A planned demolition
Why Do Leaves Change Colour?
Autumn colour is a tree dismantling its own leaves on purpose, before the cold arrives.
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Step 1 of 5
Night length is the signal, not frost.
Trees track how long the darkness lasts, which is the same every year regardless of weather. That is why a warm autumn still turns on time.
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Step 2 of 5
A leaf costs money in winter.
Broad leaves lose water fast and cannot photosynthesise in dim, freezing weather. Keeping them would cost more than they earn.
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Step 3 of 5
The green pigment is valuable, so it is reclaimed.
Chlorophyll contains nitrogen, which is scarce. The tree takes it apart and pulls the pieces back into its branches for spring.
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Step 4 of 5
Yellow was there all along; red is brand new.
The yellows and oranges sat hidden under the green all summer. The bright reds, though, are pigments the tree actually manufactures in autumn.
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Step 5 of 5
Then the tree cuts the leaf loose.
A weak layer of cells forms across the leaf stalk and seals the wound behind it. The leaf does not fall off — it is released.
Sunny days and cool nights make the reds brightest, which is why some autumns are far showier than others.
The short version
Longer nights tell a tree to reclaim its chlorophyll and cut its leaves loose, leaving hidden yellows and freshly made reds behind.
Try it yourself
Tape black paper over part of a leaf still on the tree in early autumn. The covered part changes colour before the rest.
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