# Would a 'World Tree' be feasible in real life?

A livescience.com article claims that trees can reach a theoretical height of 400 to 426 feet (122 to 130m). This is due to the tree being unable to carry water up to the top at that certain point due to gravity. It also claims that

at a certain height, leaves (or, in the case of redwoods, needles) are not cost-effective — the energy they rein in through photosynthesis doesn't pay for the energy it costs to bring them water

I'm interested in designing a tree that's much taller than 130 meters. Specifically, I'm interested in creating a titanic "world tree" that exceeds the theoretical height limit. The world tree would have a ludicrous amount of leaves for photosynthesis.

Provided the perfect conditions for growth, what methods could the tree use to ferry the water upwards? And what would the limit be in that case? I'm assuming that there are other factors that could limit tree growth besides the inability to ferry water to the top of the tree; for example, after a certain height the wood of the tree may collapse on itself.

To be slightly clearer, I would like a list of evolutions/adaptations that would allow this tree to exist under perfect conditions and stability and what its height limit would be in that case. I can understand that such a tree with conventional earth trees impossible, that the limitations of wood would debilitate its height, and that it would not go over the troposphere due to temperature and air conditions

Thanks for the answers,tho I don't think I'll be messing with the gravity

Thanks to Brythan and Hamlet for the edits :D

• With the physiology of the plants of Earth, no. But an alien tree could perhaps evolve a heart, which would increast the height it can take water to. Alternatively the tree could grow horizontally instead of vertically and take over the world. Jul 25 '16 at 12:37
• I imagine you could make any number of changes to other details of your world to make a tree grow to any arbitrary height. Can't draw water that high thanks to gravity? Decrease gravity. Can't get air that high? Thicken the atmosphere. Jul 25 '16 at 14:07
• It would probably make more sense as a lichen- or coral-like colonial organism that resembles a tree. Jul 25 '16 at 15:15
• What if the structure of the tree was different than other trees and instead of gather the water at the roots it gathered water in the air or collected it in pools within the branches? Then you could have a gravity pump, much like what you see in sky scrapers. The issue of energy would still exist, possibly, as well as any reaction necessary to heal if the tree were injured, but this would solve your theoretical gravity issues. Jul 25 '16 at 16:00
• With a three this large, the ecology there will be highly varied and adapted to the environment. There will probably be multiple biospheres due to the height. Jul 26 '16 at 12:51

Assuming you can fix the water problem using some of the other answers given, like having pools along the height of the tree, extracting water directly from passing clouds, etc., you will still have to deal with the tree crushing itself under its own weight. The following is an engineering kind of answer to the question.

The equation that determines the maximum height of the tree is:

$$H_\text{max} = \frac{\sigma_\text{allow}}{\rho g}$$

Where $$H_\text{max}$$ is the maximum height of the tree, $$\sigma_\text{allow}$$ is the compressive strength of the wood, $$\rho$$ is the density of the wood, and $$g$$ is the acceleration due to gravity on your planet.

What does this mean? Well, the tree's height is directly proportional to the strength of its wood, so if the tree evolved to have ultra-strong wood it could grow taller. It is inversely proportional to density and gravity, so if the tree evolved to have extremely lightweight wood it could grow taller (maybe it uses some metabolic process to make balloons of lighter-than-air gas to support its weight too? that would be cool). Finally, as was mentioned in another answer, the lower the gravity the taller the tree. Halving the gravity doubles the possible tree height.

Note that it doesn't matter how big the tree's base is, it will still crush itself as it gets taller. This falls out in the derivation below if you want proof.

# The math

Let's consider the simplifying assumptions that the tree has a constant cross-sectional area $$A$$ along its height (for example, a cylindrical tree would have $$A=\frac{\pi}{4}D^2$$ where $$D$$ is the diameter) and that we can ignore wind forces and things that could tip the tree over, focusing just on the compression experienced at the bottom of the tree.

