Fall in Chapel Hill
Chapel Hill is blooming with every color imaginable- purple, red, orange, pink, yellow, burgundy, mauve, gold, and every single color in between. However, it isn’t flowers painting the town, but trees! Chapel Hill is definitely known for its fall and it is one of the things that I love most about this town. Spring has always been my favorite time of the year, but fall in Chapel Hill is the one exception. With such a plentiful diversity of trees, when autumn arrives it brings with it a breathtaking array of colors that cascade through town over the course of several weeks. Since it is my last fall in Chapel Hill, I decided to study the progression of fall and how and why trees change color in the fall.
The tree that really got me interested in this project was the sugar maple. It was one sugar maple in particular that I passed on my way to class that, for whatever reason, began to change colors quite early in the season. The tree was a fairly small one and mostly in the shadow cast by an adjacent building so it didn’t get much sunlight except for the top of the tree. I noticed that the leaves began turning a bright, golden color at the top, on the side of the tree facing away from the building into the sun. The color spread slowly down and around the tree from the starting point and simultaneously soon faded from the gold to an extremely intense crimson color that dominated the tree. The completely red tree is very eye-catching and it sparked my interest as to why trees even change colors.
My initial thought observing the sugar maple was that sunlight had an effect on when the leaves change color since the first leaves to change colors were at the very top of the tree in direct sunlight. After some research, I found that there are two main pigments for producing fall colors in leaves, carotenoids and anthocyanin. The carotenoids produce a yellow and orange color in the leaves and are present all year round. These pigments being to appear in the fall because in response to longer nights and colder temperatures, the trees begin to slow down chlorophyll production until it ceases and then the dominating green color fades to reveal the carotenoids (Dawson).
Anthocyanin is responsible for producing red and purple, but it is produced in the late summer. Throughout the year, the tree produces sugars within the leaves through a light dependent chemical reaction. As fall begins and the tree starts to withdraw resources and chemicals from its leaves, this reaction changes due to a lack of nutrients and anthocyanin is produced as a result. Since the production of sugar is affected by sunlight, the brighter the light, the greater the amount of anthocyanin produced and the more vivid the red color. Therefore, the leaves that receive more direct sunlight will turn a brighter red and will do so earlier (Dawson).
This explains the phenomena that I observed in which the leaves that began to change colors first were located where the sunlight was most direct. I also noted that the color change on maples could start at a spot on the crown of the tree or a certain side and then gradually spread throughout the tree or the tops of distinct layers throughout the tree would begin to change and then the color would descend through the layers. Either method corresponds with the idea that the area of the tree that receives the most sunlight changes first, whether it is a certain area of the tree or simply all the upper leaves within the layers.
While this explains how the trees change color, the definite reason as to why the trees change color is not known. There have been several theories formulated as to why leaves change colors in the fall. This first theory, which was prevalent for some time, was that the leaf was simply a biological wastebasket. Most temperate deciduous trees shed their leaves in the fall because the cost of maintaining respiration during the winter is too high (Dawson). They boost their survival by withdrawing all the nutrients, including chlorophyll, from the leaf and dropping the desiccated shell. The major component of chlorophyll is nitrogen and as a tree has limited access to nitrogen, it must be able to remove it before the leaf falls. This suggests that the leaf is stripped of all its nutrients and dropped only for survival and any color that occurs is a side effect of the withdrawal of chlorophyll and the consequent lack of green color in the leaf. While this theory is consistent with the pigment carotenoid since it is already present in the leaf and only become visible when the chlorophyll is degraded, it doesn’t make much sense for the tree to produce anthocyanin in the fall.
William Hamilton provided an alternative, the warning hypothesis. He proposed the idea that trees change color in the fall to serve as a warning to insects and other creatures to stay away, very similar in fashion to the warning colors of animals. It was hypothesized that since more vigorous trees could produce more toxins, they would also produce more pigment molecules resulting in more vibrant colors. This is supported by the fact that many toxins and pigment molecules are made in a similar fashion, so a tree that could make one would easily be able to make the other. Since insects cannot see toxins, but they can see pigments it makes sense that the two could become correlated to signal to insects that the tree is dangerous. Changing colors also suggests that since the tree is reproductively fit enough to stop photosynthesis early and produce vibrant colors instead, it must be healthy and fit enough to produce toxins and thus able to fight off predators (Zimmer). Schaefer and Rolshausen tested the validity of this theory and disproved it in two separate experiments. They first performed a paint test where they took a stand of trees and painted half the leaves red and half green, but it made no difference in which trees aphids attacked. They performed a second experiment in which they found that the aphids heavily attacked the trees that produced the most seeds, but again found no correlation with color (Zimmer).
