Why Do Leaves Change Colour in Autumn?

Green leaves turning yellow, orange and red is one of the most noticeable signs that autumn has arrived.

 

But why do leaves change colour? How does a tree know that autumn is beginning, and why do some trees stay green all year?

 

In this blog, we’ll take a closer look at the science behind autumn leaves and explore what happens to plants and animals as the seasons change.

 

 

Why are leaves green?

During spring and summer, most leaves contain plenty of a green pigment called chlorophyll.

 

Chlorophyll helps plants absorb energy from sunlight. The plant uses this energy, along with water and carbon dioxide, to make the sugar it needs for food. This process is called photosynthesis.

 

Although leaves contain other pigments too, the green colour of chlorophyll is usually so strong that it hides them.

 

 

What tells a tree that autumn has arrived?

As autumn approaches, the days become shorter and the nights become longer. Trees can detect this change in daylight.

 

Falling temperatures can affect when leaves change colour, but shorter days give the tree its most reliable sign that autumn has arrived.

 

The tree begins preparing for winter. Shorter days mean that its leaves receive less sunlight, so they cannot make as much food. Leaves also lose water to the air. In winter, it can be difficult for the tree’s roots to replace this water, especially when the ground is cold or frozen.

 

The leaves are no longer as useful to the tree, so it begins getting ready to let them go. It gradually stops producing chlorophyll. As the chlorophyll already inside each leaf breaks down, the green colour begins to disappear.

 

 

Where do the yellow and orange colours come from?

Once the green fades, other coloured substances inside the leaf become visible.

 

These yellow and orange pigments are called carotenoids. They are present in leaves throughout much of the year, but the stronger green colour of chlorophyll usually hides them.

 

Carotenoid pigments are also responsible for the colours of foods such as carrots, pumpkins and sweetcorn.

 

 

Why do some leaves turn red?

The red and purple colours seen in some autumn leaves come from pigments called anthocyanins.

 

As a tree prepares to shed a leaf, it begins closing the tiny tubes that carry water and sugar between the leaf and the rest of the tree. On a bright autumn day, the leaf may still use sunlight to make sugar. However, the sugar cannot travel out of the leaf as easily as before, so some of it becomes trapped.

 

In some types of tree, the leaf uses this trapped sugar to make anthocyanin pigments. These pigments turn the leaf red or purple.

 

The colours we see depend on several factors, including the type of tree, the weather and the conditions in which it is growing. This is why one autumn can appear especially colourful while another looks less dramatic.

 

 

 

Why do leaves eventually fall?

Before dropping a leaf, a tree takes back some of its useful nutrients and stores them in its branches, trunk and roots.

 

While the leaf is still attached, the tree begins forming a special layer of cells at the point where the leaf stalk joins the branch. This is called the abscission layer.

 

As this layer develops, it gradually blocks the movement of water, nutrients and sugar between the tree and the leaf. It also weakens the leaf’s attachment to the branch. Eventually, the leaf is held in place so loosely that wind, rain or its own weight can cause it to fall.

 

At the same time, the tree seals the area beneath the leaf. This helps protect it from water loss, disease and winter weather.

 

 

 

Why do some trees stay green?

Not every tree loses all its leaves in autumn.

 

Most evergreen trees keep each leaf for more than one year. They still shed old or damaged leaves, but they lose and replace them gradually rather than dropping them all at once. Because an evergreen always has some living leaves, it continues to look green.

 

Conifers such as pines and firs often have narrow, needle-shaped leaves with a tough, waxy coating. Their small surface area and protective coating help reduce water loss and protect them in cold conditions.

 

An evergreen tree and a deciduous tree in the Autumn. 

 

 

What happens to fallen leaves?

A fallen leaf may look like waste, but it remains an important part of the woodland ecosystem.

 

An ecosystem is a community of living things and the environment in which they live.

 

Fallen leaves form a protective layer over the soil and provide shelter for small animals. Fungi, worms and other decomposers gradually break them down, returning nutrients to the soil. Those nutrients can then support new plant growth.

 

The leaves that helped a tree grow during summer eventually help support new life around it.

 

 

How do animals respond to autumn?

Trees are not the only living things responding to seasonal change.

 

As temperatures fall and food becomes harder to find, animals use different strategies to survive:

• Some birds migrate to places with warmer weather or more plentiful food.
• Animals such as hedgehogs prepare for hibernation.
• Squirrels collect and store food for the colder months.
• Some animals grow thicker coats or change their feeding behaviour.
• Insects may survive winter as eggs, larvae, pupae or dormant adults.

 

These behaviours are examples of how living things adapt to seasonal conditions.

 

 

 

Investigate autumn leaves

Learning about autumn leaves can lead to lots of interesting discussions and activities in the classroom. If possible, take your class into the school grounds or visit a local park and ask them to look carefully at the trees and leaves around them.

 

Questions could include:

• Can you find leaves in different stages of colour change?
• Do all the leaves on one tree change at the same time?
• Which colours can you spot most often?
• Can you find any evergreen trees?
• Where have the fallen leaves collected?
• Can you see any signs that animals have visited?
• What do you think this place will look like in winter or spring?

 

Encourage pupils to look up into the branches, examine the ground beneath the trees and compare leaves from different places. Remind them to collect only leaves that have already fallen.

 

 

Create an autumn leaf report

One simple activity is to ask each pupil or group to choose a tree to investigate. They can record what they discover in an autumn leaf report.

 

Their report could include:

• a drawing or photograph of their chosen tree;
• examples of the leaf colours they found;
• a tally chart or graph showing the number of leaves of each colour;
• notes about where fallen leaves had collected;
• any signs of animals living in or visiting the tree;
• a prediction of how the tree might look later in the season; and
• an explanation of why its leaves are changing colour.

 

If possible, return to the same tree once a week and ask pupils to update their reports. They can compare their observations, look for changes and find out whether their predictions were correct.

 

The finished reports could also be displayed together to create a record of how autumn changed the trees around the school.

 

 

Final thoughts

The changing colours of autumn leaves reveal how trees respond to daylight, temperature and the approach of winter.

 

Looking more closely at these changes can help children understand photosynthesis, plant survival, animal behaviour and the connections between living things and their environment.

 

 

Want to explore more?

Changing leaves are just one way to explore how living things respond to the world around them.

 

At Little Moose, we have Plants and Animals resources across Early, First and Second Level, helping learners explore topics such as plant growth, life cycles, habitats, adaptation and seasonal change.

 

Take a closer look at our Plants resources and continue exploring the science behind the natural world with your class.