The weight of the tree would be density × volume × acceleration due to gravity:

$$W = \rho V g = \rho A H g$$ (where $$\rho$$ is the density of the wood and $$H$$ is the height of the tree)

The ability of the tree to carry a load is described by its "compressive strength" which is basically the amount of "stress" the tree can take before breaking. Compressive stress at the base of the tree is the weight divided by the area of the base:

$$\sigma = \frac{W}{A}= \frac{\rho A H g}{A} = \rho H g$$

The area in the numerator and denominator cancels out. Now we can rearrange to find the maximum allowable height:

$$H_\text{max} = \frac{\sigma_{allow}}{\rho g}$$

# Example

An oak tree on Earth could grow to a max height of $$691\ \mathrm m$$, given a wood density of $$600\ \mathrm{kg/m^3}$$, a strength of $$4\,070\,000\ \mathrm{N/m^2}$$ and acceleration due to gravity of $$9.81\ \mathrm{m/s}$$:

$$H_\text{max} = \frac{4\,070\,000\ \mathrm{N/m^2}}{600\ \mathrm{kg/m^3}\times9.81\ \mathrm{m/s}} \approx 691\ \text{m}$$

As mentioned in other answers, there are additional factors limiting tree growth. This paper from Nature describes the limiting factors in some detail. It mentions that the theoretical maximum water transport height may not be reachable because as the pressure drops it can lead to embolisms within the xylem, which can cause nearby branches to die. Trees can try to mitigate this risk by reducing the size of their stomata, but that also decreases photosynthesis, meaning the top of the tree has a much harder time generating nutrients. According to their estimates, this limits the maximum height of trees on Earth to somewhere around 130 m.

• "the tree's height is directly proportional to the strength of its wood, so if the tree evolved to have ultra-strong wood it could grow taller" The reasonable way to do this would be for the Tree to produce polymers, carbon fibers or even carbon nanotubes incorporated into its cellulose. They are all made of carbon so there is no good reason why the complex organic processes of a living being could not be adapted to produce them. These could conceivably give strength increases of 100-1000x, resulting in a Tree of 100KM or more. Jul 25 '16 at 18:03
• @DanHenderson I respectfully disagree. The weight of the tree is mass times acceleration, a well known formula in physics. If you follow the rest of the small derivation, you will find that the weight is not squared at any point, therefore the height does not scale quadratically according to the assumptions I have made. I have given a detailed description of my assumptions, I don't know what dotancohen's assumptions were that led to $g^2$ Jul 25 '16 at 18:06
• @RBarryYoung Good suggestion with the nanotubes and polymers, however the analysis I used looked at compressive strength at the base of the tree. Nanotubes have fantastic tensile strength (i.e. you can pull on them really hard) but not as great compressive strength (if you push on them they tend to buckle, so you have to brace them together somehow). I think you'd be better off with a diamond base for the tree (opening up some worldbuilding possibilities of greedy miners risking collapse of the tree :) ) Jul 25 '16 at 18:27
• @BarbalatsDilemma I think I figured it out - I believe dotancohen's calculation is based not on compressive strength, but on the ability to draw water up, which may be quadratically proportional to gravity. I checked your maths and found that for what you're evaluating, it is indeed a direct proportion. My mistake! Jul 25 '16 at 19:01
• You say the size of the base doesn't matter. But what if the tree is a flattish cone or pyramid? Wouldn't that help to distribute the pressure somewhat and reduce compressive forced at the base? Jul 26 '16 at 5:42

Water Basins If every 300ft or so the tree grew water basins to catch rain, (or maybe as a place to deposit water that it had already pulled up from below?) the higher parts could draw from these basins instead of directly from the ground.

This can also lend itself to the fertilization as well. If these basins were large enough they could become small ponds or lakes that could support life. The animals/plants/fungi/whatever that migrate, live, and eventually die there could become the primary source of nutrition for the world tree.

