Every autumn, trees seem to paint themselves. They don't. Most of the yellow was there all summer, the red is freshly made, and the whole show is a side-effect of a tree packing up before winter.
A summer leaf is a stack of solar panels. The main pigment, chlorophyll, soaks up red and blue light and reflects green, and there is so much of it that it drowns out everything else. But the leaf also carries carotenoids, the same family of pigments that colour carrots and egg yolks. They help catch light and protect the leaf from getting too much of it.
Chlorophyll is fragile. Light damages it, so all summer long the leaf keeps breaking it down and building new. When the days shorten in late summer, that rebuilding slows and stops, while the breakdown carries on. The green drains away and the carotenoids, which are far more stable, are left behind. That is why birches, aspens and ginkgos turn gold: nothing new was added. Something was taken away.
Why not just let the leaf fall green? Because a leaf is full of valuable parts. The chloroplasts, the tiny compartments where chlorophyll works, hold roughly half to 60 percent of a leaf's nitrogen, locked up mostly in proteins such as the enzyme that fixes carbon. Nitrogen and phosphorus are scarce in most soils, so before dropping a leaf the tree strips it and pipes the nutrients back into its twigs and trunk for next spring.
A 2012 analysis of plants worldwide by Vergutz and colleagues found that, on average, leaves send back about 62 percent of their nitrogen and 65 percent of their phosphorus before they are shed. The colour change is what that recycling looks like from the outside.
The chlorophyll itself holds only 1 to 2 percent of the leaf's nitrogen, so it is not the prize. It has to be dismantled carefully anyway: once its protein scaffolding is pulled apart, loose chlorophyll in sunlight produces reactive oxygen that damages cells. The tree breaks it into colourless fragments, which is why the green simply fades instead of turning into some other colour.
Scrub through the season, or press play. Then drag the dot on the weather pad to change the autumn: sunny or grey days, warm or chilly nights, or a hard frost. Switch between a maple, which can make red pigment, and a birch, which cannot.
Weather pad — drag
Pigments in a sunlit leaf
Red and purple come from anthocyanins, the pigments in red cabbage and blueberries. Unlike carotenoids, most of them are not waiting underneath. The leaf builds them late in the season, out of sugar, and it needs bright light to do it.
That explains the weather rule every leaf-peeper knows. As autumn arrives, a corky layer grows across the base of the leaf stalk, the future break point, and it gradually chokes the veins. Sugar made on sunny days gets stuck in the leaf, and cool nights slow its escape further. Lots of sugar plus lots of light means lots of anthocyanin. Sunny days and cool, but not freezing, nights give the deepest reds. A grey, mild autumn gives duller colour, and a hard early frost kills the leaves before they can make pigment, so they go straight to brown.
Only some trees play this game. Across temperate forests worldwide, about 10 percent of tree species make red autumn pigments, but in places like northern New England up to about 70 percent do, which is part of why its hillsides look like they are on fire.
This is still argued about. Building a pigment in a leaf you are about to throw away looks wasteful. The leading explanation is sunscreen. While the tree is taking the leaf apart, its light-harvesting machinery is half dismantled and cold slows repairs, so bright light can do real damage. A red layer shades the inside of the leaf and may let the tree finish pulling nutrients out.
A rival idea, proposed by the biologist W. D. Hamilton in 2001, is that red is a warning to insects. Aphids pick trees to lay eggs on in autumn, and a bright tree might be signalling that it is well defended. Evidence for both exists, and they are not mutually exclusive.
Once the recycling is done, the stalk's break point finishes forming and the leaf lets go. The cells die, the remaining pigments break down, and leftover compounds such as tannins give the familiar brown of oak leaves on the path. What falls is mostly an empty shell. The good stuff is already tucked away in the twigs, waiting for spring.
Sources: Vergutz et al. (2012), Ecological Monographs, global leaf resorption efficiencies; Estiarte et al. (2023), Plant Communications, on nitrogen in chloroplasts and chlorophyll; Wikipedia, "Autumn leaf color" and references therein; university extension guides from Illinois and Purdue.