already the leaves are starting to change color. The red maples catch my eye first, especially those growing along streams and wetlands, with their bright red leaves shining in the afternoon sun like a warning flag – winter is coming. Soon, each day will start with cooler temperatures lingering longer into the morning, and afternoons will cool as the sun sets earlier in the western sky.

Trees represent so many different emotions. They stand, always in one place, silent, solitary eternal witnesses to our daily lives. Like old friends we recognize our trees from a distance. In our yards and neighborhoods, they are familiar faces, reliable, steadfast and comforting. And yet, it is in autumn, as the uniformity of summer’s green gives way to multi-colored foliage splashed across the landscape, that the distinctiveness of each tree species is revealed.

Countless writers have described the many colors of leaves in autumn. One of my favorite nature writers is Hal Borland. Here is his description of fall foliage from his book “A Country Man’s Woods” published in 1983.

“There is so much color and such vivid color, that it overwhelms the adjectives. New Englanders simply refer to it as “the color.” The sugar maples turn yellow, sunshine yellow, with now and then a tinge of pink. The swamp (red) maples turn red, almost all shades of red, so that the valleys are like carmine rivers. The birches and aspens turn golden and coppery. The ashes turn tan – at least the black ashes do. But the white ashes go through that incredible series of colors that range from yellowish-green to blue-tan to greenish-blue, honest-to-God purple, and then to bronze to tan, and to rust.”

An interesting book, found at a used book sale, is “An Almanac For Moderns” by Donald Culross Peattie, published in 1935. Here is his entry for Oct. 2.

“Sometimes, in midsummer, we see a single leaf that has turned yellow or red. A tree, most often a hickory, in a time of August or drought, will suddenly color while all its kind stand green. Here is the clew to autumn forest hues. They are not due to frost, as it would seem. The red and gold, the orange and purple, are but the running together of many chemicals in the leaf, held apart when the foliate is in full vigor. With the first retreat of vitality, the withdrawal of sap into the deep cellars of the roost, the raw chemicals, useless now, are spilled together by the tree as it prepares for hibernation. The maple’s orange and the tulip tree’s gold, those are but waste products.”

Culross Peattie, in his poetic description of fall foliage, hints at something here with his reference to many chemicals in the leaves. I found an excellent description of the science and biology of fall foliage on the American Forests website with an article written by Dylan Stuntz.

“To understand why leaves are the color they are, you first need to become familiar with the inside of a leaf. Leaves get their green color from a chemical called chlorophyll, which helps the tree take in sunlight. The tree uses the sunlight in a process called photosynthesis, which is how the tree eats, so to speak. It uses the sunlight to break down carbon dioxide (CO2) and water (H20) it absorbs, turning the CO2 and H20 into oxygen, which gets expelled, and glucose, which the tree consumes for energy.”

“If you imagine a tree as a factory, then the leaves are seasonal workers. They do their job when resources are coming into the factory (sunlight, water, carbon dioxide), but when resources stop coming in, there’s not much for the workers to do, so the tree sends them a pink slip. Leaves require energy from the tree, so like any good factory, the tree engages in a cost-benefit analysis. When the days become shorter, the tree no longer wants to waste energy on leaves. This starts the internal chemical process that creates fall foliage.”

“The change in leaf coloration is dependent on the amount of sunlight that the tree takes in. As the seasons change, the days get shorter, and the nights get longer. Eventually, when the nights reach a certain length, chemical processes in the tree will start to block off the connection between the tree and the individual leaves, by creating a corky layer of cells known as the abscission layer. This layer is to protect the branch when it inevitably becomes exposed to the open air once the leaf has fallen. The abscission layer protects the tree, but it also disrupts the flow of nutrients and chemicals that move from the branch to the leaf and back. Chlorophyll breaks down when exposed to sunlight, so as a result it needs to be constantly replaced. The abscission layer interrupts this renewal process, so as a result once the chlorophyll starts to fade, other colors start to emerge.”

“Two chemicals are responsible for the fall coloration of leaves. Carotenoids create orange and yellow pigments, and anthocyanins create shades of red and purple. The carotenoids are present in the leaf all summer long, but they’re masked by the green of the chlorophyll. As soon as the chlorophyll renewal is halted, the green begins to fade, and the vibrant fall colors appear. The second chemical, anthocyanin, forms as a result of the glucose formed by the remaining, faded chlorophyll. The glucose then becomes trapped in the leaf by the abscission layer, resulting in the formation of anthocyanin.”

“The colors of a particular tree are a result of the carotenoids and the anthocyanins reacting to each other in different amounts, in combination with any chlorophyll left. The formation of these chemicals and the amount of each of them are dependent on temperature, moisture and sunlight, so every foliage season is unique, because every season the chemical balance found inside the leaf changes.”

The full article can be found at: https://www.americanforests.org/blog/science-behind-fall-foliage/

What will the coming foliage season bring to us here in The Last Green Valley National Heritage Corridor? Will the lack of rain this past spring when the leaves were first emerging bring dull colors, or will the seemingly constant rains of summer create a brilliant autumn? How will the “carotenoids and anthocyanins” react to each other in next few weeks? We’ll have to wait and see.

We live in a beautiful place called The Last Green Valley. Each season is a gift to be enjoyed, each day is a new beginning, and each of us has the opportunity to care for it, enjoy it and pass it on.

Bill Reid is the Chief Ranger of The Last Green Valley National Heritage Corridor. He can be reached at bill@tlgv.org

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