From hedgerow, lawn and wooded hill
Departs the summer's chlorophyll;
The elms and hickories lose their green
And glow instead with carotene,
While sumacs, maples redden in
A burst of anthocyanin—
And TIME is moved to tell its clients
The reason why in terms of science.
When the deciduous (non-evergreen) leaf begins to die and disintegrate, the molecules of its unstable green pigment, chlorophyll, disappear. Its departure reveals the yellow carotene and xanthophyll which have been present but masked by the green all summer.
Red appears in maples, sumacs and some other plants when slowdown of the trees' physiological processes prevents carrying away of the sugars (made with the aid of the fading chlorophyll out of air, water and light) from the leaves. These sugars turn into a class of glucosides called anthocyanins, which are bright red and purple pigments. Anthocyanins develop best where 1) soil is acid. 2) nitrates are scarce, 3) light is abundant. Thus the light-bathed tips of maple leaves and the sunny sides of apples are reddest.
By late autumn, the yellow and red pigments, following the green, disintegrate in the leaves. This final unmasking reveals the dull brown tannins, which are chemically so stable that they remain till the leaf rots to powder. Unlike flower pigments, which have the vital function of attracting pollen-spreading insects and birds, autumn's colorful foliage is just a meaningless, glorious show.