Why do leaves turn red in autumn?

Leaves turn yellow and red for two very different reasons. Yellow is uncovered. Red is made.

All summer, a leaf is packed with green chlorophyll, which the tree keeps replenishing while it makes food from sunlight. But the leaf also holds yellow and orange pigments called carotenoids. They are simply drowned out by the green. In late summer, as days shorten and cool, the tree starts dismantling its leaves to recover the nutrients inside. The chlorophyll breaks down, and the yellow that was there all along finally shows.

Red is a different story. The red pigments, anthocyanins, are not in the leaf during summer. The tree actively makes them at the end of the season, from sugars broken down in bright light as the leaf's phosphate drains away. That's why bright, cool autumn days followed by chilly but not freezing nights usually produce the most brilliant reds.

Why would a tree spend energy colouring a leaf it's about to drop? Scientists aren't sure, and there are competing ideas. One is sunscreen: the red may shield the leaf so the tree can pull back its nitrogen more efficiently. Another, proposed by the biologist W. D. Hamilton in 2001, is a warning: bright leaves may tell aphids looking for a winter home that this tree is well defended. In some maples, the red pigments even seem to stunt the growth of nearby saplings. The answer may differ from tree to tree.