The Sound of Science - "Fall Leaves"

Nov 9, 2018

M: Welcome to the Sound of Science on WNIJ. I'm Mackenzie Thompson from NIU STEM Outreach, and today I'm joined by my good friend, Sam Watt. Sam, today we have a question from Rhea, who asks, "Why do the leaves change colors in the fall?"

S: Good question - and a timely one. I was raking all those pretty colored leaves out of my yard last week.

M: Yeah, they certainly are everywhere this time of year, but they sure look nice while still on their branches.

S: So to answer Rhea: When the leaves start changing color it's a sign that the trees are getting ready for winter.

M: That's right. The trees use their leaves to produce energy from sunlight. And the chemical they use to do that is called chlorophyll. Chlorophyll is what gives leaves, and other plants, their green color. 

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S: But when the weather gets cooler and the days get shorter, that triggers a change in the way trees use energy. Most leafy trees go dormants during the cold months when there's less sunlight, so their leaves stop producing chlorophyll as they prepare for winter.

M: As the remaining chlorophyll begins to break down, the green color fades and we can see the actual color of the leaves show through. Most tree leaves are yellow, orange, or brown due to a pigment called carotenoid. Carotenoid is the same pigment that gives carrots, bananas, and corn their color. Some leaves can also have brilliant red colors, caused by the pigment anthocyanin, which can also be found in apples, cranberries, and strawberries. The colors we see depend on the type of tree, and what other pigment the leaves contain.

S: The weather can also affect the leaf color. Cold, rain or clouds can increase the intensity of some colors, while an early frost can actually decrease the amount of red colors we see.

M: But not all trees lose their leaves in the fall. Conifer trees have needles that stay green all year round. That's why we call those types of trees evergreens.

S: And there are even some deciduous, or leafy trees, that don't change color - especially farther south where it doesn't get as cold. Some types of trees stay leafy and green all year round, while others may turn yellow for the winter as energy production slows down, but keep their leaves until they turn green again in the spring.

M: So there you have it, folks: the reason for the seasonal color is science! Thanks for the great question, Rhea. Everyone else, please keep your questions coming at STEMOutreach@niu.edu.

S: This has been the Sound of Science on WNIJ. Where you learn something new every day.