Autumn beauty – It’s all down to chemistry

What causes the natural crescendo of autumn beauty each year? Where do these colours come from? We look into the science of autumn leaves.

Autumn can seem a bitter-sweet season. The days are shorter, the weather is growing colder. But oh, how the trees are giving us a technicolour display to warm the heart.
Yellows, oranges, reds, browns and, yes, still some greens adorn the wooded skyline.

… hidden by the greens – until autumn

It’s a time of outstanding beauty, when the natural world treats us to a last burst of colour before the onset of winter. But where on earth do all these colours suddenly come from and why do they differ from one tree to another? Well, it’s down to chemistry.

The familiar green of summer is caused by chlorophyll – the agent that converts water and sunlight into energy for growth. But the leaf requires a constant supply of chlorophyll to maintain this process. Without it, the sunlight soon uses it up. In autumn, lower light levels reduce the need for photosynthesis, and so the supply of chlorophyll from the tree to the leaf is shut down.

As the chlorophyll dies away, it’s time for other pigments – the xanthophylls (yellows) and carotonoids (oranges) – to have their moment in the sun. Perhaps surprisingly, these rich colours have been there all  the time, but hidden by the greens – until autumn.

In preparation for leaf-fall, a corky deposit builds up between leaf and tree. The resultant concentration of sugars in the leaf produces the reddy purple anthocyanin pigment to compliment the array.

Each tree species contains its own mixture of pigments. It’s the relative dominance of one pigment over another that will determine a leaf’s colour. In this way, for example, the yellow of birch leaves contrasts with the deep red of maple or the orange of beech.

… hearten yourselves as you head inside…

The autumn display is influenced not just by the blend of chemical processes, but also by the weather. For the greatest variety and intensity of colours, sunny, dry autumn days with cold, but not freezing, nights are best. Cloudy, wet days, on the other hand, lead to muted autumn colours.

Regardless of weather, most of the pigments eventually break down, leaving the soul survivor – the muddy brown of tannin. But frost, wind and rain hasten the eventual curtain call to the show. The leaves fall and the tree is laid bare for the months ahead.

But hearten yourselves as you head inside to shelter from the chill. Whatever winter throws at us, beneath the surface, new life and new leaves are already in the making.