All About Tree Roots

In this tutorial, we will learn about tree roots.

Roots are the structures found at the bottom of a tree and they can be divided up into four different parts:
 
  • Big roots
  • Smaller roots
  • Rootlets
  • Root hairs
While they might not look like much, roots, rootlets and root hairs are very important – and there are a lot more to them than the eye can see.
 
In fact, the eye can see only the tip of the iceberg.

What do tree roots do?

Roots are important to the health and wellbeing of a tree and have three very important jobs to do.

  • Firstly, they help the tree stand up tall and strong by rooting it deep into the ground so it does not fall over
  • Secondly, they help the tree reach water, without which it would dry out and die.
  • Thirdly, they help the trees to communicate with each other.

How far do tree roots grow?

The answer to the question, how far do tree roots grow, is, it depends.

Some trees tend to have wide, shallow roots that stay close to the surface, but spread out wide around the tree – like pine trees. While, other trees have roots that do not spread as wide around the tree, but prefer growing down deep into the earth – like oak trees.

How far roots grow also depends on the soil and where the tree is growing. If the soil is deep and loose, it is easier for the roots to grow than if the soil is shallow or rocky. For this reason, a tree growing in a riverside meadow will probably have much deeper roots than a similar tree growing high upon a mountainside.

We also need to remember the second reason why trees need roots – to absorb nutrients and water. So, if a tree starts growing far from a suitable water source, the roots will reach out further, trying to find water.

Also, just like us, trees need to breathe – so do their roots. Yes… their roots breathe too! So they grow much better in soils that are light, airy, loose and well-hydrated. (Although there are some trees, like alders, that are quite happy growing in heavy, water-logged soils).

How do root hairs absorb water?

Although we think of ‘roots’ as absorbing water for the tree, it is actually the root hairs that are responsible for absorbing most of the water which the tree needs. These are narrow, thread-like roots, which sprout off rootlets and smaller roots.

The root hairs have a very thin membrane, or skin, around them with little holes all over it (called a semi-permeable membrane). Semi-permeable means that some things, like water, can pass through them, but other things, like tree sap, cannot pass through them. This allows the water in the soil to enter the roots, but stops the tree sap from draining away.

When water moves, it tends to move from somewhere with a lot of water, to somewhere with less water. Imagine dipping a sugar cube into some water. The water is wicked up into the sugar cube because the sugar has less water than the spill. This movement is called osmosis and also happens inside a tree.

How does water travel up a tree?

The cells inside the root hairs are full of tree sap, or tree sugar, which has much less water, and much more sugar in it than the water in the soil. So, it wicks up water from the soil. But now they have wicked up water from the soil, the cells next to the membrane have more water in them than the cells deeper inside the root hair.

So, the sugary cells in the middle of the root hair wick up water from the cells on the outside of the root hair. Now, these cells have more water relative to the cells further up the tree in the rootlets. And so the water keeps moving by osmosis – being wicked up the tree from the cells with more water into the cells with less.

While the water could continue randomly skipping up the tree like this, trees, being clever, instead have a system of tubes, called xylem, running all the way from the roots right up to the leaves at the top of the tree. And once the water reaches these tubes, it is pushed inside and forced up the tube by a force called, root pressure, which creates a column of water.

What is a water column?

This water column inside the xylem, is a bit like a rope, but now it is in the tube, instead of being pushed or absorbed, it is pulled up instead. That is right. The water column is pulled up the tree by the leaves, where it helps to replace any water lost from the leaves by evaporation.

So, on a hot, sunny day when the leaves are losing lots of water by evaporation, the water column travels up quickly to try and replace the lost water. A bit like us, gulping down a whole cup of water when we are hot and thirsty after sweating in the sun.

But on cold, wet days, when there is not much evaporation, the water column moves more slowly – like us slowly sipping a cup of cocoa as we sit and watch the rain come down.

How do roots help trees communicate?

Hidden deep beneath the ground, all the roots, rootlets and root hairs are busy chatting away to their friends and family using a special network of fungi, called the mycorrhizal network.

This network happens because, fungi do not have leaves. And because they do not have leaves, they cannot photosynthesis or make their own energy. So they tap into the tree roots and borrow energy from the trees.

In exchange, the fungi use their vast network of underground roots (which is many times bigger than a tree’s root network), to link the trees together and help them communicate and connect with each other.

If one tree is attacked by a pest or insect, or becomes disease, it sends special chemicals down to its roots and into the mycorrhizal network. The fungi then pass this chemical along to the neighbouring trees, warning them about the pest or disease, so they have a chance to protect themselves.

Likewise, if a tree is struggling due to drought, or lack of sunlight, other trees in the network help it out by sending sugars, nutrients and water. In a similar way, the older trees even help out the baby trees, to try and help them survive.

Summary

Well, I hope you have learnt all about tree roots? Perhaps you have been suprised by some of the things you have learn today? But, can you remember the three main reasons for a tree to have roots? They are:

  • To stop trees from falling over
  • To help them absorb water and nutrients
  • And to communicate with other trees.
We also learned about how roots absorb water through osmosis and how the water travels up a tree in a column that is pulled up by evaporation or transpiration from the leaves. And we learned all about the mycorrhizal network.

Tree Roots Nature Study

Now it is time for your tree roots nature study. Are you ready?

Your first task, is to download the resource that goes with this lesson and look at the structure of a tree root.

Your second task, is to go out and about into some woodlands, or parkland and look for fallen trees. If you cannot access a woodland, or you cannot find a fallen tree, it is fine to look up some pictures in a book or on the internet.

  • I want you to examine the roots of a fallen tree.
  • See how they spread out and branch, getting finer and finer the further they are from the trunk of the tree.
  • Can you trace them far enough to find their little rootlets?
  • If the tree has recently fallen and the ground is still disturbed, you can try and spot its mycorrhizal network too.
Just make sure when doing this activity, that there are no wobbly trees or loose branches around you. This means:
 
  • Wait until the windy weather is over and go on a calm day, then check the area for hazards first.
  • Try to stay in a more open area rather than under overhanging branches if you can.
  • Always keep your ears open for the sound of any cracks, rustling or activity above you.
  • If you hear any cracks or sounds, step away and move back as quickly as you can, since there might be a branch about to fall.
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