Why Leaves Change Colour
Through summer a leaf is a factory running on chlorophyll, the pigment that captures light and reflects green. As days shorten, the tree stops maintaining that factory — and the colours you see in October were there all along.
The reveal
Carotenoids — the yellows and oranges — are present in the leaf throughout the growing season, masked by the stronger green of chlorophyll. In autumn the tree breaks down chlorophyll and reabsorbs its nitrogen, an efficient way to recover nutrients before discarding the leaf. With the green gone, the carotenoids show.
The reds are different
Anthocyanins, which produce reds and purples, are not simply revealed. Many trees synthesise them in autumn, apparently as a sunscreen and antioxidant while nutrients are being withdrawn. Their production depends on weather: bright sunny days with cool nights tend to produce the strongest reds.
Why trees bother
Deciduous trees drop leaves to survive winter: broad leaves lose water and are damaged by frost, and the cost of maintaining them outweighs the benefit in low light. Autumn colour is a side effect of that shutdown.
Knowledge check
Two questions, no score
Where do the yellow pigments in autumn leaves come from?
Now prove it