Autumn riddle solved

Seeing red: Scientists solve riddle of why autumn leaves fall off and are different shades in Europe and North America

By Rob Waugh/ Daily Mail Science And Tech

Last updated at 12:59 AM on 1st October 2011

Leaves changing colour and then floating to the ground before winter sets in is a familiar and beautiful autumn sight.

And captured here is the contrast between birch trees (yellow), beech (red) and evergreen pines in a forest near Gdynia in Poland's Pomerania.

But scientists now believe they have answered the question of why plants might expend energy producing red pigment in leaves only for them to fall off and why they tend to be a brighter red colour in North America than in Europe.

As leaves die, the chorophyll inside them - used by the plants to photosynthesise - diminishes. Yellow and orange pigments become visible. But some plants actually produce a new pigment called anthocyanin - a vivid red colour, seen here in the beech trees on the outside of the picture.

A view over a lake in a forest with trees changing colour in Autumn in Kashubia. The contrast between the evergreens and deciduous trees shows how different plant families evolved to cope with local conditions

A new study has found trees evolved to become deciduous after a series of ice ages and dry spells and their leaves were turned red to help protect against insects that attacked them.

But the key difference in North America was as the ice age went on plants and insects were able to migrate fairly freely while in Europe the plants became trapped in ice, along with herbivorous insects, meaning the majority of these bugs largely died out.

So according scientist Simcha Lev-Yadun of the University of Haifa, Israel. 'The anti-herbivore component in red leaf colouration was relaxed, and Northern Europe became dominated by trees with yellow autumn leaves,' but North American trees continued to need the bright red to fight the insects, he says.

A person walks past the trees at Westonbirt Arboretum, near Tetbury, Gloucestershire, where the leaves are beginning to change to their Autumn colours.

A dogwalker passes trees in the same area, which will turn from red to yellow quickly compared to U.S. trees

It seems nonsensical that a plant might 'decide' to spend energy on a pigment for leaves that are about to drop off. But a series of recent scientific papers provide the answer, according to an essay, published in New Scientist this week.

The pigment isn't an unnecessary 'expense' - in fact, for some plants, it's an essential tool for survival. It also 'explains' why the changing of the seasons can have markedly different colours depending on where in the world you are.

William Hoch of Montana State University found that the pigment had a function - it allowed the plants to send nutrients to the roots for winter. If they 'blocked' plants from producing it, their leaves became vulnerable to sunlight.

The reason for this was revealed in a study of sweetgum and red maple by a team in North Carolina, led by Martha Eppes. She found that leaves in nutrient-poor soils tend to be redder - to shield leaves for longer, and allow them to 'keep doing their job' of sending nutrients to the roots.

This American tree stays a distinct red for longer as it fights insects not really found in Europe.