From Green to Gold: What’s Behind Our Fall Foliage?
By Robert Windham
As our days get shorter and nights get cooler throughout October in the Tennessee River Gorge, forests lining the mountain tops, ridgelines, and valley bottoms begin to shine with their magnificent fall foliage. Not only is it breathtaking to see an ocean of oranges, yellows, reds, and browns blanket our once entirely green forests, but the science behind why this change in color occurs is also just as intriguing! Before we delve into this color change, let’s explore what makes leaves green in the first place.
Plants making their own food and producing the oxygen we breathe through the process of photosynthesis need an especially important pigment called chlorophyll to do so. Chlorophyll’s job in a plant cell is to absorb sunlight, however, they also reflect green wavelengths of light, giving them their green appearance. The enormous amount of chlorophyll in a single plant cell and the tens of millions of plant cells present in a single leaf give leaves their green coloration.
Photo Credit: Peyman Ebrahimi
While chlorophyll is the predominant pigment in leaves during spring and summer months, carotenoid pigments are also present and responsible for the yellow-orange coloration of things like carrots or persimmons. In response to a decrease in daylight length and cooling temperatures, leaves slow down their food making processes in the fall and chlorophyll eventually breaks down. Once the chlorophyll is gone, the yellow to orange colors from remaining carotenoids shine through, as evident in the bright fall yellows of tulip poplars, sycamores, and hickory trees.
Similar to carotenoids, tannins are another compound always present in tree leaves that only express their brown hues once most the chlorophyll is gone. Tannins protect plants from infection by bacteria or fungi and also cause the bitter flavor of unripened fruit. The copper to golden leaf tones of species like beech and oak can thank tannins for their splendor.
Another group of pigments, anthocyanins, are responsible for the stunning pinks, reds, and purples and come about from a chemical reaction in the leaves themselves. As the flow of nutrients in and out of a leaf slows down and more chlorophyll degrades, sugars can build up in the leaf and react with proteins to form these anthocyanins, the same pigments giving coloration to fruits like apples and grapes. Brilliant bright reds and purples seen in sourwoods, dogwoods, and red maples can be attributed to this sugar driven reaction!
You can already see our fall colors expanding through the Tennessee River Gorge by scanning the sides of Signal, Aetna, Raccoon, or Elder Mountain. Horizontal bands of yellows, oranges, and reds along the sides of these mountains are evidence that even the slightest change in altitudinal temperature can have an effect on the breakdown of chlorophyll. Our fall colors are predicted to peak around October 28th, so we hope you make time to get out, crunch some leaves along the way, and enjoy the beautiful autumn palette. Check out our Scenic Overlooks blog for some striking spots to view fall colors in the gorge. For more information regarding fall foliage throughout Tennessee, visit this TN state parks blog or view an interactive fall foliage prediction map on Smoky Mountains website. For more detailed information on leaf color change, check out The Chemistry Behind the Color by the U.S. Forest Service.
Photo Credit: TN State Parks