John Keats wrote To Autumn in September 1819 –

Season of mists and mellow fruitfulness
Close bosom-friend of the maturing sun
Conspiring with him how to load and bless
With fruit the vines that round the thatch-eves run;
To bend with apples the moss’d cottage-trees,
And fill all fruit with ripeness to the core;
To swell the gourd, and plump the hazel shells
With a sweet kernel; to set budding more,
And still more, later flowers for the bees,
Until they think warm days will never cease,
For Summer has o’er-brimm’d their clammy cells.

Who hath not seen thee oft amid thy store?
Sometimes whoever seeks abroad may find
Thee sitting careless on a granary floor,
Thy hair soft-lifted by the winnowing wind;
Or on a half-reap’d furrow sound asleep,
Drows’d with the fume of poppies, while thy hook
Spares the next swath and all its twined flowers:
And sometimes like a gleaner thou dost keep
Steady thy laden head across a brook;
Or by a cider-press, with patient look,
Thou watchest the last oozings hours by hours.

Where are the songs of Spring? Ay, where are they?
Think not of them, thou hast thy music too,-
While barred clouds bloom the soft-dying day,
And touch the stubble-plains with rosy hue;
Then in a wailful choir the small gnats mourn
Among the river sallows, borne aloft
Or sinking as the light wind lives or dies;
And full-grown lambs loud bleat from hilly bourn;
Hedge-crickets sing; and now with treble soft
The red-breast whistles from a garden-croft;
And gathering swallows twitter in the skies.

Almost two hundred years later we see the same thing very differently. SJ Jeong wrote about autumn leaf coloration in July 2014 (J Global Ecology Biogeography). As photosynthesis slows down at the end of the hot and light summer, so chlorophyll drains from the leaves. This gives prominence to the remaining carotenoid and xanthophyll pigments.

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In sugar maples, extra red anthocyanins are produced in the lower temperatures. In turn this stops parasitic insects infecting the leaves.

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Anthocyanins may also protect the broken scar at the leaf axils from infection, and prepare it for the next growing season. But other factors add to the complexity: drought, for example, lessens the colour intensity and encourages early leaf fall