Why do leaves change colour in autumn?
The yellows and oranges were there all summer, hidden under green — and the tree dismantles the green deliberately to recover the nitrogen locked inside it before dropping the leaf.
Simple intuition
The plain reason, in everyday words
A leaf is a solar panel, and it is only worth running while there is enough light and warmth to pay for it. As the days shorten, a deciduous tree decides the leaf is no longer earning its keep and prepares to let it go. But the leaf is full of valuable material — especially nitrogen, which is hard for a tree to obtain and is bound up in the green pigment doing the photosynthesis. So before dropping it, the tree takes the green apparatus apart and pulls the useful pieces back into its branches for the winter. As the green disappears, the yellows and oranges that were in the leaf the whole time become visible, having been drowned out until now. Reds are different: those are made fresh in autumn, which is the genuinely puzzling part.
Autumn colours appear because the leaves are dying and rotting.
It is an actively managed process, with enzymes dismantling chlorophyll and nutrients being transported back into the tree. Rotting happens later, on the ground, and looks quite different.
All the autumn colours are revealed rather than made.
Yellows and oranges are revealed, since carotenoids were there all along. Reds and purples are synthesised during senescence itself, which is why red intensity varies so much with weather.
Frost causes the colours.
Shortening day length is the main trigger. A hard frost actually cuts the process short and produces dull brown leaves, while the best colour comes from sunny days and cool, frost-free nights.
Evergreens keep their leaves because they do not shed.
They shed continuously rather than all at once, replacing needles over several years. It is a different investment strategy, not an absence of leaf turnover.
It reframes something everyone has seen as an economic decision rather than a seasonal accident: the tree is closing an unprofitable operation and recovering its assets first. It also shows how a visible effect can have two completely different causes running side by side — one colour appearing because something was removed, another because something new was built — and how a well-known phenomenon can still have an unsettled explanation at its centre.
Who worked it out
The pigments were separated before they were understood: Mikhail Tsvet's invention of chromatography around 1900 was developed specifically to separate plant pigments, and it revealed that leaves contain several at once.
What problem forced it
Twentieth-century plant physiology established that chlorophyll turnover is continuous and that autumn colour reflects the cessation of resynthesis rather than sudden destruction.
How it changed since
The chlorophyll breakdown pathway was mapped in detail from the 1990s onward. The function of autumn anthocyanins became an active argument after William Hamilton proposed a signalling explanation in 2001, and the debate between that and photoprotection continues.
Why plants are green in the first place
Chlorophyll reflects the wavelength the Sun delivers most of, which is a genuinely strange choice and has its own explanation.
How trees survive winter without leaves
Dropping the canopy is only half the strategy; the rest is about water, freezing and stored reserves.
Written for Curio rather than collected from a forum — it is part of the curated corpus that ships with the platform. The references it draws on are listed under Sources.