It is possible that anthocyanins aid in protecting the leaves from ultraviolent rays. Normally chlorophyll absorbs all the UV radiation that a leaf is exposed to, but as fall approaches and the leaf breaks down the chlorophyll and reabsorb the nutrients, the leaf becomes much more sensitive to UV damage. This could possible destroy the chlorophyll before it’s nutrients can be reabsorbed. Since anthocyanins can absorb UV light and act as antioxidants to prevent further light damage, it is plausible that they are produced to protect the leaf as it breaks down chlorophyll and withdraws nutrients (Zimmer). A recent study showed that yellow leaves dominate in soil rich in nitrate and red leaves dominate in nitrate poor soil. Poorer soils overall also made for redder hues across the board (Lovett). When I considered that a major component of chlorophyll is nitrogen and that trees with less nitrogen are redder, I hypothesized that the anthocyanins could be produced to protect the leaf from UV rays during the fall so that more of the chlorophyll can be reabsorbed before it is destroyed, especially so in trees deficient in nitrogen, a main component of chlorophyll. It has actually been shown that in the sugar maple species Acer Saccharum that trees with lower nitrogen levels have more anthocyanins and they will produce them sooner in the fall as well. The presence of anthocyanin has also been proved to slow the process that drops the leaf, hence giving the tree more time to withdraw nutrients. (Cambridge). Further supporting this hypothesis, trees that have symbiotic nitrogen-fixing bacteria present have no need to reabsorb leaf nitrogen and as a result, they drop their leaves while they are still green (Cambridge). Overall, all of this evidence points towards the hypothesis that anthocyanins are produced to aid in UV protection.
The UV protection hypothesis supports the observations I made throughout the fall that the leaves to turn red first were the leaves with the most sunlight exposure. However, this was only with certain species of trees, most notably the maples. I actually noticed that there were a variety of different patterns of color change for different trees. The maples would seem to change colors based upon the amount of sunlight the tree received, so the tops and outer sides would always turn first. The beeches seemed to follow the same pattern at first, with the outer and upper leaves turning yellow before the lower, inside leaves. However, as fall progressed the entire beech tree would eventually take on a uniform color. The oak trees seem to change color almost in clumps of leaves. It would appear that certain branches, evenly dispersed throughout the tree, would change colors independently, leading to a general, gradual change in color. The eastern redbuds, one of the earliest trees to start changing colors, showed a more even distribution of color change. The leaves on an individual all began to turn yellow around the same time and did not seem influenced by any physical factor.
The ginko tree was particularly difficult to identify a pattern of color change. Rather, it seemed that entire branches would randomly change to the brightest yellow color. There seemed to be no reason or rhyme as to which branches changed first either. However, none of the leaves would drop after they changed colors, but rather they would stay on the tree until a cold snap, when all the trees would drop their leaves at once. This is how every ginko tree loses its leaves resulting in spectacular halos of yellow leaves around all the ginkos one morning. My absolute favorite tree to observe the color change in this fall was the Crape Myrtle. Starting off green, the tree would start to turn yellow at the very bottom, gradually changing into orange further up, ending in a brilliant red at the very tops of the branches. The rounded shape of the tree in conjunction with the wispy appearance of the top branches, made it appear as if the tree was a flame. The colors were so vibrant and flowed so effortlessly that it truly appeared as if someone had set all of the trees on fire. As fall progressed, the red would gradually dominate the tree until the leaves dropped.
All of the different species began changing colors at different points in the fall and they all had very distinctive colors. The very first trees I noticed changing colors in the fall were the tulip poplars, which turned a pale golden-yellow. The eastern redbuds followed soon after, turning a more fluorescent yellow. Next, the maples began to change to yellow, orange and red, although the individual trees all followed relatively distinct timelines. The maples would continue to change and drop leaves for the rest of the fall, seeming to have a much longer period of color change that most of the other species. The crape myrtle’s started changing after the maples, although once their color change began they didn’t retain their leaves for very long. The oaks on the other hand, which started to change colors soon after the crape myrtles, did so very slowly and lost their leaves at an even slower pace. The ginkos started to turn bright golden relatively late into the fall and didn’t drop any leaves until the end of November, when a cold snap caused all of the leaves on all individuals to drop at once. The beech tree retained its green color for a very long time and was the last species to begin changing color as far as my observations. By December, the majority of the trees sport dead branches, except for the oaks and beeches, whose branches still don their dead leaves. With a relatively long fall and with such a plentitude of tree species that all gradually change into a wide range of colors at different times, it is no wonder that Chapel Hill is known for its fall.
Class of 2014
Hometown- Fayetteville, North Carolina
- Neufeld, Howie. "Department of Biology." Fall Color Report. Appalachian State University, n.d. Web. <http://biology.appstate.edu/fall-colors>.
- Zimmer, Carl. "The Semiotics of a Leaf : The Loom." The Loom. Discover, 30 Oct. 2005. Web. <http://blogs.discovermagazine.com/loom/2005/10/30/the-semiotics-of-a-leaf/>.
- Zimmer, Carl. "Autumn Leaves: The Search for Purpose : The Loom." The Loom. Discover, 17 Oct. 2006. Web. <http://blogs.discovermagazine.com/loom/2006/10/17/autumn-leaves-the-search-for-purpose/>.
- Lovett, Richard A. "Leaves' Fall Colors Have "Dirty" Secret, Study Finds." National Geographic. National Geographic Society, 30 Oct. 2007. Web. <http://news.nationalgeographic.com/news/2007/10/071030-fall-leaves.html>.
- "Map of LifeConvergent Evolution Online." "Autumn Leaf Colouration" : Map of Life. University of Cambridge, 6 May 2009. Web. <http://www.mapoflife.org/topics/topic_410_Autumn-leaf-colouration/>.
- O'Keefe, John, and David Lee. "Autumn Foliage Color." Harvard Forest. Harvard University, 2004. Web. <http://harvardforest.fas.harvard.edu/autumn-foliage-color>.
- Dawson, Jeffrey O. "Why Tree Leaves Turn Color in Autumn." Illinois Forestry. University of Illinois, n.d. Web. <http://web.extension.illinois.edu/forestry/fall_colors.html>.