Why do leaves change colour in autumn?

Uncover the science behind the changing season and discover why leaves change colour in this magical season. 

Phase 1: Green

The green leaves you see throughout summer already contain most of the colours of autumn! Throughout most of the year the colours of autumn are ‘covered’ by the dominant green chlorophyll.

Chlorophyll helps trees to absorb the energy of the sunlight - the tree is essentially 'charging' during the summer months. Chlorophyll changes carbon dioxide (CO2) + water (H2O) into sugars which 'feed' the tree.

Beech tree leaves close up

Why is chlorophyll green?

Chlorophyll is green because it allows trees to absorb blue and yellow wavelengths from the light, but not the green which is reflected back. 

Woman using phone app in Autumn

Phase 2: Turning yellow

As the weather cools in autumn, the leaves start to receive less sunlight.

Less sunlight triggers chlorophyll to break down (and the tree does not replenish it) revealing the new, colourful pigments underneath and treating us to beautiful autumn yellows and oranges.

The yellows and oranges which were previously masked by chlorophyll are called xanthophylls and carotenoids. These pigments are present year-round in the leaves, but are usually masked by the green chlorophyll.

Enter your best shots in the autumn photo competition with #AutumnLeafWatch

Phase 3: Fiery reds

In some species and in certain special years, you'll see another stage of colour as the leaves turn a vibrant red. This is caused by anthocyanins.

These reds are produced in years when lots of sunlight and dry weather have increased the concentration of sugar in tree sap, triggering the tree to release anthocyanins in an attempt to grab the last of the energy from its leaves, powering it up to get through the winter. Interestingly very cold weather, acidic soils, and other stress factors occasionally trigger higher levels of anthocyanins to be produced.


Autumn Maple Leaves
Photo credit: Paul Box
Crunchy autumn leaves

Phase 4: The fall

At the end of this colourful process, leaves die. Trees shed their dead leaves because they are no longer able to get energy from them through winter. By shedding their leaves, trees are able to survive because they make room for new growth in the spring.

Trees are great at recycling! Fallen leaves contribute to the health of the forest after they fall, through decomposition. Though they have been released by the tree, some carbohydrates and sugars remain in the leaves. On the forest floor, these carbohydrates and sugars give nutrition for 'feeders' to flourish. Once fungi and bacteria have begun to break down the leaves, insects get in on the dinner party too.

Once the leaves are broken down, they provide a source of fibre which helps the soil retain moisture and improve drainage.

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