People worldwide enjoy the changing colors of tree leaves in autumn. Not only are the colors spectacular, but they herald the onset of cool crisp weather and fun fall activities. Brilliant yellows, oranges, and reds draw tourists from all over to areas known for fall foliage, such as New England, the NC Mountains, and much of the East Coast. But did you ever wonder WHY the leaves change color?
It turns out that the yellows, oranges, and browns of fall are actually always present in leaves, just masked by green during the spring and summer. The reds are a fall-specific phenomenon. Several pigments come into play in the cycle of leaf color during the year. Chloropyhll, carotenoids, and anthocyanins are the key pigments involved. Chlorophyll reflects light so that we see green; carotenoids (such as beta carotene and xanthophyll) appear in shades of yellow, brown, and orange; and anthocyanins produce the stunning scarlets and purples that can make or break a fall color experience. Carotenoids are always present in tree leaves; chlorophyll is abundantly present during the growing season; and anthocyanins are produced in late summer and autumn in certain trees.
During spring and summer, the leaves on trees appear green because of the presence of large amounts of chlorophyll, the molecule instrumental in photosynthesis that allows plants to make their food. As chlorophyll is used up during the growing season, it is rapidly replaced because the plants are growing and need nutrients. Chlorophyll causes the leaves to appear green and as long as it’s abundant, that’s the dominant color. It masks the always-present carotenoids, hiding the yellows, oranges, and browns until fall.
As the days shorten and the temperatures drop with the approach of autumn, chlorophyll production slows down and the tree begins to slowly close off the veins that carry nutrients into the leaves, in preparation for shedding its leaves for winter. Chlorophyll is no longer replaced at the previous rate and the green color begins to disappear. As the chlorophyll levels dwindle, the yellows, oranges, and browns produced by the carotenoids are unmasked. While the magnificence of fall color may vary year to year, the yellows and oranges are fairly stable. Carotenoids also occur in other familiar plants and animals: carrots, canaries, egg yolks, and buttercups all owe their flashy shades to these pigments. You can look for brilliant yellows and oranges in poplar tree leaves, birches, hickories, and certain species of maples.
The less predictable player in the fall color line up is anthocyanin. This pigment is produced in late summer and fall due to a complex of factors related to the presence of certain minerals and the breakdown of sugars. As the veins in the leaves begin to close off, the change in presence of various minerals affects sugar levels and the production of anthocyanins begins. Bright days and cool (but not freezing) nights will lead to the best production of this pigment and the flashiest reds and purples for fall foliage. However, not all types of trees produce this pigment. Part of the reason New England is famous for its fall color is that a higher percentage of the trees in that region produce these brilliant shades. The color from anthocyanins is familiar to us in cranberries, red apples, cherries, and plums. You can look for red foliage on sourwood trees, dogwoods, and certain species of maples.
As veins close off in the leaves, the plant also begins to form cork cells at the base of the leaf that will eventually close it off entirely from the rest of the plant. During this transition of decreasing chlorophyll levels and slowly closing veins, you may notice some leaves that are a mix of green and other colors…look to see if the green is focused around the veins. Once the cork cell layer is formed, the leaves can fall off and land on the forest floor, where they are recycled into the soil and provide nutrients and moisture retention for future growth. There are various explanations for why deciduous trees lose their leaves, but the traditional one is resource conservation because the sensitive leaf tissue would not withstand the harsh conditions of winter. New research is showing that other factors may also come into play. Only deciduous trees change color and drop their leaves each year. Evergreens such as spruce, pines, and firs do not- their needles are waxy and contain substances resistant to cold temperatures, and they do not shed them yearly.
Temperature, length of day, rainfall, and other factors all play a role in the fall color experienced in a given year and when the trees will drop their leaves. It can be very difficult to predict both the “quality” of fall color and when it will peak. However, you can check out a fun fall color prediction tool on the Smoky Mountains website (https://smokymountains.com/fall-foliage-map/). Scrolling through the timeline on this map, you can see that the peak fall color for our region in 2016 is predicted to occur around the third week of October, though it will begin to peak the week before. Depending on where you’re traveling to, you can also call the U.S. Forest Service’s Fall Color Hotline (1-800-354-4595) for details on the progression of fall color.
Looking for something local to do to enjoy the fall color? Visit any of our parks and trails during the last few weeks of October and you’re sure to see some bright foliage. We are also holding a Fall Festival on October 15th at Cedarock Park with old time games for children, candy, hayrides, and face painting. This will be the week that the color is beginning to peak. If you like arts and crafts, consider joining us earlier that morning at Great Bend Park for pumpkin painting and nature art. While you’re at Great Bend, grab one of our self-guided Tree ID Cards and see if you can match up the different tree species on the trail with their fall colors! For more details about these events, visit our special events page or call (336) 229-2410.
Resources: The information for this article was gathered from several resources, including Wikipedia: Autumn Leaf Color, USDA Forest Service: Why Leaves Change Color, ESF: Why Leaves Change Color, and the Smoky Mountains’ Fall Foliage Prediction Map. Click on any of the links to refer to these resources.