Red Leaves or Yellow Leaves in Autumn?

The BBC today highlights an interesting new theory to explain why trees in Europe tend to produce yellow leaves in autumn, whilst those in America and eastern Asia produce red leaves.

Various theories for why leaves change colour have been proposed over the last decade, although the evidence for many has been inconclusive. One suggestion is that different coloured leaves serve to deter insect predators. Red leaves may signal to potential predators that the leaves contain higher concentrations of toxins, or that they are about to die. Indeed, one review of 262 tree species showed that those with red colours had a long evolutionary history with aphids, which try to lay their eggs on the trees in autumn.

This does not however explain why Europe has more trees with yellow leaves, but a review of leaf colour and its causes by two botanists suggests that the series of ice ages which began in the Tertiary Period may form part of the answer. They suggest that all temperate forests initially had the ability to produce red leaves, having evolved from tropical trees which frequently produce such a colour. However, from about 35 million years ago, the series of ice ages which covered much of Europe and North America meant that plants could often only survive in southern refuges. Whereas North America and East Asia have mountain ridges that run north to south, allowing red-leafed trees to migrate south into refugia where they survived, in Europe, the mountain ranges run east to west. This meant that any red-leafed trees north of the mountains were trapped, and went extinct. Moreover, the selective agents of herbivory that caused red leaves went extinct too, resulting in lower selection for red pigments amongst the trees which recolonised Europe.

Further support for this hypothesis comes from the fact that dwarf shrubs with red leaves, rather than trees, dominate the northern parts of Scandinavia. Such low-lying shrubs may have been able to better survive the cold conditions than the trees above them.

For further information please see:
Lev-Yadun, S., & Holopainen, J.K., ‘Why red-dominated autumn leaves in America and yellow-dominated autumn leaves in Northern Europe?’