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I'm working on a fantasy setting that's literally a giant tree on a planetary scale. Magic is in effect, so concerns of how a tree that large could survive are irrelevant, but I need to figure out what water sources usable by it's inhabitants could feasibly exist on a tree of that size. I know bogs could exist, because bogs actually occur in the canopies of real life redwoods, but what about other sources? There's a highly diverse epiphyte community, including some plants that are normal-sized trees, as well as both normal-sized and giant equivalents of real life epiphytes, so there might be bodies of water inside the cups of giant bromeliads or something similar, but could lakes form on the surface of the tree itself? What about rivers and streams? I probably don't want anything so large as a sea, let alone an ocean, but, if possible, how big would lakes be able to get?

If it matters, the tree in question isn't attached to any true planet, when you reach the base of the tree, you find a field of spongey material that can't be seen through a thick, surrounding mist, and if you travel on that field more than a couple miles from any point of the trunk on this surface, you end up more or less back where you started. On the other hand, there is a fairly regular weather system, with rain and other precipitation falling on localized regions of the tree, and some areas being typically drier or wetter than others.

Though a root mass exists, it is, for want of a better way to describe it, "out of phase" with the plane of existence the rest of the tree is on. A few surface roots protrude in the area around the base of the tree, extending about half the distance that can be traveled out from the trunk. The other plane where most of the roots are could in theory be reached via magic, but no one's ever tried due to the sheer uselessness of the surface around the base of the tree, which inhabitants refer to as "the mist ring". There is no other effective mass of the "planet" other than the tree and the various lifeforms growing on it. From the base of the tree to the top of the canopy is roughly 10,000 miles, with a canopy spread of maybe 12,000 miles. It has definitive "upward" direction rather than growing from a central mass. The main trunk is relatively short and thick before masses of branches start splitting off - about 1,600 miles in diameter and branching off at a height of 2,000 miles. Some one race has a fairly extensive tunnel network inside the trunk and some branches, though not reaching the heartwood.

Gravity has a definite downward direction, but there's also a very slight pull toward the trunk. Something that falls off a branch will tend to "drift" towards the center of the tree as it falls. It's also worth note that gravity isn't nearly as strong as Earth's, somewhere between 0.2 and 0.5 Gs. The tree is fully stationary. I'm as of right now undecided as to how weather patterns move, but some clouds can definitely enter the canopy in a way that some precipitation might fall on lower branches but not upper ones.

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    $\begingroup$ This has the makings of an interesting question. 1) Is there any actuall rootball mass? 2) What percentage of the mass of the "planet" makes up the tree? 3) Does the tree grow in all directions from the center of mass, or is there really only one direction defined as "up?" 4) When you say "planetary scale," do you mean the distance from the root to the canopy is on the order of 8,000 miles (center mass, my Q#3) or 16,000 miles (actual tree shape)? Please add a paragraph that specifically describes the tree. $\endgroup$ – JBH Dec 10 '17 at 23:14
  • $\begingroup$ My main world I'm working on at the moment is a world tree too (though more like the one @JBH is imaging I think), but most of the action takes place on continents suspended in the branches. Is there any actual landmasses in the tree or is it just all tree? $\endgroup$ – adaliabooks Dec 10 '17 at 23:26
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    $\begingroup$ OK, we're close. A) Do we care about the effects of gravity? or is gravity always "down" toward the roots? B) Is the tree "fixed" in space such that it rotates in no direction, or is there rotation? (yaw, pitch, roll?) If so... C) Do we care about centrifugal forces? $\endgroup$ – JBH Dec 11 '17 at 0:56
  • $\begingroup$ I think weather is an importnt point to think about. If it rins then you have an easy source of water $\endgroup$ – Slarty Dec 11 '17 at 1:30
  • $\begingroup$ @Slarty Like I said, it does rain. Overall precipitation levels would average around that of northern Europe, with some region of the tree being drier or wetter. In particular, the upper branches tend to be more arid. $\endgroup$ – Cowrie Dec 11 '17 at 2:13
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You've already mentioned accumulation in bog material, leaves, and flowers. So I'll ignore all that.

branch bowls Where two or more branches exit the trunk and are near to each other, they will create depressions or bowls as the base of the branches grows outward. On a tree this size, those bowls could create sizable lakes (several square miles at least). Such bowls can occur with diminishing size anywhere from the trunk to the tip of the branches wherever two or more branches diverse perpendicular to the rootball. A bowl could also form at the very top of the trunk between branches. Such a bowl would be nearly unique in that it would have the least amount of taint from sap.

