Autumn Colour: Why Leaves Change and Fall

Every autumn, the woods around us perform a magic trick in slow motion. The English oak that has been a solid mass of green since May starts to glow russet. A field maple on the edge of the copse turns the colour of butter. Then, without fuss, the whole canopy lets go. It looks like decline, but it is one of the most efficient pieces of housekeeping in nature — a deliberate shutdown that gets the tree safely through the winter ahead.

What Happens Inside a Leaf

The trigger isn't really the cold. It's the light. As September turns into October, the nights get longer, and trees in Britain measure that shortening day length with great precision. Once the darkness passes a certain point, a chemical signal tells the leaf to begin dismantling itself.

Chlorophyll, the green pigment that captures sunlight, is the first thing to go. It's a fragile, nitrogen-rich molecule, and the tree can't afford to waste it. So it breaks the chlorophyll down and pulls the nitrogen back into the trunk and branches, where it will sit safely until spring. As the green drains away, other pigments that have been hidden underneath all summer suddenly show through. That's the yellow and orange of carotenoids, which have been in the leaf all along, quietly doing other jobs.

At the same time, a thin corky band of cells — the abscission layer — forms at the base of each leaf stalk. It cuts off the water supply and, eventually, the leaf's own weight and a puff of wind finishes the job.

Why a Beech Turns Copper and an Oak Goes Russet

Different trees produce quite different autumn palettes, and it's worth learning to read them:

• Field maple and birch give the cleanest, brightest yellows, because they rely almost entirely on carotenoid pigments.
• Beech turns a soft copper, and young trees and clipped hedges often hold their dead leaves right through until spring — a habit called marcescence.
• Pedunculate oak is more likely to go brown and tawny than brilliant red, though the leaves turn a handsome bronze before they fall.
• Rowan, wild cherry and guelder rose produce the deep crimson that makes a mixed hedge so good in October.
• Alder and willow, the trees of the riverbank, often stay green late and then yellow or brown without much fanfare.

The reds and purples are a different pigment group altogether: anthocyanins, which are made fresh in autumn from sugars left stranded in the leaf. A crisp, sunny day followed by a cold night traps the most sugar, which is why the best autumn colour usually follows a bright, sharp spell in late October.

Falling Leaves Are a Survival Strategy

A broad, thin leaf is a brilliant solar panel and a terrible thing to carry through a British winter. It loses water constantly, and the ground is often too cold for roots to replace it. Frost would rupture its delicate tissues. So the tree sheds the whole apparatus and goes into a low-energy holding pattern.

But the leaf isn't simply discarded. Before letting go, the tree recovers most of the nutrients it invested. What's left falls to the woodland floor, where it becomes food for earthworms, fungi and a great host of invertebrates — the beginning of the soil that will feed the tree's own seedlings decades from now. Nothing is wasted.

Autumn on the Riverbank

Along our streams and rivers, the same process does something remarkable. Alder, willow, ash and poplar drop their leaves directly into the water, and in a chalk stream that litter is the foundation of the entire food web. Fallen leaves are trapped by twigs and woody debris, where freshwater shrimp, caddis larvae and stonefly nymphs shred them into finer particles. Those invertebrates then feed brown trout, grayling and young salmon parr. A river without autumn leaves is a river running on empty.

It's also a season of arrivals. Redwings and fieldfares come in from Scandinavia to strip hawthorn and rowan berries, joining our resident blackbirds. Woodcock start appearing in damp woodland, and on the wet meadows wigeon, teal and snipe return to the flooded grass. The hedgerow that looked plain in August is suddenly the busiest larder in the parish.

Gardening for Wildlife as the Season Turns

Autumn is when a wildlife garden earns its keep, and the most useful thing you can do is far less tidying than you think.

• Rake leaves onto borders rather than into the bin. A cage of wire netting or a few pierced bin bags in a corner will give you leafmould in a year or two — oak and beech take longer, birch and lime are quicker.
• Leave a leaf pile in a quiet corner. Hedgehogs, frogs, toads and overwintering ladybirds all use them. Never strim a leaf pile without checking it first.
• Build a log pile or dead hedge with prunings from the autumn cut. Fungi, beetles and solitary bees will find it within a season.
• Stand back from the pond. Skim some leaves off a small pond, but leave the sludge and the margins alone — that's where frogs and dragonfly larvae spend the winter.
• Plant bare-root natives between November and March. Hawthorn, blackthorn, hazel, guelder rose and rowan establish best in dormant season and pay you back with blossom, berries and autumn colour.
• Leave seedheads standing on teasel, knapweed and evening primrose. Goldfinches will work them all winter.
• Keep feeders topped up and leave ivy alone — its late flowers and black berries are a lifeline for insects and birds alike.

Making the Most of the Colour

The best way to enjoy this season is to walk the same stretch of woodland or riverbank a couple of times a week and watch the order in which things turn. Note which tree changes first, which holds its leaves longest, and where the leaf litter gathers in the current. Keep a rough record in a notebook and you'll soon be able to predict the peak colour in your own patch, often within a week or so.

It's worth remembering, too, that the tree is not dying. It is closing down, tidying up, banking its resources and settling in for the cold. Everything it drops this month becomes next spring's soil, next summer's mayfly, and next autumn's colour. Few things in a British garden or wood are quite so generous with their endings.