Why do some leaves change colour in autumn?

Green hides the other colours

A leaf is green because it is full of a pigment called chlorophyll. A pigment is a substance that gives something its colour. Chlorophyll catches energy from sunlight. The leaf uses that energy to make sugar, which is food for the tree.

But green is not the only colour inside a leaf. Leaves also hold yellow and orange pigments called carotenoids. They are the same kind of pigment that makes a carrot orange. All through spring and summer there is so much green in the leaf that it hides them.

When the days get shorter

In autumn the days get shorter and the weather gets cooler. Some trees get ready for winter by letting go of their leaves. They are called deciduous trees. First the tree switches off its food factory. The leaf stops making new chlorophyll, and the old chlorophyll breaks down. The green fades away. The carotenoids were there all along, so now we can see the yellow and orange.

The tree does not waste what is in its leaves. It takes useful materials back into the rest of the tree, to use again in spring. Then a special layer of cells grows across the bottom of each leaf stalk and slowly cuts the leaf off. The wind blows the leaf away, or it drops by its own weight.

You may have heard that Jack Frost paints the leaves. That is a myth. Some leaves begin to change before there has been any frost at all.

Where do the reds come from?

Yellow and orange were hidden in the leaf all along. Red is different. The red pigment is called anthocyanin, and some leaves make it new in autumn. It is made from sugar that gets trapped in the leaf. Sunny days help the leaf to make lots of sugar. Then cool nights stop the sugar from leaving. So a run of sunny days and cool nights gives the brightest reds.

Why would a leaf make a new pigment just before it falls? Scientists are still testing ideas. One leading idea is that the red works like sunscreen. While the tree is taking nutrients back out of the leaf, the red pigment helps to protect the leaf from too much light.

Different trees make different mixes of colours. Ginkgo leaves turn golden, and sweet gums can turn red. Weather matters too. A warm, cloudy, rainy autumn gives less red. Even on one tree, leaves in full sun may turn red while the leaves in the shade turn yellow.

Why are most Australian trees still green?

Now look at a gum tree. A eucalypt does not go bare in autumn, because it is evergreen. That means it always has leaves. It drops old leaves a few at a time, and it grows new ones. Nearly all of Australia's native trees are evergreen.

Gum trees do not follow the autumn calendar. Scientists measured how much litter, mostly leaves, fell in eucalypt forests and woodlands in Queensland and New South Wales, and in forests of tall mountain ash trees. In those studies, the most fell in summer or early autumn. So the dry leaves you scuff through under a gum tree in January are not really an autumn story.

A few native trees do lose their leaves, but usually for other reasons. Red cedars, white cedars and some native figs lose leaves when the weather is dry. Flame trees can lose theirs just before they flower. Many of these trees save their best colours for new growth. The new leaves of the Australian red cedar are a beautiful red in spring.

The fagus, and autumn colour in cool places

Only one Australian native tree puts on a true autumn show in response to the cold. It is the fagus, or deciduous beech, which grows in the mountains of Tasmania. It is Tasmania's only winter-deciduous tree. Its crinkly leaves turn from gold to orange to rust red. The colours peak around Anzac Day, 25 April, and stay bright for about a month. Visitors flock to Lake Fenton and Tarn Shelf at Mount Field National Park, and to Dove Lake at Cradle Mountain.

Most of the autumn colour in Australian towns comes from introduced trees. They come from cooler parts of the world, and people have planted them in parks, streets and gardens. You can see them in autumn in places such as Canberra, the Blue Mountains and Albury. In the Blue Mountains, the best colour is usually in April and May.

The weather has to be right. In Sydney's mild autumns, many of the best deciduous trees do not grow well. In Brisbane, it rarely gets cold enough for them to put on much of a show.

These trees do a useful job as well. In Canberra, where summers are hot, a deciduous tree gives shade in summer. When its leaves fall in autumn, the low winter sun can warm the house and garden again.

📚 Words to know

Chlorophyll

The green pigment in leaves. It catches energy from sunlight so the leaf can make food.

Carotenoid

A yellow or orange pigment in leaves. It is hidden by the green all summer, and it is also what makes carrots orange.

Anthocyanin

A red or purple pigment that some leaves make in autumn, from sugar trapped in the leaf.

Deciduous

A tree that loses all its leaves for part of the year, usually in autumn or winter.

Evergreen

A tree that always has leaves. It drops old leaves a few at a time and grows new ones.

🧪 Try this: be a leaf detective

1. Find two trees near your home or school. One should be a gum tree, or another native tree that stays green. The other should be a deciduous tree, such as a maple, oak, ash, ginkgo, sweet gum or ornamental pear. Ask a grown-up to help you if you are not sure which is which.
2. Look at the ground under each tree. Collect up to five fallen leaves from each, but only pick up leaves that are already on the ground. Do not climb the trees or pull leaves off them. Wash your hands afterwards.
3. Make a colour chart. Stick or draw each leaf, and write down every colour you can see: green, yellow, orange, red or brown. Which pigment do you think made each colour? Remember that carotenoids make yellow and orange, and anthocyanins make red.
4. Make leaf rubbings. Put a leaf on a table with its underside facing up, because the veins stick out more there. Lay a thin sheet of paper on top, hold it still, and rub the side of a crayon across it. Compare the shapes of the gum leaf and the deciduous leaf.
5. Challenge: from March to May, check the same deciduous tree once a week and draw its leaves. When do the first leaves change? Do the leaves in the sun change before the ones in the shade? Check a gum tree every week as well. Does it ever go bare?