Sap Whether or not sap is consumable is story dependant, but there would be a never-ending supply of it (well... assuming your population doesn't grow too large). This can have a fair number of story elements as a tree this large may have different kinds of sap depending on how deeply you "mine" the bark or tree. Since the sap is a nutrient-carrier for the tree, it can have a variety of healing/healthy properties... not the least of which is coating the biggest honking stack of flapjacks the universe has ever seen. Note that most of the standing water I describe here is likely to be tainted with the sap(s).

Decantation Water falling on or condensing on the upper branches will (often) flow back along branches to the trunk, then down the trunk to the rootball. You could have substantial rivers flowing along the trunk and underside of larger branches after rainfall. Smooth bark would allow the water to spread out and flow shallow. Rough bark would form pools, waterfalls, and channels.

Hollows Branches die, and when they die they can fall away and leave a gaping hole in the tree. That hole could hold a lot of water... maybe even thousands of square miles of water. Rivers decanting down the side of the tree would decant along the "roof" of the hollow for a ways before falling in a glorious waterfall to the inner sea. Cracks in the wood would allow water to seep in in a manner similar to terrestrial springs.

Man-made hollows If your people can mine, they can create their own hollows. They wouldn't be large — call them ponds — but they'd be perfectly useful for drinking water.

Bark tears Areas where the bark has separated from the tree could cause water acculumation.

Barrels Your people have an awful lot of wood available to them. Barrel-making (if they can find a suitable replacement for the metal straps... leaf fiber, maybe) would be a booming business, and you can let them accumulate water from leaves and other flows. Flows can be created with tarps made out of leaf material. Your people can divert rivers fairly readily.

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    $\begingroup$ The idea of different saps at different depths could be useful, since this world tree blends the elements of 4 species of tree that have been mythically represented as "world trees": Ash, Yew (alternate interpretations of Yggdrasil), Kapok (Mesoamerican world tree) and Mulberry (one of several Chinese world trees). So I've got options of sweet/edible, highly toxic, medicinal, and mildly toxic hallucinogen, in that order. Though it probably won't have super-toxic yew sap, I like the idea of surface level ash sap, deep-flowing kapok resin that needs to be mined and mulberry sap in leaves. $\endgroup$ – Cowrie Dec 11 '17 at 3:30
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    $\begingroup$ Your comment on barrels made me think of other artificial containers. Due to lack of metal and the inherent danger of using fire, biotech is prevalent among the races of this world. Some real plants make seed pods that hold water, giant versions of those are probably widespread. Actual barrels might be held together by woody lianas grown around the boards, or they have a species of epiphytic tree that has resin that can be "cured" to amber-hardness in a short period and heated to flexibility or melted and cast. That amber might be suited to making barrel fittings. $\endgroup$ – Cowrie Dec 11 '17 at 3:40
  • $\begingroup$ One more thing -- About decantation, how long do you think those temporary rivers would persist after rainfall? $\endgroup$ – Cowrie Dec 11 '17 at 17:43
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    $\begingroup$ That would depend heavily on where the storm occured, how thick the branches/leaves are in the area of the storm, the distance of the branches from the trunk, and the location of your natives. The leaves and branches are acting like a watershed. Watersheds can be small, feeding rivers that are 1-100 miles long, or massive, feeding rivers like the Columbia, the Mississippi, and the Amazon. The time it takes to drain a watershed can be, therefore, days to months... and if the rain occurs again, the rivers can be permanent. $\endgroup$ – JBH Dec 11 '17 at 19:03

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