Why Do Leaves Change Color in Autumn?
The yellow and orange were in the leaf all along. Autumn just takes the green away — and the reds are a different story.
The short answer
Leaves change color in autumn because they stop making chlorophyll, the green pigment used in photosynthesis. As days shorten and nights cool, the tree breaks chlorophyll down and seals off the leaf. That reveals yellow and orange pigments called carotenoids, which were in the leaf all summer but hidden by the green. Red and purple come from anthocyanins, which some trees make fresh in autumn from sugars trapped in the leaf.
Transcript
Why do leaves change color in autumn? Not because the cold paints them. The yellow and orange were in the leaf all summer.
A leaf is green because of chlorophyll, the pigment that captures sunlight for photosynthesis. Chlorophyll breaks down constantly, and in summer the leaf keeps replacing it. When days get shorter and nights get cooler, the tree stops. It seals the base of each leaf and pulls nutrients back into its branches.
As the green fades, other pigments show through. Carotenoids, the same pigments that make carrots orange, give the yellows and oranges.
Reds are different. Some trees make new pigments called anthocyanins in autumn, from sugar trapped in the leaf. Sunny days and cool nights make the reds brightest.
Test yourself on Biology
5 questions, easy to hard. No account needed to try it.
What gives leaves their green color in summer?
Where do the yellow and orange colors of autumn leaves come from?
What mainly triggers a tree to stop making chlorophyll in autumn?
How are the red and purple colors in some autumn leaves different from the yellows?
Which autumn weather tends to produce the brightest red leaves?
The longer answer
The short answer is that autumn takes the green away. The answer worth having is what is underneath the green and why a tree throws away its own leaves, because those two ideas are what biology questions on this topic actually test.
A leaf in summer is a solar-powered sugar factory. Its key pigment is chlorophyll, which absorbs red and blue light and uses that energy to drive photosynthesis, turning carbon dioxide and water into sugar. Chlorophyll reflects green light, and there is so much of it in an active leaf that it masks every other pigment present. Chlorophyll is also fragile: sunlight breaks it down continuously, so a working leaf has to keep rebuilding it all season.
The leaf has other pigments as well. The most important are the carotenoids, the same family of molecules that makes carrots orange and egg yolks yellow. Carotenoids help the leaf by capturing some extra light and by protecting it from damage, and they are present in the leaf throughout the summer. You simply cannot see them, because chlorophyll overwhelms them.
As autumn approaches, the tree reads the shrinking day length as a signal that winter is coming, and cooler temperatures reinforce that signal. For a broadleaf tree in a cold climate, keeping thin, water-filled leaves through winter would be a liability: they would freeze, lose water the tree cannot replace from frozen ground, and catch snow that could break branches. So the tree prepares to drop them. It forms a special layer of cells, the abscission layer, at the base of each leaf stalk. This layer gradually cuts off the flow of water into the leaf and the flow of sugar out of it, and it later becomes the clean break where the leaf detaches.
Before letting go, the tree recycles what it can. It breaks down chlorophyll and moves valuable nutrients, especially nitrogen, back into the branches and roots to store for spring. Because chlorophyll is no longer being replaced, the green fades. The carotenoids break down much more slowly, so for a while they are left standing on their own. That is the yellow and orange of autumn: not new color, but old color finally uncovered.
The reds and purples are a different process. They come from pigments called anthocyanins, which, unlike carotenoids, are mostly not present in summer leaves. Some species, maples among the best known, actively make them in autumn. Once the abscission layer begins to block the leaf stalk, sugar made by the remaining photosynthesis gets trapped in the leaf, and the tree uses that sugar to build anthocyanins. Their exact purpose is still debated by scientists; a leading idea is that they act as a sunscreen, protecting the leaf while the tree finishes recovering its nutrients. Not every tree makes them, which is why some species turn only yellow while others turn scarlet.
That also explains why some autumns are more colorful than others. Bright, sunny days let the leaves keep producing sugar. Cool nights, above freezing, slow the movement of that sugar out of the leaf, so more of it stays behind to feed anthocyanin production. The result is vivid reds. Warm, cloudy autumns produce less sugar and duller color, and an early hard frost can kill leaves before the reds develop at all. Drought can also make leaves drop early.
Brown is the last stage. When the other pigments have broken down, what remains are tannins, compounds that are brown and are left in the dead leaf tissue. That is why most leaves end up brown on the ground no matter how bright they were on the tree.
Two points are worth separating clearly because exams like to mix them up. First, the yellows and oranges are revealed, not created; they were there all along. Second, the reds are created; they are new pigments built in autumn. Cold does not paint the leaves, and frost is not what causes the color change. The trigger is mainly shorter days, and the visible change is the loss of chlorophyll.
So when a question asks why leaves change color in autumn, the complete answer is this: shorter days and cooler temperatures signal the tree to stop making chlorophyll and seal off its leaves; as the green breaks down, the yellow and orange carotenoids that were always there become visible; and in some species, sugar trapped in the leaf is turned into new red anthocyanins.