Autumn Leaves

Autumn Leaves

Autumn leaves (c) Jon Hawkins - Surrey Hills Photography

Our best natural autumn spectacle is the changing colour of leaves in our woodlands. But why does this happen?

A popular American autumn pastime is ‘leaf peeping’ – travelling to witness and photograph the glory of the New England woodlands in the fall. It’s a horrible phrase, but a fabulous experience.

Here in Britain we may not be able to boast the expanses of deciduous forest they have in the States, but there’s still plenty of glorious glowing autumn colour around for us to wonder at. If you’d like to know the whys and hows, read on.

Beech trees in autumn

Beech trees in autumn (c) Don Sutherland

Leaves are a tree’s way of collecting energy from the sun. They’re green due to the chemical that allows them to use the sunlight to convert carbon dioxide and water into carbohydrates – the building blocks for growth.

All through spring and summer the tree benefits from daylight and warm weather, but as autumn draws on the days get shorter and the weather colder and windier, so the tree has to get ready for winter.

Leaves are not frost-hardy – if the water inside the cells freezes, the cells rupture, much like a frozen lettuce leaf, leaving the tree covered in useless, unproductive leaves. To avoid such damage and loss, the tree withdraws all the useful stuff from its leaves and then sheds them. Thus it actively protects itself from frost damage and the danger being blown down as winter gales catch the leafy canopy like a ship in full sail, ready for a fresh start with new, undamaged leaves in the spring.

How exactly do they do it?

Autumn leaves don’t just get blown off the tree. Triggered by the shortening day length, the tree produces hormones which start the process of abscission – literally, ‘cutting off’. Chlorophyll production ceases, and the vessels carrying water to the leaf and sugars to the trunk close. A layer of cells, known as the abscission layer, develops between the leaf stalk and the twig, and as it gradually weakens the tree reabsorbs any nutrients from the leaf before the leaf drops.



The loss of chlorophyll in a leaf allows the yellow pigment xanthophyll, already in the leaf, to show through. Red and purple pigments – anthocyanins – are produced when sugars remain trapped in the leaf. These act as a temporary sunscreen, blocking out harmful radiation, and as antifreeze, protecting the leaves until their nutrients have been fully absorbed and they’re ready to fall.

Eventually the leaves turn brown, losing all pigment and showing the naked colour of the cell walls.

English oak acorn and fallen leaves

English oak acorn and fallen leaves (c) Ross Hoddinott/2020Vision

Leaves in autumn

Gillian Day

So that’s the hows – what about the whys? Why do different species have different autumn colours? Colour is partly climate related. As chlorophyll is destroyed by low temperatures, frosty nights encourage yellow leaves.

The red pigments, however, are weakened by cold, and more prevalent when the weather is dry and the temperature above freezing. Another possible factor is evolutionary. It has been suggested that red colouration wards off insect pests which would eat more appetising yellow leaves.

Japanese Maple

Japanese Maple

When trees first developed the deciduous habit, around 35 million years ago during a period of ice ages and dry periods, they also began to develop the red pigmentation against insect attack. In North America, the geography of the continent allowed ‘migration’ south to avoid the extremes of the ice ages as the climate fluctuated. As the trees ‘moved’ to southern refugia, so did their insect predators.

However, in Europe the mountains tend east to west rather than north to south, so no available protected areas were created and any tree species that couldn’t survive the extreme cold died out, along with their dependent insect predators.

Japanese Maple

Japanese Maple

At the end of the repeated ice ages, most tree species surviving in Europe no longer had need of the red pigments as their insect pests had become extinct.

The exception, which adds weight to this theory, is in the dwarf shrubs such as dwarf birch, bilberry and bearberry growing in Scandinavia and northern Europe, whose leaves still turn red in autumn. These managed to survive the ice ages under the snow layer, along with those pesky insects, so the battle goes on.

Most of the truly red leaves we see in autumn are on non-native trees from America and Japan, such as maple, liquidamber (sweet gum), tupelo (black gum) and dogwood, but a few natives also put on fiery hues.

Rowan and Guelder-rose both develop bright orange-red foliage to complement their red berries and are a splendid choice for a colourful autumn wildlife garden.

Autumn leaves

Autumn leaves (c) Jon Hawkins - Surrey Hills Photography

Don’t forget to make use of those lovely leaves when they fall – leafmould is an invaluable soil conditioner.

Kept for over two years it can be used for seed sowing or in a mix as potting compost, but even poor quality or less rotted leafmould is a great mulch or top dressing, or can be dug in to improve soil structure. Best of all, it’s absolutely free, and recycles the transient glories of the autumn trees into the lasting glories of a healthy garden. While it decomposes, leafmould offers a wonderful habitat for worms, snails, spiders and all those tiny decomposers and a hibernaculum for reptiles and amphibians too.

Make a bit of space for the golden wealth of autumn in your garden!

Dr Cath Price

Engagement Officer, Shropshire Wildlife Trust