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In my fantasy story, elves live high up in giant trees (or at least something similar to a tree). These trees are so massive, that entire cities can be built on their branches. The branches have flattened to accommodate these elves, as their presence is beneficial to the trees. This has the double bonus of flattening the 'ground' and providing more of it.

I recently added up the size of the kind of city I would like the elves to have on one of these branches. Combined with the right amount of open space around it, I've deduced that the branches need to be around 1 - 1.5 miles wide.

What do I need in place for a tree of this size to occur?

Notes:

  • I've included some diagrams below, if you are having trouble picturing things.
  • The branch only needs to be 1.5 miles wide at the widest point (the base of the branch, next to the trunk). I have nothing against the branch tapering off normally.
  • The branches have flattened (at least near the trunk) as can be seen in the diagrams below.
  • Magic is present, but I would prefer for it to have as little a role as possible. If such a tree simply cannot occur naturally, then I can work with magic tweaking things slightly until such a tree could exist. Just tell me what I need.
  • The planet is earth-like.
  • While the branches are huge, the tree itself does not need to be super high. I have nothing against it towering to the sky, but if it's a problem, the branches only need to be about 650 feet up.
  • It seems that 1.5 miles is a bit of a stretch. Is roughly 300 feet any more realistic?

Diagrams:

enter image description here

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  • $\begingroup$ I'm not certain this is possible. At least not on an earth-like planet. $\endgroup$ – Bellerophon Mar 14 '17 at 22:36
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    $\begingroup$ I'm not a biologist so I don't know for sure but I suspect that a) gravity is too string/wood to weak to hold the weight. Also, in plants, not sure about trees, water and dissolved salts/minerals are carried through branches by diffusion. On such a large branch this would be pretty slow. Possibly that could be solved if the elves build artificial water-ways to support their city. $\endgroup$ – Bellerophon Mar 14 '17 at 22:45
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    $\begingroup$ Related: worldbuilding.stackexchange.com/q/10793/7351 $\endgroup$ – apaul Mar 15 '17 at 1:45
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    $\begingroup$ Must the city be built on one branch? I'd find it more likely that the city spreads between many branches, each of which is relatively small (10-30'), and would make it much more easily reality-checkable. $\endgroup$ – Delioth Mar 15 '17 at 18:05
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    $\begingroup$ Maybe it's a Dyson Tree? Or this $\endgroup$ – JDługosz Mar 15 '17 at 20:28
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The scale that you are referring to (1.5 miles) is definitely non-scientific. Maybe in a world with much lower gravity, but still, your trees would need to be taller than Mount Everest.

What can possibly make things easier is "multi-trunk" approach. Instead of one humongous tree, you have a multi-trunk forest which is all interconnected. No single branch will be nearly as wide as you want, but together they can form a strong support base that can be very large.

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    $\begingroup$ just look at redwood forests with flying buttresses connecting trees, lots of potential of civilization in something like that but a bit denser. $\endgroup$ – John Mar 14 '17 at 23:56
  • $\begingroup$ It looks like this might be the way to go. However, if I remove the extra space around the city, we're left with a branch about 300 feet wide. Is that any more realistic? $\endgroup$ – Thomas Myron Mar 15 '17 at 0:22
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    $\begingroup$ Look at the banyan trees. I think their multi-trunk setup would make your city viable. $\endgroup$ – SRM Mar 15 '17 at 2:52
  • $\begingroup$ About how thick would each individual branch be if I use this approach? $\endgroup$ – Thomas Myron Mar 15 '17 at 18:57
  • $\begingroup$ that depends more on what is ON the branch. cities are heavy. Humans have built wooden structures up too a 1000 feet wide and buildings up to 50 meters (165 ft wide) so it is possible something could grow that size. the biggest effect on the thickness will be the length of the branches (the unsupported span) and what else is on top of it. $\endgroup$ – John Mar 16 '17 at 2:34
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1.5 miles definitely won't work, but it becomes exponentially easier the smaller the radius is, so let's look at 300 feet:

We'll operate under the assumption that the branch forms a perfect half cone (with a flat top). Let's say the top is 300 feet across at the base and then tapers off as you go up, ending in a point 1200 feet from the base of the branch. If we do the math (or plug it into a calculator), we get that the branch is:

14,150,000 cubic feet/400683.37928 cubic meters

Next we take the density of wood. Here's where we can start fudging things to make it work better. The weight of the American Redwood (the tallest trees in the world) is usually about 45kg/m3, somewhat light for typical wood. However, this is a fantasy world, and since these trees will need to be obscenely tall they would need much less dense wood to not buckle in on themselves. So, let's pretend that these elven trees are only 20kg/m3. Assuming that, the total weight of the branch is:

