By Taylor Tibbs
It is that time of year again: the morning air is crisp and you can feel a bit of a chill; the summer trees that were once a lush and deep green are beginning to turn into a splendor of reds, yellows, and oranges. This colorful phenomenon seems to occur every year without fail, begging the question – How exactly do the leaves change color in the fall?
Image 1. Photo of trees during the fall in the Blueridge Mountains near Asheville, NC. Image Source.
Well, to understand how leaves change color in the fall it is best to understand what makes them green during the spring and summer. Leaves are green due to the presence of chlorophyll. Chlorophyll is a green pigment that allows the plant to photosynthesize (turn energy from light into food). Light energy absorbed by chlorophyll is stored in a cellular organelle called a chloroplast – the plant version of mitochondria in animal cells (the powerhouse of the cell!). This energy is used in combination with water and carbon dioxide (CO2) to produce sugar that the plant uses for energy. A byproduct of this process is oxygen (O2), which animals like us breathe and use to generate our own energy.
Image 2. Diagram of how a plant cell uses the chloroplast (chlorophyll-filled organelle) to photosynthesize energy. Author’s own work.
So, what does all this talk about chlorophyll and photosynthesis have to do with the spectacle of colors we see during the fall? Well, as the temperatures begin to drop, many trees begin to slow down their energy usage and production in preparation for receiving less sunlight during the winter. As the trees slow down, their chlorophyll begins to break down. Once the chlorophyll breaks down, the green color of the leaves begins to fade and other pigments start to show through, including flavonoids (yellow), carotenoids (orange), and anthocyanins (red). Some pigments are always present in the leaves, but are only seen once the chlorophyll is absent and some pigments are produced in response to the changing temperatures. These same pigments are responsible for the hallmark colors we see in common fruits and vegetables like carrots, strawberries, and beets. Mixtures of these different pigments vary in different species of tree, giving them each their own unique color palette!
Image 3. How pigment levels affect leaf color. Author’s own work.
Other environmental factors affect leaf color as well, including temperature, sunlight, and water. The amount of sun and water a tree received during the summer and spring can affect how vibrant and how long their fall colors last. For example, if the tree didn’t get enough water during the warmer months, its leaves will change color earlier in the fall and not last as long. Similarly, colder and sunnier days during the fall promote more anthocyanin production, resulting in redder leaves.
Fall is a great time to enjoy nature’s ever-changing beauty. Next time you are admiring the beautiful colors of the fall leaves, you can appreciate the science behind the show!