The limiting factor would come down to how high can you stack wood before the bottom most block is crushed by the weight. Because this is an alien(magic?) world you could always say as the tree grows old instead of outright dying, it carbonizes into some carbon fiber structure that allows it to withstand the immense weight of itself?

• Water basins are an amazing idea, I can actually see waterfalls from multiple basins and stuff. Thumbs up for you.
– Skye
Jul 25 '16 at 13:05
• The water basins will only fill with water if they are below the level where clouds form AND the temperature is above freezing. Max cloud height is 6000 metres and those are not rain clouds. Above a certain height the water basins will be filled with ice and snow. If you really want your tree sticking beyond the clouds into the upper atmosphere (as in your picture) you need another method to get water above 6000m. Jul 25 '16 at 13:06
• doesn't this mean that water will freeze in its xylem? So it's limit is slightly less than 6 km?
– Skye
Jul 25 '16 at 13:15
• Wolverine tree?
– Skye
Jul 25 '16 at 13:52
• "The limiting factor would come down to how high can you stack wood before the bottom most block" - i'd like to add that if you are using wood blocks, then the highest stack is 255, minus what bedrock you have beneath. Jul 25 '16 at 18:51
• Transport : Due to transpiration, water can go upwards in trees, however after a certain height the pressure won't be enough. A way to counter this would be if the tree had a heart of some sort to act as a pump. Also, the tree can absorb water from outside, so the leaves at the very top can still receive water. ( You can also decrease gravity in that world.)
• Lifespan : It will take a really long time for a tree to grow to that height, it can't be done if it lives only for 30 years. So the tree would have to be pretty much immortal and fast-growing, because even 3000 years is too little. A way around this would be a tree whose offspring grow from itself (it is self-fertilizing, and the seed remains in the tree). So the tree will continue growing.
• Nutrition : A tree that big needs lot of minerals. Which is hard to get if it's stuck in one place. A counter would be if there were bacteria that excrete the minerals required for the tree, and also depend on the tree in some way. Also it can have some sort of 'basins' which emits a scent that attracts bugs. The bugs fall in and are dissolved to provide the nutrition.
• Comments are not for extended discussion; this conversation has been moved to chat. Additionally, keep it civil. Several users made impolite remarks; insults have no place on Stack Exchange. Jul 27 '16 at 12:03
• @HDE226868 : Well I didn't start it :) (though I did continue it). Hey, why do I have this warning --> "Some of the information contained in this post ..." ? The stuff I posted does not really require links to wiki. Jul 27 '16 at 12:14
• See the comments here. Jul 27 '16 at 14:00
• @HDE226868 : I see. However what part of my answer requires additional info? Because the counters I gave, don't exist in real life. So I can't link it to anything. Jul 27 '16 at 14:47
• Well, you could start with citing something for your lifespan numbers (i.e. why 30 and 3,000 years aren't enough); that likely has a foundation somewhere in basic biology. You could also back up your statement about water traveling through trees, as well as how you think a "heart" could work. Jul 27 '16 at 20:59

If you significantly relax your concept of "tree", then consider this: an enormous buoyant "crown", attached to the surface via a long, lightweight, flexible "trunk". On a planet like Venus, with its very dense CO2 atmosphere, there are a variety of gases that could provide the necessary buoyancy.

One gas that comes to mind - oxygen! It has a lower molar mass than CO2 and, as a bonus, is a natural product of photosynthesis. The tree could start out growing on the ground, and then grow out some kind of bladder that captures the oxygen produced by photosynthesis. As the tree grows older, both the stalk and bladder grow, lifting it higher into the atmosphere.

For water, your atmosphere could have a reasonable water vapor content - not so high as to make the atmosphere lighter than the gas bladders, but enough that your "trees" can extract the water they need directly from the atmosphere. If your atmosphere is fairly well-stratified, the denser CO2 would sink to the bottom where it can provide more support, while the lighter water vapor would rise to the top, where your crown/bladder apparatus can extract it. This would give a double-incentive for trees to grow as tall as possible - more light and more water the higher up you go. The limiting factor would be at the point where the atmosphere no longer has sufficient density.

