Science Puzzle
Where Do Autumn Leaves Get Their Colour?
In June the leaves on a silver birch are a deep, even green. By October the very same leaves have turned bright yellow, a week or two before they drop.
Where does the yellow come from?
The Answer
The yellow was there all along. In summer it's simply drowned out by green.
A leaf holds several different pigments. The green one is chlorophyll, which captures sunlight for photosynthesis, and in summer there's so much of it that it hides everything else. Alongside it sit pigments called carotenoids, the same family that colours carrots and egg yolks. They're yellow and orange; they spend the summer helping to gather light and protecting the leaf from too much of it.
As the days shorten, the tree stops making chlorophyll and starts breaking down what's already there. Chlorophyll is packed into proteins that are rich in nitrogen, which is valuable, so the tree pulls it back into its branches to reuse next spring. Carotenoids break down more slowly. As the green fades away, the yellow that was underneath all summer finally shows through.
Reds are a different story. Trees such as some maples really do make a new red pigment in autumn, called anthocyanin, which is why a red maple and a yellow birch can stand side by side in the same cold snap.
The principle: Pigment masking. A leaf's colour comes from the mix of pigments inside it, and when the green chlorophyll is broken down in autumn, the yellow and orange carotenoids that were present all summer become visible.