Why Do Leaves Change Color? The Secret Rainbow Hiding in Every Leaf
Summary
Fall colors are glowing across North America right now, but many of those yellows and oranges were hiding inside green leaves all summer long! Join our leaf detective mystery to find out how trees reveal their hidden colors, and why the reds are a brand-new surprise painted fresh every fall.
Transcript
0:00The Hidden Rainbow
Right now, all across North America, trees are bursting into red, orange and gold. But here is something surprising: many of those bright colors are not new at all. They were hiding inside green leaves the whole summer long. Right under your nose! A big tree can grow hundreds of thousands of leaves, and every single one was keeping the secret. Every time you walked past a green tree in July, a hidden rainbow was tucked inside. So how does a tree keep a rainbow secret for months? And how does it finally let the colors out? Grab a leaf, detective. We have a mystery to solve. And our first clue is the color green itself.
0:41Why Leaves Are Green
Every leaf is a tiny food factory. It takes in sunlight, water, and a gas from the air called carbon dioxide, and it turns them into sugar. That sugar is the tree's food. The leaf's secret helper is a green material called chlorophyll. Say it with me: KLOR-oh-fill. Chlorophyll soaks up sunlight to power the factory, and it is so green that it covers up almost everything else. Here's a wow-fact: chlorophyll wears out in bright sunshine, so all summer long the leaf keeps building fresh chlorophyll to replace it. It's like a painter repainting a wall every single day. So if green covers everything, what exactly is hiding underneath? Our second clue is orange. And you might have eaten it for lunch.
1:29Clue Two: Carrot Colors
Underneath all that green, the leaf has other colors: sunny yellows and bright oranges. Scientists call this family of colors carotenoids. That word might remind you of a vegetable you know. Yep, carrots! The same family of colors that makes a carrot orange is sitting inside green leaves right now. You'll find it in pumpkins, sweet potatoes, and the yellow of corn too. The yellows and oranges help the leaf catch extra sunlight and help protect it, like a helpful little team working behind the scenes. But in summer, the green is so strong that you never notice them at all. Picture a yellow wall painted over with thick green paint. The yellow is still there. You just can't see it. So what makes the green paint start to fade? Clue number three is hiding in the sky.
2:19Clue Three: Longer Nights
Have you noticed it getting dark earlier lately? In the fall, the days get shorter and the nights get longer and cooler. Trees notice too! They can sense the longer nights. It's like a quiet alarm clock telling the tree: winter is coming, time to get ready. Winter is tough for a leaf. Thin leaves can't handle freezing weather, and when the ground freezes, water is hard to get. So trees like maples, oaks and birches get ready to drop their leaves and rest until spring. Wow-fact: a tree has no eyes, but special materials inside its leaves can still tell when the nights grow long. And once the tree gets that message, something amazing begins inside every single leaf.
3:03The Green Fades Away
Remember our painter who repaints the green every day? In the fall, the painter stops. The tree stops making new chlorophyll. The old green breaks down, bit by bit, day by day. And as the green fades away, guess what starts to show through? The yellows and oranges that were there all along! Look at the same leaf side by side. Summer leaf: lots of green, with yellow hiding underneath. Fall leaf: the green is gone, and the yellow finally shines. That's why birches and many maples glow golden. The tree didn't paint them yellow. It just stopped covering them up! But what happens to all that green? Does the tree just throw it away?
3:42Trees Recycle
Nope! Trees are amazing recyclers. Before a leaf falls, the tree pulls back many of the useful bits from it, including pieces of the old chlorophyll. It's like packing up your snacks before you leave the picnic. The tree stores those good things in its branches, trunk and roots, saving them for new leaves next spring. Wow-fact: leaves are full of nitrogen, a nutrient trees need to grow, and a tree saves a lot of it before its leaves drop. So now our mystery seems solved. The yellows and oranges were hiding, the green faded, and the hidden colors appeared. Case closed? Not quite. Because there's one color that doesn't fit this story at all.
4:25The Red Twist
Red. Those blazing red maples, the deep red of some oak leaves, the purple of some leaves too. Here's the twist: most of those reds were NOT hiding all summer. The tree makes them brand new, right in the fall! Scientists call these red and purple colors anthocyanins. They're the same kind of colors that make blueberries blue, raspberries red and red cabbage purple. So the hidden rainbow has two parts. The yellows and oranges were tucked inside all along. But the reds are a last-minute surprise the tree paints fresh, just before the leaves fall. Why would a tree make a whole new color right when its leaves are about to drop? And what does it make red from? The answer is sweet. Really sweet.
5:11Sugar Makes Red
Remember the sugar the leaf makes? In the fall, the tree begins sealing off the spot where the leaf joins the twig. So sugar made on sunny fall days gets stuck in the leaf. Now add bright sunny days and chilly nights, but not freezing ones. Sunny days help the leaf make lots of sugar, and cool nights help keep it in the leaf. The leaf turns that extra sugar into brilliant red. Here's something you can see for yourself: look at a maple tree. Very often, the side that gets the most sunshine glows the reddest, while the shady side stays more yellow or orange. One tree, wearing two colors at once! Scientists think the reds may work like a sunscreen, shielding the leaf while the tree pulls back its good stuff. That's why a sunny fall with cool nights gives the most dazzling reds. But every leaf's show has a final act.
6:05Brown and Letting Go
After the bright colors fade, many leaves turn brown. That brown comes from materials called tannins, which are left over after the other colors break down. It's the same kind of material that helps make tea look brown. So here's the whole show, step by step. Step one: the nights grow long. Step two: the green fades and the hidden yellows and oranges appear. Step three: trapped sugar turns some leaves red. Step four: brown appears, and the leaf lets go. That letting go is planned too. A special layer of cells forms at the bottom of each leaf stem, like the dotted line where you tear a page from a notebook. A breeze comes along, and the leaf drifts down. Wow-fact: fallen leaves break down into the soil and help feed plants and tiny creatures, so the tree's old leaves help the whole forest grow. So, detective, let's look at that leaf in your hand one more time.
7:02Mystery Solved
Remember the secret rainbow we started with? Now you know where it was hiding. The yellows and oranges waited under the green all summer, and the reds were painted fresh with trapped sugar on sunny fall days. Every bright leaf you see outside right now is a tree showing you its secret, one leaf at a time. But here is a mystery for another day: while maples and oaks put on their colorful show, pine trees stay green all winter long. How do their needles survive the freezing cold without ever changing color?
Also in other languages
- हिन्दी: पतझड़ की पत्तियों में छिपा इंद्रधनुष: पत्तियाँ रंग क्यों बदलती हैं?
- संस्कृतम्: शरत्पत्रेषु निगूढम् इन्द्रधनुः: एते वर्णाः कुतः आगच्छन्ति?
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Series: science (English)