This summer, I left the verdant landscape of Canaan Valley to embark on an urban escapade in St. Louis. I spent ten weeks working at a newspaper and becoming thoroughly entrenched in city life: biking in the park, barhopping, and eating tons of barbecue. On Sunday nights, I would hear about all the amazing outdoor adventures back home. From paddling to climbing to creek stomping, summer was in full force in West Virginia and I was missing out.
Anxious to take advantage of the last dregs of summer, I started counting down the days until I could return home. But with a late August homecoming, autumn had already arrived. The air was crisp and cool, the humidity was far less palpable, and the trees glimmered with tints of yellow, orange, and red.
This time last year, the days were warm and the trees were green. It seemed like fall would never begin. And when the leaves finally changed, they quickly transitioned from green to rusty yellow and burnt orange. Although it may seem like trees are passive entities, they actively change the color of their leaves in response to external cues. For instance, when a branch of a tree is cut off, the leaves don’t turn yellow or red before turning brown, they just die and fall off. “That’s a good sign that the tree is doing something to make those leaves change color,” says Zach Fowler, West Virginia University Core Arboretum Director.
Winter is Coming
Trees regulate the process of leaf color change in response to daylength and weather conditions that signal the onset of winter. Because cold weather traps water in an icy form, many trees would be stressed if they retained their leaves. “Color change is evidence of the tree beating winter to the punch,” says Fowler.
To prevent this stress, trees break down their leaves to reabsorb nutrients like nitrogen and phosphorous. This process involves several groups of pigments—colorful substances produced in plants that absorb light. Understanding how and why leaves change colors requires a deeper dive into three color groups present in leaves: green, yellow / orange, and red.
Leaves look green in the spring and summer due to a pigment called chlorophyll that helps plants absorb light and convert energy from the sun into food through the process of photosynthesis. In the fall, trees slow down photosynthesis and break apart chlorophyll because it contains large amounts of nitrogen and other nutrients. These nutrients are then transferred into the roots and trunk for winter storage. As chlorophyll is degraded, the green disappears, revealing other autumnal hues.
Yellow and Orange
Yellow and orange pigments called carotenoids are also present in leaves throughout the year. These pigments give carrots, pumpkins, and daffodils their colorful appearance. Carotenoids also protect plants from light damage and function as antioxidants. In the fall, trees don’t break down carotenoids because they are less abundant and nutrient-rich than chlorophyll. As a result, these pigments are left behind, allowing them to take center stage during autumn.
Red pigments, called anthocyanins, are not present in leaves year-round but are instead produced during the color change process. In many plants, red coloration functions as a sunscreen to protect leaves and flowers from sun damage. Anthocyanins give some tree species, such as sourwood and black gum, their characteristic vibrant red color. Other tree species, like beech and yellow birch, produce very few if any of these pigments, resulting in their yellow and orange fall foliage.
From Red to Shed
After trees absorb nutrients from their leaves, they go through another active process—shedding them. Some pigments are trapped in the leaves as they drop, keeping them yellow, orange, and red on the ground. Soil microbes then degrade and recycle nutrients from the leaves, replenishing the soil and providing resources back to the trees. Some trees, including oaks and beeches, don’t go through this process and will keep their dead leaves attached throughout winter.
Cues and Hues
Trees initiate leaf color changes in response to several cues, including daylength and weather. “They don’t have a calendar,” says Fowler, but plants do have fairly well-understood mechanisms for sensing changes in daylength. Once the days get shorter, trees start breaking down chlorophyll. Because changes in daylength are consistent over time and carotenoids are always present, leaves consistently turn yellow and orange every fall.
In contrast, red pigments may or may not appear in the fall because they depend more on weather patterns. The production of red pigments requires sunlight and is enhanced by cold weather. As a result, the most vibrant fall foliage tends to occur when there are bright, sunny days followed by cool, dry nights. Last fall featured rainy days followed by warm, soggy nights. “We didn’t have a very good color change because of that,” says Fowler.
Along with fall weather, summer growing conditions can also affect the intensity of leaf color. Some argue that a good growing season with plenty of water and sunlight produces more vivid fall colors.
Although scientists understand several factors that influence fall foliage, it’s not entirely clear how these cues interact and there might be additional cues that researchers haven’t discovered yet. This complexity may explain why fall foliage varies across years even when changes in daylength remain the same.
With the recent string of warm, sunny days and cool, crisp nights, signs are pointing to stunning colors this fall. For those committed to catching vibrant leaves, check out high-elevation locations first since the winter cues occur earlier in the highlands. There are also several websites dedicated to fall foliage reports. The effort that goes into monitoring leaf color changes emphasizes that fall, as Fowler says, “means something to people too, not just to the trees.”
Nikki Forrester is associate editor of Highland Outdoors and reached out to several trees for comment, but none of them responded. They’re so chlorofull of themselves!
1 thought on “Let the Leaf Drop: The Science of Fall Foliage”
Thanks Nikki for the info. Very well written. I never stopped to think a bout why the leaf change and the different colors. This year the poplar trees lost their leaves in September. Very early for them. Am told it had something to do with the very wet spring. If you have any info on that let me know. Pat parr