How and what does a tree eat? Why do leaves change colors in the fall? Why do trees drop their leaves?
Trees, just like humans, need nutrients, oxygen, water, and the sun to thrive. Also, adaptations for surviving our climates have occurred in both trees and humans, but ours are less showy in the fall.
Let's start with the first question: how and what does a tree eat?
When we eat carbohydrates, our bodies produce glucose, aka sugar, which we either use instantly or store for later. Similarly, trees also utilize carbohydrates in their growth, but instead of eating a sandwich, they use water from their leaves and roots, carbon dioxide (CO2) from the environment, and energy from their leaves through a process called photosynthesis. Photosynthesis uses the leaves’ stored chlorophyll, which produces? … You guessed it, carbohydrates! Science note: Chlorophyll is also why leaves appear green… more on that shortly!
So, how does this tie into the next question: Why do leaves change colors in the fall?
Well, simply put, the sun is to blame for the fall colors we get to enjoy. Because the sun is getting further away from our hemisphere, the days become shorter, and the temperatures start to drop. This causes the tree’s adaptation to enter the chat. Remember the tree needs water, sunlight, and CO2 to make its food (glucose), and what is beginning to decrease? … That's right, the amount of sunlight the stored chlorophyll can use to make energy with.
For the tree to survive the coming months of less water absorption from frozen soil, no rain, and reduced sunlight, the tree must dramatically reduce its energy needs. The cooler weather and shorter days trigger the trees to stop producing and start reclaiming the existing chlorophyll nutrients.
Remember the science note from earlier? This reduction and reclaiming of the existing chlorophyll cause the leaves to change colors. Chlorophyll absorbs red and blue light, causing most of us to see leaves as green. As the level of chlorophyll continues to decrease, we get to see the other colors in the spectrum. Remember ROY-G-BIV? Once green gets out of there, the fall show can really start.
My friend Kim Frye is an environmental scientist, and while I was chatting with her about this article, she shared a fun detail about leaf pigments: "[trees produce] xanthophyll (yellow orange) and anthocyanins (red purple blue), and they also help all year long when the light changes in the mornings and at nightfall. Those wavelengths become more predominant as we head into fall and maximize photosynthesis beyond the wavelengths chlorophyll uses."
Now onto the last question: Why do trees drop their leaves?
Ginkgo tree leaves (Ginkgo biloba)
Once the tree has reclaimed all the available chlorophyll stored in its leaves, an important step in the tree’s survival happens: it drops its leaves. It does this for a few reasons, but the main one is to reduce energy needs and conserve as much water and stored energy as it can. This increases its survival during the winter months when it can't produce energy from the sun and absorb water through the frozen ground. This also ensures the tree has enough energy come springtime to grow and produce new leaves to begin the photosynthesis cycle again. I bet that it is a re-leaf to trees each spring!
Think of the complex and inspiring work all of the colors mean as the leaves put on their show, the next time you are enjoying leaves. Whether that is in the spring when everything is green and energetic, or in the fall with the beautiful multi-colored show ushering in a restful period for plants.
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Laura Howell | Human Resources Generalist, Russell Tree Experts
Laura joined Russell Tree Experts in January 2025. She supports our staff through her role in the HR department. Laura is an alum from Franklin University and CSCC with degrees in forensic accounting and finance. Outside of work she enjoys traveling, gardening, and camping.