Nature Walks Made Fun

Why Do Trees Lose Their Leaves In The Autumn?

Last week, we discovered how and why leaves change their colour in the autumn - giving us all those beautiful reds, golds and orange leaves. But, have you ever wondered why trees let go of those leaves? Why don’t they just change colour, stay on the tree, then turn green again in the spring? In this tutorial, we will find out why trees lose their leaves.
why do trees lose their leaves in the autumn?

Topic Overview

  • Why do trees lose their leaves in the autumn?
  • Trees shed their leaves in response to the lengthening nights and cooler days of autumn.
  • Leaf drop is triggered by chlorophyll, auxin and ethylene.
  • It starts by the growth of an abscission layer between the leaf stalk and its twig. 
  • This layer cuts the leaf off from the rest of the tree, so it no longer has food or water. It also weakens the cells in that area.
  • Eventually, the leaf attachment weakens so much, it detaches and falls from the tree.

Are you ready to learn more? Let’s go!

Why do trees have leaves?

To understand why trees drop their leaves, we must first understand why trees have leaves.

As we discussed in our tutorial, why leaves change colour, leaves are effectively the kitchen stoves of the tree. They capture and use solar energy to make up food for the tree. This process is called photosynthesis and happens because of  a pigment called chlorophyll in the leaves.

Chlorophyll is only found in the trees’ leaves. So, the leaves have evolved to help the chlorophyll capture as much sunlight as possible, by being thin and flat. This helps provide lots of energy for the tree.

What triggers autumn leaf loss?

In the autumn, the lengthening nights mean less sunlight exposure, and less work for the chlorophyll. Producing chlorophyll takes up a lot of the tree’s energy, so, it stops producing it.

The absence of chlorophyll, stimulates a hormone in the leaf, called auxin. Normally, auxin flows from the tree branches, through the twigs and out into the leaf. But, the loss of chlorophyll causes it to change direction. Now, it flows back from the leaf into the tree.

Auxin has protective properties – especially on a special layer of cells, called the abscission layer, between the leaf stem and its twig. Its absence causes this abscission layer to become sensitive to an enzyme called ethylene.

Ethylene breaks things down. It weakens the cells in the abscission layer, causing them to break and fracture. This makes the leaf weak to the point where wind, or the leaf’s weight by itself, causes the leaf to detach and fall. This is called abscission. 

What is auxin?

Like insulin or adrenaline in humans, auxin is a hormone that can be found inside plants. In fact, it is a very important hormone, and is the reason why roots grow downwards and stems grow up. It also makes twigs and branches lengthen, and leaf buds expand into leaves. 

In order to nourish the growing leaves, the tree needs to get food and water to them – which it does through little vessels – just like our own arteries and veins. Auxin is responsible for growing these, too.

But, once the leaf becomes old or damaged, or when there is no more chlorophyll, the auxin stops going there. This causes the little vessels feeding the leaf to block off. It also makes the leaf stem become sensitive to another hormone – ethylene.

What is the abscission layer?

The abscission layer, or scissor layer, is the name given to a special layer of cells that grows underneath the leaf stem. The cells in this layer are tough and corky and work a bit like a scar over a wound. Protecting the tree once its leaf falls off. This tops fungi or bacteria from entering the tree.

Why do trees shed their leaves?

Okay, so now we know how leaves drop, but now we need to ask – why? Why should the tree lose its leaves? 

If you look back at the section above, we can see that leaves have evolved to have a large surface area in order to absorb as much sunlight as possible. They are also very thin. During the spring and summer, this is great. But not in the autumn and winter.

Having a huge surface area of thin leaves, is like a ship in a storm with a huge, open sail. The leaves will catch the wind, potentially causing the branches to crack or break. They could also act like a huge umbrella, catching the weight of heavy rain or snow.

In addition, because the tree loses water through its leaves, if the ground is frozen and it cannot absorb enough water, this can cause the tree to dehydrate. So, the tree sheds its leaves through the cold, dark winter months to protect itself and stay alive. It enters a state of dormancy or hibernation and waits until spring to make new leaves.

Do all trees lose their leaves in the autumn?

While it may look like most deciduous trees drop their leaves in the autumn, if we look closely, we will see this is not true. At least, not all trees shed their leaves at the same time.

Some trees, such as ash, do lose their leaves very quickly. But other trees, such as hornbeam and beech, hang onto them for a lot longer. These trees tend to have redder leaves in the autumn, indicating a higher sugar content. So, they may be more resilient to loss of sunlight and the cooler weather.

And while we may think that evergreen trees do not lose their leaves,  this is not quite true. They do shed their leaves too – but not always in the autumn. Coniferous leaves abscise little by little, throughout the year, instead of all in one go, like deciduous trees. So, they only appear to be green all year round.

Summary

So, why do trees shed their leaves? Like changing leaf colour, it is all down to decreasing daylight, cooler weather, pigments and hormones. The tree sheds its leaves to protect itself, then hibernates until it is ready to burst forth new leaves in the spring.

Your autumn nature walk task

Now it is your turn. The next time you are out and about, pay attention to the trees. 

  • Which trees have already dropped all their leaves?
  • Which trees are still hanging on to theirs?
  • Have they dropped because of windy days?
  • Or, have they dropped by their own weight?
  • Can you see leaf scars on the ends of the twigs, where the leaves used to be?
  • Finally, did you enjoy this tutorial? If so, leave us a comment and a rating of 5 stars!

Have fun with your nature walk x

P.S. If you would like to learn more, there is a nice little video about leaf abscission here. And if you are curious about why leaves change colour, we have a little tutorial on that too.

 

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