## Potential problems

### Gravity

You may need to play with the mass of your world such that the buoyant forces are enough to overcome gravity.

### Respiration

Plants need to respire just like the rest of us - this means consuming oxygen. They could consume some of the O2 they've stored in their flotation bladder(s). They might start to slump a bit at night, when at peak metabolic rate ;-)

• And these bladders could contain valuable gases, which are harvested by humans. If the bladders should go low on gas... Jul 26 '16 at 5:47

The tree's ability to lift water would scale quadratically if the gravity of the planet were lower. Thus, on a hypothetical planet with half Earth gravity (g = 5 m/s^2) the tree could attain a height four times as it would on Earth.

If your planet had the Moon's gravity, for instance, the tree could reach a height of four kilometers high.

Note that BarbalatsDilemma mentions in her answer that once we remove the limit of water transport, the limiting factor of tree growth will become the wood's ability to withstand compressive stress, which increases linearly with height. In fact, the strongest woods are from those trees which grow the tallest. Therefore once the water transport limitation is removed, the tree may have reason to develop yet stronger woods.

To address concerns of lesser gravity affecting the planet's atmosphere, I present Titan, a moon of Saturn. This moon has an atmosphere thicker than Earth's atmosphere. To address arguments that the atmosphere may be possible only due to the lower temperate at Titan's location, the leading theory of the origin of Titan's atmosphere is that it is caused due to low silicate content in Titan's crust, so everything is volatile. Thus, a higher temperature would actually increase the sublimation rate and thicken Titan's atmosphere. The higher temperature would expand the atmosphere, but so long as that atmosphere remains inside Titan's Hill sphere it would remain and cause even higher pressures. Titan's Hill sphere is about 50,000 KM, current atmosphere reaches about 1000 KM above it's surface, and volume increases by the third power with increasing radius. Note that these volatiles are organic materials, in particular hydrocarbons, so they would actually support plant growth very well.

• Doesn't work. decreasing gravity decreases the amount of atmosphere which decreases temperature which decreases the height possible. Jul 25 '16 at 16:30
• @Durakken: Not necessarily. See Titan, Saturn's moon with gravity comparable to the Moon yet an atmosphere thicker than Earth's. Jul 25 '16 at 16:36
• it's also got a much lower temperature. Jul 25 '16 at 16:40
• @Durakken: Titan's lower temperature is generally accepted to be due to its distance from the Sun, not due to its gravity. Temperature and distance from the Sun are very strongly correlated in stellar systems, at least as supported by observations in our own solar system. Jul 25 '16 at 16:58
• The reason there is a thicker atmosphere is due to the coldness of the planet which is reduced from what it would have because it has less gravity. The reason the moon is as warm as it is has to do with orbiting Jupiter. Jul 25 '16 at 17:09

the tree can utilize some form of https://en.m.wikipedia.org/wiki/Atmospheric_water_generator which would allow it to collect water from air itself. It would get water directly from the clouds and water vapor itself. I guess the leaves can act as a net to allow water to condense and is then absorbed by the leaves itself.

I highly doubt the tree can survive past 6000 km depending on where the tree is situated as temperatures would start decreasing drastically near the tropospause and air would get thin. As for the trees support and weight, I'm thinking that it would have to evolve a kind of multiple tree trunk that winds together? A trunk that twists together might be a better support for the lower part but it would straighten out as it gets higher to avoid even more weight pressing down. For the amount of leaves needed to support a massive tree, the branches would need extra strength by either supporting each branch by intertwining[might be disastrous if the bottom branches break] or by 'braiding the branches' which should increase the weight it can hold.

Otherwise, it might be a nice little cheat to have your tree be supported by a really really tall steep cliff[part mountain part tree]?

