Why do leaves fall off trees?

There’s no denying it, as much as you may want to - summer is officially over.

Whilst that means long days in the sunshine may be behind us, another spectacle of the season is in full flow. It’s autumn, and that means the leaves on the trees are turning beautiful shades of yellow and red, before falling off entirely.

Why does this happen?

A small dog jumping in leaves falling off trees
It provides cheap entertainment for people and pets.

It’s a way of protecting the trees, and making sure they survive the winter and beyond. Leaves die during the winter months. If those dead leaves stayed on the trees, and new, working ones didn’t grow in their place, the trees would have no way of processing food for themselves.

Why do leaves change colour?

A country road surrounded by trees in Autumn
The colours are absolutely stunning.

When leaves are green they are working as organs for the tree. The green colour comes from a part of the cell called chlorophyll, which processes sunlight into sugars that the tree can ‘eat’. As the weather gets colder, the days get darker, and there is less sunlight for them to munch on, the rest of the tree starts to absorb these useful parts of the leaves. It then stores them for winter in its roots. As the tree does this, the green chlorophyll is removed, which reveals hidden colours in the leaves.

The first colours to appear are yellow, then they turn clear too, revealing the gorgeous red-brown that dominates most of the autumnal months.

Here's a quick run down of why leaves change colour.

So then why do they fall?

Simply, trees actively shed their leaves because there’s no use for them anymore. Doing this also ensures the tree’s long-term survival. If it didn’t do it every year, the water in the cells of the leaves would freeze, and this would rupture them, as water expands as it turns to ice.

By the time summer rolled around, the tree would have no working leaves, so no means to create food, and it would die pretty quickly. Also, by the time summer ends, most leaves have either been eaten by bugs or have decayed to some extent, so this gives them a chance to start fresh. To turn over a new leaf, if you will.

But Christmas trees stay green all year round - why’s that?

Fir trees in a line covered in snow
Don’t you think a few baubles would liven these up?

Some trees are what are known as ‘evergreens’. As the name suggests, that means their leaves are green all the time and they don’t fall off. The reason for this is trees such as firs have a thick, waxy coating called a resin that protects their leaves. This means they don’t freeze and rupture like other trees’ leaves would when they get cold, so they don’t need all of them to fall off and regrow in time for summer. Some do fall off sporadically throughout the year though, and some trees called semi-evergreens only shed all their leaves when it’s cold enough. If the winter months are mild, they stay green.

Wait a second - I’m constantly cleaning up needles from my Christmas tree in December. So they do fall off!

That’s because the tree has been cut down and cannot uptake water anymore. As the tree loses water, the leaves, or needles, become unable to create food anymore, so they fall off.

So, leaves always fall. There’s no getting around it. And now you know why - but it’ll never stop being annoying when you can’t hoover them out of your carpet.

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