Cosy season is upon us as we have bid farewell to summer for now, with the temperatures beginning to drop.

Over the next few weeks, you might notice the endless amount of leaves falling to the ground as they turn from green and yellow to red and brown.

But why do they change colour and what causes one of nature's annual processes? Let’s find out.

What causes the leaves to change colour in autumn?

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The main reason leaves change colour in autumn is because as the weather becomes colder, the leaves begin to receive less sunlight, explains Forestry England.

The website adds: “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.”

However, there are four phases when it comes to leaves changing colour according to the nature experts, such as:

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.

Glasgow Times: Chlorophyll helps trees to absorb the energy of the sunlight Chlorophyll helps trees to absorb the energy of the sunlight (Image: Getty)

“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.”

Phase 2 – yellow

“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.”

Phase 3 – red

“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.

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“Interestingly very cold weather, acidic soils, and other stress factors occasionally trigger higher levels of anthocyanins to be produced.”

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Phase 4 – brown

“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.”

Forestry England adds: “Once the leaves are broken down, they provide a source of fibre which helps the soil retain moisture and improve drainage.”