• Alien trees 0_0 , I'm not sure about the winding and twisting part, Any experts?
– Skye
Jul 25 '16 at 13:51
• Yes. I have heard that the tops of the redwoods on the California coast, for example, get their moisture from the air. Jul 29 '16 at 3:29

Symbiosys Life forms like little insects and birds search protection in the tree, the tree get water from animals urine and from air umidity (using sponge like leaves).

Primitive refrigeration The leaves could have a shape like a double "u" that is great for converting wind energy into mechanical energy, the side effect of that is that wind get cooler, allowing condensation of water (even if in very small amout).

Symbiosis II Certain life forms could actually evolve to bring water to the tree and get in exchange small fruits, if the tree is that big it is likely it has a complex ecosystem inside it wich would make the tree also a isolated system.

This is for getting water.

For getting instead great height instead we have more problems, the tree would have a shape that resembles a reversed cone (base on ground and vertex upside).

Wood is light but also weaker than iron, so the height maximum reachable (to avoid mechanical breaking of the tree) should be slightly above or below the highest buillding, so don't expect it to be higher than a kilometer, however you could partially cheat if the tree grows on the highest mountain in your world.

EDIT: since tag advocates are becoming increasingly boring, here's for the hard-science part:

Symbiosys: no know micro-organisms have such a behaviour, however there's a spider that is able to transfer air bubbles below water level to create a nest. Since there are already life forms that move air somewhere, it is plausible to assume are possible life forms that move water somewhere. Also, snails have a protective coating to avoid dehydratation when sleeping, the tree and its life forms could use a similiar mechanism to preserve water.

Regarding the refrigeration: simple physics law, if air do any work it spends energy in doing so and hence lower its temperature, also certain shapes (double "U") are really helpfull in draining thermal energy and converting it in kinetic energy

Since a big life form is highgly susceptible to die by any disease (single DNA), it is more realistic if the tree is in reality a composite life form so that it has enough DNA variety to not die entirely due to diseases. We have already proofs of this being possible (moss is in reality made by 2 different life forms cooperating), also it is possible to do vegetable grafting (put a plant branch to a foreign plant to obtain new type of fruits), another proof that a vegetable life form wich is in reality made by different life forms is entirely possible.

• Upvoted for first suggestion of symbiosis I've seen. Don't overlook the possibility of multi-party symbiosis (as in the lichen, which was recently discovered to consist of two type of of fungi and an algae), and even a designed symbiotic relationship. If you could design a lichen-like tree, that would grow horizontally as well as vertically, it could be an all-in-one terraforming juggernaut for an advanced society. Think of "the creep" from Starcraft, but maybe more like the jungles of Kashykk (Star Wars)? Jul 27 '16 at 16:40
• Then the sentients who designed the tree could care for things like water and nutrient delivery to the upper levels, and even pruning and reinforcement to enable the tree to exceed even its designed structural limits. Jul 27 '16 at 16:46

Wind is a killer. There is a reason that the tallest trees have few branches below their crown. Catchment basins would act like sails and tear the tree apart. Sorry to rain on your parade. The highest building in the world is 830 meters tall and I doubt that mother nature could do more than double that with a living organism, but I'm just guessing...

• you're missing the fact that theoretically the planet could have very little or no wind. Jul 25 '16 at 20:24

Just passing through and thought I'd leave my thoughts on this topic...

Have a look at the latest (last) book in Terry Pratchett and Steven Baxter's Long Earth series, "The Long Cosmos". One of the worlds there has ginormous trees. Water transpiration to the upper branches is effected by the trees capturing / generating hydrogen, and using this to float sacs of water up to the upper levels.

Of course, the wood of the trees were highly imflammable, being chock-a-block full of hydrogen. The existence of such a world and such curious trees was never reported, probably because any traveller who stepped in, and stayed long enough to note the size of the trees, probably stayed long enough to collect some wood and make a fire... BOOM!

