Revealing Fall’s True Colors
By Daniel Manget
It is that time of year again. Time to prepare for the “big sleep” of winter, as with nature, and reflect upon all that has happened throughout the long eventful year. The trees of the Southern Appalachians are popping with golds, oranges, and reds to the delight of any and many an onlooker. Leaf-lookers are flocking to the mountains to witness the vivid color displays. The more curious ones might be asking themselves “how do they create such beautiful colors,” referring to the miraculous process of what scientists call leaf senescence. The extra curious people, who are thinking from more of an ecological and evolutionary standpoint, are asking WHY are they doing that? For something that humans have been witnessing since our migration away from the equator, we know surprisingly little about the reasons behind the much-loved phenomenon of leaf color change.
The entire process starts with somewhat of a mystery. Scientists know that the length of daylight hours is the trigger for trees to start cutting off water and nutrients to their leaves, but predictions as to when the colors will peak seems harder than predicting snow fall amounts in a specific area. This year’s color was more than 2 weeks later than leading experts in the Southern Appalachians predicted which is due to many changing factors such as temperature, moisture, pH and tree species variety. The large biodiversity of trees in the Southern Appalachians creates more variability and adds to the complexity of the color science. One year the Red Maples may do well but not the Yellow Poplars for example. This variety is beneficial because it allows the leaf show in the South to last longer than in the Northern Appalachians which are known to have a really bright but short color season due to their forests being dominated by Sugar Maples.
As the days get shorter and the tree decides that it’s tired of maintaining these energy-intensive leaves, the tree grows specialized cells to block the flow of water and nutrients to the leaf. This is called the abscission layer. Once this layer is formed, the green Chlorophyll quickly fades away to reveal the true colors of the leaf. The cause of these colors can be narrowed down to two important chemical pigments, Carotenoids and Anthocyanin.
Carotenoids are a group of pigments that includes the commonly known Lycopene, which makes tomatoes red, and Beta-Carotene, which can be found in many common foods, such as carrots, sweet potatoes, and corn. It’s is taken as a dietary supplement to help with vision and arthritis issues among others. The Carotenoids are present in leaves throughout the growing season but are covered by the dominating chlorophyll green until fall arrives. This group of chemicals plays a vital role in the function of leaves.
Chlorophyll is the rock star of any green plant and all processes are built around the process of photosynthesis but, like any rock star, they wouldn’t be anything without their band members, backup singers, and stage crew, the Carotenoids. Chlorophyll is a delicate chemical and can only be made and maintained when there is direct light. It can hold on to energy for only a moment before it is lost, damages the leaf, or turned into sugar. Carotenoids, on the other hand, are tough, can be made at night, can absorb wavelengths of light that Chlorophyll cannot, and can store energy that it then passes on to the Chlorophyll at night or during rain to make food. It also protects the chlorophyll from getting hit with too much energy by absorbing or reflecting radiation that is outside the wavelength that chlorophyll can use (that will damage the chlorophyll cells).
Carotenoids Color Spectrum
The yellows, oranges, and lighter reds are Carotenoids and are always there hiding in the leaf under the green. However, the part that is so hard to predict each year are the dark reds. That is because they are not hidden there all along, the reds are created as the leaf is being cut off from the tree based on weather conditions. Anthocyanins are chemical pigments that that are responsible for the reds, pinks, purples, and blues such as in blueberry’s, beets, cherry’s, and apples. Around the time that the abscission layer is made, if the days are sunny and the nights are cool, chemical reactions occur with sugars stored in the leaf to produce the Anthocyanin reds of fall. If it’s a cloudy dreary warm fall than you can expect there to be few reds or at least duller reds.
Anthocyanins Color Spectrum
So, that’s the “how” but now, “why” do leaves change color at all? So humans will take pictures of them and boost the economy’s of small mountain towns? What evolutionary purpose is there to creating Anthocyanins and leaving perfectly good sugar in a leaf so that it will turn a bright beautiful red? This is something that is still debated by scientists, with no clear answer. Here are a few theories.
If there is something we know about bright colors in nature it’s that the creature possessing them is usually warning that it will put a hurting on you if you get near it. But leaves don’t bite and they aren’t poisonous, but could they still be trying to send a signal? There are still perfectly good nutrients inside those leaves (especially Nitrogen) that the tree wouldn’t mind having back once they fall on the ground and decay. However, in fall the usual suspects (insects) are cruising around looking for dying leaves to munch on and on which to lay their eggs as their winter death inevitably nears. Scientists theorize that the red colors are a signal showing the insect world that they are healthy and strong and “don’t even think about eating my nice bright red leaves.” So the red colors help trees to compete against insects for the nutrients in their own dead leaves. A problem with this theory is that the data shows that this works best for yellow colors and therefore would explain the Anthocyanin mystery.
Something known about Anthocyanin is that it is a strong Anti-Oxidant and slows the aging process by protecting and repairing cells. This is the good healthy stuff in blueberry’s remember. There is also valuable Nitrogren in the leaves that the tree could really use. So, when Anthocyanins are being produced in the leaf, and the abscission layer is slowly cutting off the leaf, one theory is that the tree does take in and store some of these nutritional goodies before the leaf is totally cutoff. Evidence that supports this shows that red leaves on the ground have less Nitrogen than their on-tree counterparts.
The reds created from Anthocycanins act as a sunscreen to protect the leaf from damage while the leaf is dismantling it’s machinery. This is connected to the Anti-Oxidant theory but has a different twist. It states that trees that are considered to be early successional species or “pioneer” species are adapted to be able handle a lot of direct sunlight (since they grow in recently disturbed areas) and therefore don’t need the “sunscreen” of Anthocyanins. Older successional species, however, are adapted to more shady conditions and therefore need more sunscreen to protect their leaves while the tree gets drains them from all of their nutrients.
The last theory claims that animals that usually eat leaves might not eat brightly colored ones because they would lose their camouflage in order to do so. Something that is usually camouflage to earth tones would stick out like a sore thumb (or sitting duck) when eating in front of a brightly colored tree.
It is fascinating to debate reasons and theory’s behind our ever changing world but there is apart of all us that doesn’t want to fully understand. Wonder is the basis to fascination. As scientists puzzle over the mystery’s of the universe, perhaps some of nature’s mysteries are best left unsolved.
“I begin to see an object when I cease to understand it.” Henry David Thoreau
“Mystery creates wonder and wonder is the basis of man’s desire to understand.” Neil Armstrong