Fall Foliage Fervor: The Science Behind A Beloved Autumnal Phenomenon
To kick off our new section Everyday Science, contributor Addison Parker tells us about the science underlying the brilliant colors of autumn.
Imagine a red and gold so brilliant, so extraordinary, that it stops you in your tracks, makes you freeze, an orange so bold, a blazing fire up high in the trees. The leaves of Autumn so staggering that you can hardly breathe.
And you think to yourself... just how can that be?
Introduction
One of nature's most beautiful color displays is the changing leaves in autumn. There are songs, paintings, and an entire tourism industry dedicated to the fall foliage phenomenon. There is no doubt that it captures our imagination. But, have you ever wondered how and why leaves change color in the fall? This piece explores the science behind the changing colors and other related questions. What role do pigments play in leaves? Why do some trees undergo this dramatic metamorphosis while others don’t? Do leaves actually fall? What happens to them once they are on the ground? Is climate change affecting any of this? With so many questions to answer, let’s get into the science.
Photosynthetic Pigments: The Chemistry Behind The Color
Tree leaves contain pigments that absorb energy from sunlight, which is then used to drive the conversion of carbon dioxide and water into glucose and oxygen. Okay, this fact alone should make you lose your mind. Consider it for a moment. Through the process of photosynthesis, plants take energy from the sun and convert it into glucose, which gets incorporated into their cell walls and seeds. Then, animals eat those plants and seeds and other animals eat those animals, and so on. This means that our everyday existence is fueled, ultimately, by the sun. But because we humans do not possess the ability to convert sunlight energy into chemical energy, we rely on plants and their photosynthetic pigments. Each of these pigments is ‘tuned’ to absorb specific wavelengths of light, also known as that pigment’s absorption spectra. The most abundant pigment, chlorophyll, absorbs blue and red wavelengths of light while reflecting green–that reflection is why plants look green to us. The accessory pigments, the carotenoids, absorb violet and blue-green light while reflecting yellow and orange. While the carotenoids absorb light for photosynthesis, they also play a protective role by absorbing excess energy and dissipating it as heat to prevent damage to the photosynthetic machinery.
At the start of fall, the sun excuses itself from the sky earlier and earlier, and the molecules that comprise the local atmosphere begin to cool down to a slow pace, signaling to the tree that winter preparations must begin. In response, the production of chlorophyll ceases and any remnants present in the leaf get broken down, slowly revealing the underlying carotenoid pigments that were shielded from view throughout the growing season. Sugars trapped within the leaf react with resident proteins in the cell sap and produce the pigment anthocyanin, which turns the leaf either red or purple depending on the pH–the measure of how basic or acidic something is–of the sap.Â
While color change is emblematic of the fall season, not all trees that lose their leaves go through this dramatic color display, and not all trees lose their leaves in the first place. The evergreens, examples of which include fir, spruce, and pine, are well-adapted to regions with harsh winters. Their leaves and needles contain less water and are fashioned with a wax-like coating that prevents water loss. These adaptations allow them to keep their leaves during the winter months and remain, well, ever-green. Though there are exceptions to the rule, typically trees that lose their leaves are called deciduous–the term deciduous coming from the Latin decidere, which means to fall off or down. Deciduous trees, examples of which include birch, ash, and sugar maple, typically have broad leaves that are vulnerable to damage by harsh weather conditions. Leaf shedding in advance of the winter months gives them a chance to sequester nutrients and water, valuable resources that would otherwise be destroyed if the leaves are left intact. For deciduous trees, the transition from green to yellow, orange, and red signals the beginning of the end for the leaves… or does it?
The Fate of The Leaves
The autumn colors are like the leaves taking their final bow before exiting the stage. They take on their most beautiful form and then they fall. But, what is interesting is that the leaves are not so much falling as they are being deliberately ‘told’ to leave. You know that overripe banana decaying on your counter? The plant hormone responsible for your rapidly ripening banana, ethylene, is also responsible for a process called abscission–the separation of the leaf from the stem at the abscission zone, an area comprised of both a protective layer and a separation layer of cells. Shorter days and cooler temperatures cue the tree to ramp up the production of ethylene, which triggers the release of hydrolytic enzymes that break down the separation layer, freeing the leaf from the rest of the tree.Â
Once separated, the leaf descends, drifting along cool wind currents until it reaches its final resting place. Leaves fall layer upon layer. Dry, brittle, and lifeless they sit, the top litter layer seemingly useless, subjected to scattering by blowers and rakes. Beneath this is the fermenting layer. Here, water gets trapped and the leaves begin to rot. Further down is the humus–the nutrient-rich layer comprised of fully decomposed plant and animal matter. For a group of small animals and insects known as detritivores, this matter actually serves as their main source of nutrients. These organisms mechanically break down detritus (non-living matter) into smaller pieces and the decomposers–bacteria and fungi–enzymatically break down matter into its chemical constituents, releasing nutrients like nitrogen and phosphorus back into the soil. Thanks to the detritivores and decomposers, the leaf takes on a new life, its molecules becoming integrated into the broader ecosystem. A sort of reincarnation, if you will. While nutrient cycling occurs in the lower layers, the litter and fermenting layer create a microhabitat full of a diverse array of critters–snails, centipedes, spiders, and frogs, to name a few–who rely on the leaves for shelter, moisture, and hunting prey. Additionally, they serve as nesting materials for birds and insulate rodent burrows during the winter months. Fallen leaves enrich the soil, help it retain moisture, and prevent the growth of weeds. So, maybe skip the garbage bag, rake, and especially the leaf blower, and let nature do its thing. The seemingly lifeless leaves indeed serve a purpose.
Trees in a Changing World
In order to orchestrate this complicated process of abscission, trees depend on various environmental cues like weather and length of day to tell them when to begin nutrient sequestration. However, climate change is likely affecting the cues that trees receive. Studying the impact of climate change on autumn phenology–the timing of leaf coloration and drop–is challenging because the timing of autumn phenomenological events are difficult to standardize and definitions of the various phases rely on subjective observations. But what does seem clear is that, on average, the timing of leaf color change is delayed by increasing temperature. We could imagine a scenario where more frequent early-autumn freezes are coupled with delayed leaf shedding, resulting in damaged leaves and an altered nutrient cycle. Additionally, more frequent droughts could mean earlier shedding and missing the color display entirely. This is the scary reality that we live in. If no real action is taken to slow global carbon emissions, then we will have to face the consequences.
While we humans might be doomed, I have hope for the trees. They witnessed the rise and fall of the dinosaurs. They re-established themselves in the aftermath of the fifth mass extinction, a resilience that is likely to help them rise out of the rubble of the current sixth mass extinction. The science tells us that trees are incredibly dynamic, complex, and resilient organisms, capable of interacting with and responding to their environment. They are brilliant in their strategies to survive harsh winters, stunning in their fall color displays, and are integral members of the broader ecosystem, worthy of our study and attention. While the reds, yellows, and oranges of fall are extraordinary, even more so is the science that makes it all possible.
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