Travelling Trees

They may seem still. But actually, they’re always on the move.

Ants move around while travelling. But when trees travel, they also stay put exactly where they are.

Trees don’t travel like water droplets down a window-pane, but more in the way a crack travels through glass: by staying still, but growing new bits that do the travelling.

Trees begin their lives as a small seed up in the air. How that seed gets back into the ground is a different story — but when it does, that’s when the travelling begins.

Up until this time, the seed has everything it needs to live. Seeds are born packed with food and water, which they can live off for quite a long time. But that time is not a time for relaxing. That reserve of food and water is meant to give you a head-start, a chance to find a food-supply of your own.

That’s what all good seeds do after reaching the ground: start travelling out in search of food and water, so they can live off it once their initial reserves run out.

Some humans think Planet Earth is one big seed for humanity, meant to support them until they travel to space and colonise other planets.

The Earth has everything humanity needs to survive. It also has some reserves of coal, natural gas, metals, and other fossil-fuels and minerals — all meant for building rockets and launching them into space. Without those materials, there would be no way for humans to travel out. Humanity would be stuck on Earth until it died.

Humans who think that way are worried about what other humans are doing. They’re worried that their seed’s precious resources are being wasted on cars and smartphones and gas-stoves and plastic-bags — and that at this rate, when they finally design a spaceship good enough for colonising other planets, there won’t be anything left to launch it with.

Of course, that’s a very human-centric way of looking at things. Humans do things very fast, and they still get impatient about how slowly things are going.

What’s more, humans are very bad at colonising places. They can’t adapt easily to a new world. So they have to spend a lot of time and effort making the whole world adapt to them.

If humanity travels in little bits called “humans”, then the bits trees travel in are “root tips”.

The moment the outside world seems ready to travel in, many tiny root-tips emerge from their seed and start burrowing through the soil. These root-tips have hard “root caps” in front, to shield them from the soil. Their job? To explore the world as fast as possible, and find a good source of food and water to live on.

Through the soil they go, avoiding any hard objects and probing for the softest soil to travel through. As they move, they sometimes split into two smaller halves: each half becomes a new root-tip, they can now do their searching twice as effectively.

Root-tips don’t travel like humans. They travel more like snails, leaving a trail behind them as they go. The difference is that, instead of a shiny line, what root-tips leave behind are solid, usable tubes of “root”.

Those tubes, like the trails of snails, stay put and do not move. But they don’t vanish either. Instead, they sit there, growing fatter over time, and ferrying food and water to wherever it’s needed.

But how does the food and water get collected in the first place? That’s the job of the “root hairs”.

For animals, hair is a very diverse thing. Primates have it as many thin strands, coming up from their bodies to form a mossy layer. Cats, on the other hand, also have a few straight shoots near their noses. These shoots, called “whiskers”, help to feel the world around them — similar to the hairs of a Venus Flytrap. Porcupines have their hair as straight, long thorns called “quills”. And pangolin hair actually forms flat sheaths of armour.

However amazing these kinds of hair may be, they all have one thing in common: they’re not alive. When cells near the animal’s skin die, they get pushed up through certain spots to join the hair — which then rises up slowly like a molehill.

A root hair is a bit like an animal hair, in the sense that it’s usually long and stringy, and only grows from a certain place. That place is the side of the root-tip, which leaves behind hairs as it travels.

But a root-hair is not made of dead cells. It’s made of one very long cell that is very much alive.

When a hair grows out of a root-tip, it immediately clings to the soil. It begins its job of absorbing nutrients and sending them back up the root. In a few days’ time the hair would have dried up, and its job replaced by younger hairs further down the road.

But those old hairs will still be working on another, equally important, task: keeping the “trunk” of the tree standing.

You can’t just “eat” food. You need to have some energy to eat it with.

Animals get their energy from the other animals, fungi, or plants they eat. Plants, on the other hand, usually have to get it directly from the Sun. But sunlight can’t travel through soil. That’s why every tree has a “trunk”, a thick bit that shoots up, right out of the ground, and stands exposed to the atmosphere. There, with the help of sunlight, it processes all the food that gets sent up and turns it into something the tree can actually use.

