Why Do Leaves Change Color in the Fall?

Hook

In summer, this leaf is green. A few weeks later, it's bright yellow. Where did the yellow come from?

Puzzle

Here's what you might expect. Maybe the cold paints the leaves. Maybe frost turns them yellow. But lots of leaves change before the first frost ever comes. Maybe the tree makes a new yellow color in the fall, and pumps it into the leaf. That sounds right. But it's mostly wrong. To see what really happens, we need to zoom in. Way in.

Zoom In

A leaf is made of tiny boxes called cells. Inside each cell are lots of little green beans. Each bean is a food factory. It catches sunlight and uses it to make sugar, the tree's food. The green stuff that catches the light has a name. Chlorophyll. Sunlight is a mix of colors. Chlorophyll soaks up the reds and blues, and bounces the green back to your eyes. That's why leaves look green. But here's the strange part. Chlorophyll wears out fast in sunlight. So all summer long, the leaf keeps making new chlorophyll to replace it.

Aha

Now look closer. The bean holds more than green. There's yellow and orange in there too. It was there the whole summer. You just couldn't see it, because there's so much green. It's like a bag of candy that's mostly green ones with a few yellow ones mixed in. From far away, the bag looks green. Then fall comes. The nights get longer. The tree senses the long nights, and it stops making new chlorophyll. The old chlorophyll keeps wearing out. Nothing replaces it. And the yellow that was there all along finally shows. The tree didn't add the yellow. It took away the green. So a yellow leaf isn't a new leaf. It's the summer leaf, with the green taken out.

Red, and Why It Matters

Red is different. Red really is new. On sunny fall days, some leaves still make sugar. But the tree is closing off the leaf's stem. The sugar gets stuck. The leaf turns that trapped sugar into a red color. That's why sunny days and cool nights make the brightest reds. A cloudy, rainy fall means fewer reds. And brown? That's what's left when the other colors are gone. So why go to all this trouble? Winter. In winter, the ground freezes. The tree can't get water. Its leaves would let water escape, and the tree would dry out. So before the leaves drop, the tree pulls the useful stuff back into its branches, and lets the leaves go. Fallen leaves even feed the soil for next year. And in spring, new green leaves start the whole thing over.

One-Line Recap

Leaves change color because the green fades away, and the other colors finally get their turn. Next time: why is a negative times a negative positive?