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As a tree grows new layers are added each year so that the trunk expands in radius. The living part of the tree is a relatively thin layer on the outside just beneath the bark, and the wood further inside is dead, just there for support. In trees that live long enough, some of the dead wood (heartwood) can be removed without harming the tree, leaving it with more than enough to support its weight. This can happen naturally, in the form of hollow trees.

Grown from scratch

In a hypothetical tree that could survive for tens or hundreds of thousands of years, it could take on the form of a ring that would be limited in height but continue to expand outwards surrounding an ever growing hollow. For a very large ring I would expect this area to be open to the sky. This suggests the possibility of a walled city where the wall is the very slowly expanding outer ring of a single tree.

Grafted to save time

A similar result might be achieved on a much shorter timescale by deliberately grafting small trees side by side along the required path of the city wall so that a single outer ring is formed without having to wait for all the inner rings to grow first. This would involve grafting trees side to side with vertical cuts rather than grafting the top of one tree onto the lower trunk of another.

Are these realistic?

Are there any flaws with either of these approaches to a city surrounded by a single tree? Are there any consequences that I have not considered that would limit this or require changes? Although the first approach would require a tree with a far greater lifespan than we see on Earth, I still want everything else to be realistic and consistent. The second approach I would expect to be realistic even on Earth, so I would like to know if there are reasons why this would not be the case.

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  • $\begingroup$ Norse mythology features this theme. The entire universe is in and around a giant ash tree call Yggdrasil. Have a look at wiki if you are interested. The Poetic Edda and The Prose Edda are available from amazon. Both excellent reads. Great stuff. $\endgroup$ – Epsilon Oct 18 '14 at 3:42
  • $\begingroup$ @NickR that is a different thing from what I'm asking about, but interesting as a separate tree-themed idea. $\endgroup$ – trichoplax Oct 18 '14 at 15:36
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I'll address the two scenarios separately as they have different issues:

Single Tree:

As you've already identified your main problem is time. Most trees grow very slowly. The fastest growing trees might grow 1 meter a year. Lets say your hypothetical tree grows outwards at that rate.

This means that it would take 100 years to be 100 meters across. To be a km across and make room for a settlement it would need to be 1000 years old.

It would be fairly robust and able to repair itself but still catastrophic damage could kill or permanently damage large sections of this. Faster growing wood tends not to be as strong as well, so if you want the walls to be strong it would need to grow slower.

You also have to consider root structures and branches. How does this tree actually get the resources it needs in order to live and to grow? Can it even gather enough resources in the space it has?

Multiple Trees:

The main problem here is resources, these trees are going to be fighting each other for land, for light, for water. Each will try to crowd out the others and there is not going to be enough for all so sections of your wall would start growing slower or even die. Think about the spacing between trees in a normal forest, it's much further than what you are thinking about.

Additionally if the trees are ones that keep on growing now your wall is growing both inwards and outwards. The inner space for the city will start shrinking every year!

A third option

There is another option, certain wood will sprout new trees even if you just plant a stake into the ground. If a conventional pallisade was built using that wood it would turn into a ring of trees by accident if not design!

Yew stakes for example placed into the ground will sprout new roots and branches, growing into new trees if given a chance.

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  • $\begingroup$ Wouldn't the new trees in the palisade also crowd each other out and die eventually? $\endgroup$ – Cerberus Jul 26 '16 at 6:00
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I would suggest looking at the Baobab tree, Ta Prohm tree and Banyan Tree for models.

The baobob tree grows hollow, it stores water there and is often wider than it is tall. The Ta Prohm is the famous trees of ankor wat that seem both towering and made of flowing wax. The Banyan tree "walks" by dropping brace branches to the ground and spouting roots and then growing out further.

A tree with large intentionally hollow trunk, like the Baobab tree that has grown runners like the Ta Prohm for stability which then spreads out limbs, drops tendrils to the ground and starts a new trunk. The large hollow trunks would now be linked side to side like a beam and girder bridge, plus sharing all the water, sap and minerals produced by all the other parts of the tree. The main trunks could grow to 200ft or more tall (real baobab do in some circumstances) maybe 100ft in diamets with lots of intermeshed lateral trunks.

There would be lots and lots of space inside. Housing could be build on the inside of the truck for protection, treating the trees like miny tower forts. The gaps in the braches could be places for woven tents or bird nest like houses. Agriculture might be largely about things grown on the canopy layer. It would be something like people living in the girders of an indoor sports stadium.

All in all it would be like a city of close packed smaller skyscrapers, connected by flying bridge ways all merging at the top to form canopy dome over head.

Wouldn't like to attack the place with anything save modern weapons. It would a vast series of tall towers to penetrate, having to lay siege to each one in turn. Each lateral limb was be an easily defensible bridge. Moving anywhere on the lower level would expose you to gravity attacks from above.

All and all, fairly realilistic would be very realistic.

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  • $\begingroup$ Sounds very cool. But fire... $\endgroup$ – Cerberus Jul 26 '16 at 6:01
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Cities might be pushing it.

Let's suppose that it does so start. The tree grows to a certain size much like a ring fungus in your yard, expanding on the outer edge. One you get large enough that i light gets inside the ring, then other things will start to grow.

In passing: In light of the previous answer with rooted cuttings: Most of the willows and poplars do this already. You can make living walls, called fedges. (fence x hedge) by planting a 45 degree angles, and lashing them where they cross.

However, all you are doing is making a fancy city wall.

Go 3d: Let's suppose that you have a relative of the giant redwood. Call it Sequoiadendron pyrimdalis. It's trick is that it will grow to 200 meters high. This is twice the height of normal sequoias. It needs a way to get water to the top. S. pyrimidalis by deliberately maintaining a 60 degree angle cone, disolving wood from it's interior surface and replacing in the outer one, in addition to the new wood created by photosynthesis.

It's bark is highly corrugated, but the corrugations run in shallow chevrons on the surface so that fog and rain run to small openings at the bottom of each chevron.

With a 60 degree angle, the base would roughly the same as the height. So you would have a 200 meter space inside the cone.

However the prime real-estate would be the outside wall. The corrugations make for easy climbing. This has several times the area of the space inside.

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There already be some example of locals (like a bar) in an hollow baobab (in South Africa if I remember correctly), so yes, a single house can be inside a hollow tree.
But to have a city inside a hollow tree is really impratical for at least a couple of reasons:

  1. As big as a tree can be, a city is still way more bigger, so a single tree is out of question, maybe a large number of trees that form a circle can form a space big enough to host some exclusive residential zone
  2. A city is not static: it grown faster than a tree (or a circle of trees), so it is only viable as long as the city does not grow.
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