Why do leaves change colour in autumn?

Why do leaves change colour in autumn?

It’s officially the start of autumn! Many of us were left feeling confused during the summer when leaves started dropping everywhere. This was ‘summer leaf drop’, also known as ‘false autumn’ which is a tree’s natural response to the hot and dry weather. Trees react to their environment, so when and why leaves colour change and drop depends on many different things. So why do we experience a kaleidoscope of colour in autumn? Forestry England explains:

For trees to grow, they need sunlight, nutrients, and water. The nutrients and water come from the soil. The sunlight is captured by the leaves.

In spring and summer, to capture the sunlight, the leaves use a chemical called chlorophyll, which is what makes leaves green. Chlorophyll turns sunlight into food, which the trees need to grow, through a process called photosynthesis. The food they make is sugar, which they use to grow new leaves, flowers and seeds.

As well as chlorophyll (greens), leaves contain two different types of pigment. Carotenoids (oranges), xanthophylls (yellows) and most of the year these colours are masked by great amounts of green colouring.

As autumn arrives the days get shorter, it gets cooler and there is less sunshine. With winter on the horizon, trees start breaking down the chlorophyll (green) that is stored in the leaves to get the nutrients out. This allows trees to store the nutrients from their leaves in their roots, which are protected from the cold during the winter months.

As the plants break down the chlorophyll, the green colour disappears from their leaves. What is left behind are other pigments which you normally cannot see, the carotenoids and xanthophylls. Depending on which pigments are found in the leaf, they can turn different shades of yellow or orange or even red.

Anthocyanins (reds) form in some trees in autumn when there is a concentration of sugar in the sap. The more sunlight a leaf is exposed to, the more sugar is produced; the more sugars that accumulate in the leaf, the more anthocyanins are produced, and the redder the leaf becomes. 

The weather plays a significant role in the duration of the magical colour of autumn and with climate change, the milder autumn weather experienced allows leaf colour to persist well into November. However, heavy wind or rain can cause the leaves to fall before they fully develop full colour. If we experience an early frost, it can cause the leaves to die or fall off before they’ve even turned colour.

This year the public will be able to view the colour transformation - from green to golden - as it is happening by using Forestry England’s interactive map. This will be displayed on their website throughout autumn, allowing you to find the best colour in your area.