The problem with the trunk is that it’s a very tall thing, without any soil to support it. So the rest of the tree needs to hold very tightly to the ground to prevent the trunk from toppling.

What’s more, trunks are exposed to the atmosphere. And the atmosphere is a dangerous place. There are many animals up there that may eat your leaves, damage your bark, or even chop down your whole trunk and use it for timber.

There are other dangers lurking in the atmosphere too. It’s the place where lightning can strike you down, destroying your inner layer. Forest-fires do a slower job, and don’t always kill you, but they spread around affecting large numbers of trees at once.

Then there are some rare but dramatic events that we don’t know much about. In 1908, a sudden explosion in the atmosphere, near the Tunguska river in Siberia, knocked down hundreds of tree-trunks in a single blast.

Trunks are the most unique, and also vulnerable, part of a tree. If a tree loses it’s trunk, it will certainly die.

Certainly? Well, not quite. There are times when neighbouring trees get together, to help the trunkless one continue living.

Root-tips are not solitary creatures. To start with, there are many of them belonging to the same plant. Even a tiny rye plant can have fourteen million root-tips. For a tree, that number would be many times more.

These root-tips are always in touch with each other. After all, they do belong to the same plant. They share the same trunk, and whatever food they find is distributed amongst them.

But what about root-tips from other trees? Do different families speak to one another?

Yes, they do. And not just for trees of the same species: this messaging can happen across different kinds of tree as well. Though they don’t come into direct contact, many trees can communicate using the Wood Wide Web.

Micorrhyzal fungi are small kinds of fungus that live on the roots of trees and other plants. Sometimes, they even live in the roots of trees and other plants. They actually infect the root-tips, and start growing within the root itself.

This is not a bad infection. These fungi are much more efficient at collecting some kinds of nutrient than root-tips are. And they pass those nutrients on for the root-tips to use. In return, the root-tips provide a shelter for the fungus to live, and also share some of their processed foodstuff with them.

The fungi and root-tips are so closely connected that they are known as “micorrhyzae” — neither root nor fungus, but a combination of both together.

That’s not all, however. The fungi themselves are spread out all through the ground, forming their own interconnected network. Scientists call this a “micorrhyzal network”, but it’s more commonly known as the “Wood Wide Web”.

Through this network, trees have been known to connect to each other — even across species. In one study, Douglas Firs were observed sending distress signals to their neighbouring Ponderosa Pines. And they even sent across some carbon from their reserves, which they wouldn’t have any use for at the moment.

Sometimes, dying trees send their nutrients off to neighbours who’ll have better use for them. Other times, it works the other way round.

Forrester Peter Wohlleben was once walking through his forest, when he made an interesting discovery: a beech tree whose trunk hardly existed at all. It had been cut down long ago, and even its stump has rotted away. What little bit was left looked like a circle of mossy rocks.

But when he scraped away the surface, Wohlleben found green, living tissue inside. This elderly tree could no longer make food for itself — but its neighbours had been sending in nourishment to keep it alive for hundreds of years.

The Wood Wide Web highlights the fact that a forest is more than just a bunch of trees. If you plant 2000 trees in the middle of nowhere and water them, you could call the result a “forest” — but it wouldn’t really deserve the name.

A true forest also need to have all the springtails, earthworms, creepers, mushrooms, millipedes, rodents, climbers, and whatever else goes to make up its specific ecosystem.

Particular kinds of forest also need to live in particular kinds of places. And if the climate is not right, they can sometimes move to where it is. That doesn’t mean the trees have to get up and walk. Just as root-tips can travel while their trunks stay still, so a forest can travel without uprooting its trees.

Forests travel all the time. But sometimes, the reason they’re travelling is not very clear. For example, the trees of North American forests are growing better on the western side than on east. Somehow, conditions seem to be better the more westwards you move. Over time all the old trees in the eastern side are slowly dying away, while more and more new ones are growing at the opposite end.

The entire forest is travelling west — and nobody knows why! But you can be sure of one thing: it’s travelling without moving a single tree.

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