This autumn has been dry and warm, which isn’t an ideal combination for viewing fall colors. With more moisture and cooler nighttime temperatures, the display would likely be far more vibrant. So, why do leaves change color and put on such an eye-catching show?
First, leaves appear green because of chlorophyll, a green pigment responsible for absorbing sunlight to fuel photosynthesis. When chlorophyll is healthy and abundant, it masks any other pigments in the leaf, making leaves appear green. But chlorophyll doesn’t just determine leaf color; it’s essential to the tree’s life. Chlorophyll captures the sun’s energy, combines it with water absorbed by the tree’s roots and carbon dioxide from the air, producing the tree’s food – simple sugars. This process releases oxygen, which we breathe, making us direct beneficiaries of the trees’ natural functions.
During summer, chlorophyll is constantly breaking down and is replaced by the tree, keeping the leaves green throughout the season. As autumn approaches, trees respond to reduced daylight by preparing for winter, 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 connects to the tree, a thin layer called the abscission layer begins to form, gradually blocking the flow of nutrients. As this process continues, chlorophyll production decreases, and it is no longer the dominant pigment in the leaves. Other pigments, such as carotenoids, reveal yellow and orange hues, allowing the full spectrum of colors to emerge.
Another group of pigment cells called anthocyanins, which are absent during summer, start to develop in late summer. These pigments, especially common in maple trees, produce the reds and purples of autumn. Unlike carotenoids, anthocyanins form in the sap and result from a complex reaction involving bright sunlight and a chemical called phosphate.
During summer, phosphates help break down the sugars produced by chlorophyll. In autumn, phosphate levels decrease, moving from the leaves into the tree, reducing sugar breakdown. Brighter light during this period promotes greater anthocyanin production, resulting in more vibrant red leaves.
If a tree experiences drought stress, as we’re seeing in my area this year, or suffers from fungal or viral infections, its colors won’t be as vibrant. Likewise, if autumn is dark and cloudy or warm and dry, the brightest colors may not develop. When temperature, moisture, and sunlight conditions align just right, we’re treated to a bright, colorful autumn.
Soon, the abscission layer will fully close off, causing the leaf to die and fall from the tree. Since leaves are expendable, it’s no big deal for trees to shed them. Plus, as leaves break down on the forest floor, they return nutrients to the soil, which the tree’s roots can reabsorb to help produce new leaves in spring.
Though drought conditions in my area may have dulled the autumn leaves’ vibrancy, it’s still a pleasure to see the colors of fall. Understanding why this happens adds to the enjoyment, and I hope it does for you too. Until next time …
Editor’s note: Stan Tekiela’s NatureSmart column appears twice a month in the Eden Prairie Local News. Tekiela is an author, naturalist, and wildlife photographer who travels extensively across the United States to study and capture wildlife images.
You can follow his work on Instagram and Facebook. He can be contacted via his website at www.naturesmart.com.
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