Editor, Earth News
Autumn displays produce a variety of hues
The swathes of beautifully coloured leaves produced by trees each autumn are one of nature's great displays.
But why trees in Europe tend to produce yellow leaves, while those in America and eastern Asia produce red leaves, has remained an enigma.
Now scientists have published a theory that could explain the difference.
They believe that red-leafed trees in America and Asia only exist because they and their insect pests managed to survive a series of ice ages long ago.
So say botanists Simcha Lev-Yadun of the University of Haifa-Oranim, Israel and Jarmo Holopainen of the University of Kuopio in Finland, who have published a review of leaf colour and its causes in the journal New Phytologist.
Until a decade ago, scientists understood very little about why trees produce leaves of different colours, and at different times of the year.
Then came a surge of interest into why trees in temperate forests produce a range of hues each autumn. For example, the mixed forests of New Jersey turn predominately red, as do maple trees in Japan, while beech forests in Patagonia turn red and orange.
Yet the forests of Europe turn predominately yellow.
Leaves usually look green because they contain higher levels of cholorophyll relative to their other pigments.
But leaves do not turn red or yellow because they are dying.
Instead, trees produce less chlorophyll in autumn, so the yellow and red carotenoid pigments already in the leaf come to the fore. At this time of year, the trees also produce more anthocyanins, a red-purple pigment.
When the leaves start to die, they turn brown.
Some forest turn yellow not red
There are thought to be various reasons for the colour change.
One idea is that the anthocyanins protect the leaves against the damaging effects of light, which are worse in the autumn as canopies become thinner and the plants fix less carbon due to falling temperatures.
That would allow the trees to keep healthier leaves, and to withdraw more nutrients from them before they drop. But the evidence for this remains inconclusive.
Another idea is that different coloured leaves fend off insects that might otherwise eat them.
Deep red leaves might signal to insects, such as aphids, that the leaves contain higher concentrations of toxic chemicals, lack nutrients or may shortly die.
And evidence for this idea is stronger.
One review of 262 tree species showed that those with red colours have a long evolutionary history with aphids. The insects try to lay eggs on the trees in autumn and the trees respond in an evolutionary arms race, signalling ever more strongly that their leaves are unappealing.
But these ideas cannot fully explain why Europe has more trees with yellow leaves than in America or eastern Asia.
Lev-Yadun and Holopainen have proposed that the trigger for the difference first occurred millions years ago in the Tertiary Period.
They suggested that temperate forests evolved from tropical plants during this time. Many young tropical trees produce red leaves they said, and old leaves in the tropics are also often red-coloured.
So temperate forests inherited this ability to produce red leaves.
Tourist cars flock to see Japan's autumn trees
But 35 million years ago, the world started to drastically cool and warm, as a series of ice ages covered large swathes of Europe and North America. Plants could only survive in a few southerly refuges.
North America and East Asia have mountain ridges that run north to south. As each ice age took grip, red-leafed trees migrated south along the mountain ridges into refugia where they survived, before migrating back north as the climate warmed.
In Europe, the mountain ranges run east to west. That meant that any red-leafed trees north of the mountains were trapped, and went extinct.
The insects that had been engaged in a long evolutionary arms race with these trees also died out with them.
"The selective agents of herbivory that cause red leaves went extinct, resulting in lower selection for red leaves," said Lev-Yadun.
That broke the evolutionary arms race between insects and the trees, allowing those trees that recolonised Europe to put less effort into deterring aphids from their leaves.
As a result, European forests tend to produce glorious swathes of yellow each autumn.
"Red in a way is an old adaptation reflecting stresses from the Tertiary herbivorous fauna," said Ley-Yadun.
Further support for Lev-Yadun and Holopainen's hypothesis is that dwarf shrubs with red leaves, rather than trees, dominate the northern parts of Scandinavia.
That suggests that trees in the area were wiped out by snow and ice, which the low-lying shrubs could survive.
As the trees were wiped out, so were their pests - a luxury not afforded to the generations of plants in America and East Asia.