2 min
Why do leaves change colour in autumn?
Autumn colour is not decay — it is a carefully planned chemical retreat. The green fades because the tree cuts off supplies, and the red is actually a kind of sunscreen the tree makes for itself.
Answer
Leaves changing colour is not decay. It is a carefully planned chemical retreat.
Through spring and summer, chlorophyll runs the show. It absorbs red and blue light and reflects green, which is why leaves look green. It is also busy every second doing photosynthesis, making food for the tree.
Then autumn arrives. Days get shorter and temperatures drop, and the tree reads the signal: winter is coming, time to shut down. It grows a layer of abscission cells at the base of each leaf — a kind of gate — and begins cutting off the supply of water and nutrients. With no supply, chlorophyll breaks down fast. The green drains away, and the other colours that were there all along finally get their turn.
Those colours never went anywhere:
- Carotenoids — orange and yellow. They are stable and slow to break down, and chlorophyll simply hid them all summer.
- Anthocyanins — red and purple. These are made fresh in autumn, only in certain trees such as maples, and they need bright sun and cool nights to form.
The most surprising part: anthocyanins are not waste. They are the tree's sunscreen. Autumn light is still strong, but the leaf is about to fall and photosynthesis is winding down, so extra light energy can damage the cells. Anthocyanins absorb that surplus blue and ultraviolet light, protecting the leaf while it safely recovers its nutrients for the last time.
In one line: autumn leaves are not the colour of dying. They are the palette a tree uncovers before it retreats — green was the work uniform, red and gold are the farewell dress.
A stranger fact: why do some trees skip all this and drop green leaves — pines and cypresses, for instance? Their leaves are needles: small surface area, slow water loss, and a thick waxy coating that survives winter. They do not need an autumn retreat at all, so their chlorophyll just keeps working until spring.
Michael's Thought
I always thought leaves turned colour because they were dying. It turns out the red is brand new — the tree makes it to protect itself.
Sources
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