• XD. Might be best to stick with helium then
– Skye
Jul 30 '16 at 7:20

No. As trees are today it is impossible. In trees water is drawn up the tree passively. To get the kind of heights your looking for the tree would have to pump it own water. I can't see how a tree could produce enough energy through photosynthesis to power an active method of water transportation at that height. Furthermore if the tree grew too high the air would be too thin for photosynthesis or respiration. In order to survive our tree would have to eat other plants for sufficient energy and would have to pump oxygen up to the top of the tree.

• I'm pretty sure his tree would "work" for his story even if the technical internal structure is wildly different from real earth trees, as long as it has the general outward features/looks.
– AnoE
Jul 26 '16 at 10:35
• And then the one small answer that starts with "No." Jul 26 '16 at 14:34

A smaller version is possible, especially if it is adapted for that sort of thing: It can have a very thick base and thinner at the top (so it won't break), and very long roots that are spread out (so it will get minerals).

If it is near a river, in a relatively windless area then it can grow to large heights. This solves the problem of nutrition (the waste in the water provides nutrition, as well the other stuff stated by KingOfSnakes and it can have a heart which provides additional pressure to draw the water to its leaves.

It depends on your definition of "tree".

First, there are multiple answers here that propose a heart--this doesn't overcome the problem that lifting the water costs more energy than the photosynthesis yields. This means either we obtain the needed water directly from the atmosphere (which means a humid climate) or we need some radically different chemistry. Since we want something tree-like lets start with photosynthesis--but the high energy molecules aren't sent down the tree, but only laterally to the trunk where they are converted to electricity. The products are sent back out to the leaves to be combined again--you only need to lift for growth or leakage, not for energy production.

Now, we need a very strong tree--instead of wood lets figure a core of metal foam. Once the roots find suitable ore deposits they grow a metal foam core in each trunk. For the sake of strength you need multiple trunks with many cross connections. Look at how antenna towers are built for how the tree has to do it.

As the tree gets big enough we can take some of the load off it. The tree grows a transparent but airtight canopy over itself. The leaves are inside this canopy. The canopy acts like a greenhouse--the insides are warmer than the ambient air which is good as it gets cold up there and you have in effect a giant hot air balloon. If it's big enough the only elements in compression are those holding out the waist of the bubble, the rest of it is all in tension and therefore much easier to support.

The tree needs to have full regeneration capability or else the ravages of time will kill it before it gets to the size of a world tree.

I can think of a few options, some of which will just be additional notes on existing answers:

1. Capillary Action

Capillary action could possibly circumvent gravity - it already does on a small scale in all sorts of real world situations. The basis for capillary action is surface properties and surface tension of fluids allowing them to rise (or fall) through extremely narrow channels. A fountain pen is a great example of this. If you take an empty fountain pen and rest the nib in an ink well, ink will slowly rise through the nib into the ink feed.

2. Water Collection Basins

A tree could have natural collection basins, akin to a strawberry pot. This would allow the tree to naturally collect reserves of water anywhere along its surface. This could even be done (through some twist of evolution) at each location where a branch meets the trunk. Actually, I'm almost surprised there are no Earthly trees (as far as I know) that do this. Or possibly it's leaves could be designed to funnel water into such a system. Hostas work like this on a much smaller scale - drawing water into the base of the plant.

3. Atmosphere

If your planet is not strictly Earth-like, and the tree is tall enough, a persistent or near-persistent vapor base (clouds) at or near the crown of the tree could provide ample water, especially if the tree had adapted structures that would help capture and channel water. This could even be as simple as interestingly channeled bark-like structures trapping and condensing water from the vapor.

