Why (maybe) leaves are yellow in Europe and red in North America

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Ice age Earth at glacial maximum. Based on: &q...

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Supposedly it is autumn – though I have just been standing outside in a teeshirt watching Deneb, Vega and Altair appear out of the twilight as though it was mid-July.

But leaves are turning yellow as the seasonal clock moves on, no matter what the temporary weather is like.

New Scientist offers an explanation as to why leaves in Europe are more likely to be yellow and in North America to be red.

Deciduous trees are an evolutionary adaptation to the ice ages – trees hibernated through the long cold periods, storing nutrients in their bodies as opposed to keeping them in leaves.

As winter approaches the chlorophyll in the leaves diminishes and pre-existing yellow and orange pigments become more prominent. But many plants also manufacture a red pigment called anthocyanin.

Anthocyanin protects leaves for longer and minimises insect damage.

And here’s the interesting bit…

In North America more insect species survived the ice age – they could simply move south when the ice advanced. In Europe ice advanced from both the north and the Alps in the south, so exterminating insect species.

Hence trees native to Northern Europe are adapted to produce less anthocyanin than those from North America.