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One of the cities in my world is built on the ground floor beneath and around giant tree. Giant tree cities seem to pop up fairly frequently in science fiction and fantasy, but how would they actually work and how would the city be designed?

An obvious question seems that of sunlight. How much would be blocked out by the tree, and what effect would this have on the city?

It's also possible that the tree could shed leaves in the fall, but that would depend on the type of tree.

Are there any other concerns for a tree based city?

(BTW - the world includes magic and is currently at about 1500s technology level)

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  • $\begingroup$ You're looking for a colossal bonsai in a tropical climate. $\endgroup$ – user6760 Apr 14 '15 at 4:35
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The biggest one would be growth. If you build your house between two branches, then in some period of time, your house could easily be in two pieces. Or if you anchored it to one branch, it could be simply swallowed up by the tree's growth, depending on how the house is attached.

Sunlight actually I don't think would be too much of a problem, unless the tree is extremely dense (coniferous trees, for example, might make it perpetual night). But with most other trees there's enough space for light to move around between the branches that you should be ok. It might lead to a sort of value system, where houses on the outsides of the tree, which have an actual view of the sun, are worth more than those which are on the inside and only get light that's been bounced around of few times first. People might try to build farther and farther up the tree, and farther and farther out on limbs, to make more expensive houses, but those houses would be quite unstable in any kind of wind, and the branches at either of those places are by definition much less strong than those near the trunk of the tree.

Leaf collection would indeed be a big problem, but the nature of the problem would change depending on whether this is a huge normal tree (leaves are normal size, the tree's just really old) vs. if its simply a huge version of a tree (and the leaves are scaled up to match). In the first case, you might be able to simply build a roof over every area, sloped enough to make the leaves slide off, but for the second a single leaf might be able to crush something, and you probably wouldn't want to live in a tree which shed leaves. Maybe an old dead tree would work.

Fire, obviously, would not be something you want to break out in this city. Small ones can be controlled, and even a house fire might not be able to break through the tree's bark before it went out, but some magic systems include the ability to make fires which can't be put out - and those would make short work of your tree.

Wind storms could be problematic for people and structures built on flexible areas of the tree.

The more things get built on a branch, the more it sags, and the more it bends the structures already built on that limb.

A slight misstep while walking would hurt (eventually, depending on the height of the tree).

Children's games most often require a large open space to run and throw things, and its one thing when your baseball falls in the neighbor's lawn, but a completely different story if the ball falls a couple hundred feet down out of your tree. A large pavilion could theoretically be built to give them an area to play though.

If any of the houses are actually cut into the tree, then you run the risk of eventually hurting/killing the tree if you go deep enough. Also again, if the tree doesn't die, then it's gonna keep growing and your house is going to change shape.

Traffic (even foot traffic) would require a huge 3D spiderweb of roads bridges and stairs in order to facilitate movement between branches, and that would probably greatly increase the difficulty of people unfamiliar with that section of the tree to navigate around, find a friend, or catch a criminal on the run. I'd suggest the mages look into levitation.

Also if it snows there, then you'd have to coordinate the shoveling of each person's house, so the guy at the bottom doesn't end up getting all the snow of the 25 houses above him dumped onto his roof.

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  • $\begingroup$ I tried editing my question to be more clear, but the city is not built in the branches but only on ground level. $\endgroup$ – CoolCurry Apr 15 '15 at 13:20
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    $\begingroup$ Oh. Well that changes a bit:P all I can say then is that most likely the city would be completely dark, except for in the winter. And that I agree leaves would pose an issue - fall might very well be the equivalent of a huge snow storm; the leaves could easily cover your city completely (as in, to a depth of 30ft or more) if they were normal sized leaves, and might crush it if they were proportionate to the tree. The only other thing I can image would be falling branches - it would be quite common in a tree that large. $\endgroup$ – HammerN'Songs Apr 15 '15 at 15:12
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Since I need a higher reputation (50) than I currently have to ask questions in the comments I will have to make the following assumptions:

We are talking about average human size inhabitants.

While the tree would have to be enormous to hold any size civilization under and around it (per the light being blocked impact) the leaves and or fruit that fall from it would not match in scale. (i.e. not crush a house when a 20 ft apple fell.)

With that here is my shot at it:

Significant agriculture would need to exist outside of the trees drip line to support the local population. (1500's technology would prevent mass transportation of food stuff to sustain a sizable population.)

Waste management of said population.

Nutrient management of tree. Impervious area (houses, hard packed ground/stone from roads, etc.) around the tree inhibit water penetration to the roots as well as competition from surrounding agriculture.

Canopy cover should provide relief from the heat of the day and also provide shelter from storms.

Said underground development does not undermine the integrity of the trees ability to grow and remain standing.

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  • $\begingroup$ Some average human size inhabitants, but most would be the size of, say, hobbits. $\endgroup$ – CoolCurry Apr 14 '15 at 1:58
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The closest I could come to in real life is termites infesting a tree. However, knowledge of how water travels up to the leaves through the trunk is very helpful, because you just solved your problem with elevation.

Now here comes the tricky part: does it stand up to the inverse-cube law, gravity, and material mass/weight?

Brush up on your tree and world's gravity. Light can be handwaved with magic, but it would be cool if you could find the breaking point and set it up so that there could be windows to let in the sun.

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There are a lot of things to think about.

Starting with sunlight, yes, it will be pretty dark under the tree. The closer to the trunk the darker it would be.

Some trees, like Walnut, have a natural herbicide that kills many plants around them to reduce their competition. The largest living Walnut tree is this one:

Largest living Walnut tree

It looks like a spring picture before it has leafed out.

Large trees need food and water, generally from the roots in the soil, and while the trees can find ways to get water and even some nutrients, most will still be provided from the ground. The roots also supply stability to the tree so it doesn't fall over. This means a whole 'highway' system might show up from the roots sticking out of the dirt around the tree.

It would certainly affect farming for some distance since the roots would go out at least as far as the branches and I've seen many trees reaching much farther than that.

If it is a seasonal tree, when the leaves fall it could bury a city.

No matter what, trees lose and drop limbs. On a large tree this could wipe out a whole city block.

Is it a fruiting tree? Nuts could be the size of coconuts and be deadly, even apples falling from 300 ft. could kill. A rain of berries (or cherries) could drown the town in food, also likely bringing many animals to the harvest, some wanted (animals for food) others not so much. Either way, the clean up would need to be done.

The more I think about living in/around a large tree, it seems best to either live inside of it (hollow out a living space) or live hanging underneath the large branches.

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