Why do leaves change colour?

The yellow and orange were already in the leaf, hidden under the green. Chlorophyll, the leaf’s main pigment, is green and so abundant that it masks everything else.

As days shorten and temperatures fall, the tree stops making chlorophyll and the existing supply breaks down. With the green gone, the carotenoids — yellow and orange — show through. They were present all year, playing a supporting role in photosynthesis.

Red is a different story. It comes from anthocyanins, which many trees actually manufacture fresh in autumn. That is why yellow appears in almost every leaf while vivid red shows only in certain species, and more strongly in some years than others.

Why the tree drops the leaf at all is part of the same process. Holding leaves through winter or a dry season is expensive: they lose water while photosynthesis slows. So the tree withdraws the useful materials into its stem, then builds a layer of separation at the base of the leaf and lets it go.