Leaves change colour in autumn because the tree stops producing the green pigment chlorophyll as the nights grow longer. That unmasks the yellow and orange pigments that were in the leaf all along, and in some species it also triggers new red and purple pigments built from sugars trapped inside. The colour is the visible sign of a controlled shutdown before the leaf is sealed off and dropped.
Three pigments decide the palette
Autumn colour comes from a small set of molecules that are either newly exposed or newly made. Illinois Extension and the US Forest Service describe the same three groups.
| Pigment | Colour | When it shows |
|---|---|---|
| Chlorophyll | Green | All growing season; fades in autumn |
| Carotenoids | Yellow, orange, brown | Present all season, unmasked as the green fades |
| Anthocyanins | Red, purple, crimson | Made mainly in autumn, in some species |
According to the US Forest Service, chlorophyll and carotenoids sit in the chloroplasts throughout the growing season, while most anthocyanins are produced in autumn in response to bright light and excess sugars. Because carotenoids never left, the yellows and golds are fairly predictable from year to year. The reds are not.
Shorter days, not cold snaps, start the process
According to extension.illinois.edu, shortening day length and cooling temperatures are detected by the tree, which begins producing hormones that initiate leaf senescence. Senescence breaks complex molecules down into smaller, soluble ones such as sugars and amino acids, which the tree pulls back into its twigs and trunk to store for winter. Day length is the dependable cue that sets the calendar; weather only adjusts the timing and intensity.
How the leaf seals itself off
As senescence proceeds, a barrier called the abscission layer forms at the base of the leafstalk, severing the flow of materials between leaf and tree. Illinois Extension notes that with the vein network closing, sugars made by the still-working chlorophyll are trapped in the leaf. tfsweb.tamu.edu explains that this buildup of carbohydrates, together with bright light, drives anthocyanin production.
Once the abscission layer is complete, the leaf is ready to fall and is usually removed by wind or rain. Trees that lack the genes for anthocyanin simply turn yellow, gold or brown; the browns of oak and beech come from tannins rather than from a pigment that forms in autumn.
Why some autumns glow more than others
Fall weather mostly affects the reds. According to extension.purdue.edu, warm sunny days followed by cool but not freezing nights produce the brightest reds, because daylight keeps sugar production going while the cool nights and closing veins keep that sugar inside the leaf. An early hard frost can kill the leaf before its pigments develop fully, and long spells of cloud or rain dull the display by lowering light intensity. Carotenoids are already present, so the yellow display stays more consistent than the red.
The bottom line
Autumn colour is not decoration added at the end of the season. It is a by-product of the tree reclaiming what it can before shedding the leaf: green disappears because it is recycled, yellow was there all along, and red is manufactured on the spot from surplus sugar. Weather decides how vivid the final show is, but day length decides when it begins.