8,000,000 kg

This is equivalent to 40 Statues of Liberty, so that's probably not gonna fly. However, there's another way to fudge it. If the interior of the branch is rotted away, leaving only the outer layer, we can vastly reduce the total weight. If we leave 5 feet of wood along the outside, around a hollow interior we can cut off ~93% of the total mass. This presents it's own structural problems however, so let's say the interior is mostly hollow, but still has some strong supports so only 85% of the mass is removed. With a hollow branch, this brings the total weight of the branch down to:

1,200,000 kg (6 statues of liberty)

I'd say this is a good minimum weight for the branch as it isn't even factoring in the city, assumes it's mostly hollow, and generously lightens the wood. At that point you just need to say that the type of wood is strong enough to hold the city up. I'm not a mechanical engineer and can't give you the precise calculations, but I can give some pointers to make it more realistic. Specifically, you want a short stubby branch where the bottom of the base stretches really far down. Basically, when viewed from the side it should be a triangle. Also, you can maybe have the branch curve downwards sharply and enter the ground (or enter a neighboring tree) creating an arch that would be much easier to support. Finally, you could lighten the wood even more, while assuming it retained the same strength (make it a "super material" like spider silk). Keep in mind that this sort of super strong/light wood would be super valuable in all kinds of goods, especially weapons and ships.

Edit: One other thing to keep in mind, the tree will need to be really fat if it's going to support branches of this size. Also the upper trunk should be hollow, as too much weight on the base will make the tree fall over.

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  • $\begingroup$ Good calculations. You probably also need to factor in the total weight being supported by the base of the trunk - all the branches, all the leaves, all the trunk above that point. If that bit of the tree fails, the whole thing falls over! I'm constantly amazed how much weight a regular tree supports - wood is amazing. $\endgroup$ – DrBob Mar 15 '17 at 18:02
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Shrink the miles and feet

(OK, that really means shrink the people)

The tricky thing about calling a rabbit a 'smeerp' (Reddit) is that sometimes you want to call a rabbit a smeerp, because the characters' ordinary experiences really are the right mixture of similar to and different from our own to warrant it. You want human-like characters in impossibly large trees, so I think this qualifies.

Measurement units originate as analogues to human experiences and are chosen for their usefulness at the time, not because some giant interstellar standards agency says we have to measure everything in Galactic radii.("How far is it to the corner drugstore?" "Oh, go down the street about 0.0000000000000000013GR and it's right there.")

The 'foot' started out as, well, the length of a human foot, and the 'mile' as a thousand paces marched by Roman legions.

If your people have evolved to be what we would call 'very small' in what we would call 'normal gravity', then a tree doesn't have to be physically impossibly large in order to have people measuring its size as 'very large' relative to themselves. Even large enough to build a city in. If your people are the size of, say, termites, and their feet are about 0.1mm long, then that '1.5 mile' diameter limb would be 794mm in our reckoning, which is still sizeable but by no means impossible.

James Blish, the likely originator of the smeerp concept, wrote a famous story called "Surface Tension" that has tiny people having human-analogue experiences like this.

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Short Answer: No.

Expanded Answer: As all of the previous people to answer have said, this is scientifically impossible. Now, you could argue some fantasy claim such as "The tree's life force is chained to the elves," but when your leaves are in space, it's sorta tough to make that claim.

Easiest way out: Assume you're not on earth, and on someplace with a significant difference in gravity. As long as this planet has an exorbitantly large atmosphere, and a little bit of magic mixed in there, it could happen.

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There are two approaches you could take with this.

Science

If your saying they downright don't need to be trees then it makes this a little easier to solve. I'm going to assume that all you care about is the fact that this is some sort of tree-like vegetation. This means the material the trees are made of could be different than our trees. Assuming you could make up a material that is light enough to grow this big without being crushed under it's own weight, strong enough to hold the weight of an entire city, and have a planet with a gravitational pull weak enough to make it all possible, it could work. (Also note that having a weak gravitational pull can also make it easier for elves to get around on trees such as the ones you describe)

Fantasy

This is a fantasy world and you are talking about elves, a magical race, so clearly this story contains magic, right? This makes it even easier to solve as some ancient elves may have enchanted this species of trees to make them capable of withstanding their own weight at that size. This could also add an interesting element to the story. You could even include the legend of how the trees got to be so big.


Edit: I remembered something from my science class. It goes something like this:

Trees use the energy they collect from photosynthesis to pump water into all the parts. This water helps the tree grow. If a tree grows to a certain point, the amount of energy needed to pump the water that high exceeds the energy they get from photosynthesis. At this point, trees stop growing. For these trees to grow as tall as they need to be, you will probably have to make them broad-leafed (so they get more energy from photosynthesis) and again have lower gravity (to make it easier for the water to travel up from the roots).

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