• What kind of clarification does this need? I understand the reality-check and hard science tags - but I also believe that atmosphere and capillary action are readily available for discovery (these are not esoteric science concepts or heavily difficult concepts). Additionally, the tag may need to be moderated off of the question since it's difficult to provide "hard science" for a question that specifically states that Earthly principles need not apply. Jul 26 '16 at 22:03
• Jesse - That was me. I added post notices to a number of these answers because the majority don't meet the stringent requirements of the hard-science tag (see the tag wiki). Most don't even link elsewhere - much less to a source like the ones the tag wants, like a peer-reviewed paper. This is a short-term option I'm exploring while we try to work out the details here. I'm sorry for any inconvenience this caused, but I'm trying to see if adding the post notice will work. Jul 26 '16 at 22:30
• @HDE226868 understood. That makes a lot of sense. Thanks for the clarification. Jul 26 '16 at 22:40
• Thanks for understanding. I considered saying something public about it - and I think I wrote offhand something in chat earlier today - but I wanted to keep it low-key to see how many people saw the post notices on their answers and responded with good edits; I chose this question as a test case. I have my fingers crossed for good results. Jul 26 '16 at 22:43
• I'll see what I can do. I would suggest that a post shouldn't have science-based and hard-science since the latter implies the former, but the former has looser restrictions. Also, based on some details of the question, I'm not sure the OP has a good case for hard-science as a tag. Jul 26 '16 at 22:49

All other answers assume the tree is growing in a gravity well. This need not be the case - however, other constraints will apply (read Niven's 'The Integral Trees').

There is also a question of scale. A perfectly normal tree could be considered a 'world tree' by a very very small intelligent race.

The tree could get "help" from other organisms, willy nilly. People could feed it (they'd need a motive). Or it could consume other plants. Then it could have access to more energy and different materials than a normal tree.

I highly recommend that you read the novella The Son of the Tree by Jack Vance. It also has a world tree. Its feeding patterns may be...instructive.

It could also do something with solar power in order to generate energy, either by heating water in basins through reflective leaves, or by using metals that catch light and convert it into electrical power. It may be far-fetched, but it could work in a novel.

Another idea is that the world tree could get it's nutrients from the waste products of animals that live on it.

I believe that in order to make the tree possible the tree would have to be constructed with genetically modified tissue vertically with an ascending team of 3D printers as if a spider wove its cloth upwards, and this would have as main objective to have a "living" To shelter human life like living in a biological skyscraper, I believe that with all current and future technologies and the most brilliant minds in engineering and design it would be possible to build a tree that could reach 40,000 feet, ok no :P.

• Welcome to Worldbuilding Peter! This questions asks for hard science, meaning that all answers to this question should be backed up by equations, empirical evidence, scientific papers, or other citations. Any answers that do not satisfy this requirement, like yours, might be removed if you don't edit your answer and add in scientific evidence to support it.
– fi12
Nov 12 '16 at 19:42

Lets say world trees really could have existed on earth it would need immense water to sustain itself i think of natural pure and warm spring water under it that flows up under higher pressure created under ground aiding the tree to pump the water up higher into its self and to prevent it from crushing into its own weight parts of its stem/trunk and root system must have been crystallized by the pressure of its weight together with the minerals within the spring water as it grew over time (like some of that evidence is found in tree stumps/trunks on this site https://secretenergy.com/news/are-these-giant-prehistoric-trees/)

After looking at all of the comments, this is my personal conclusion for the ultimate "world tree".

Leaves with very short hairs. These leaves would not only produce energy through photosynthesis but absorb water like roots, where the hairs act as root hairs. With slightly different physiology this is certainly possible.

Location in a valley providing relative shelter from winds. The tree would also be very thick at the bottom and get progressively thinner to avoid breaking.

No more than 2000 meters (2 km) high to avoid constant, ultra-low temperatures, but still have a massive size. This would also remove the need to make ridiculously strong wood or other, more complicated and/or unrealistic elements.

Light but relatively strong wood. The tree would also (as with most plants) have a high lignin content, from which it would somehow produce its own carbon fibre and implement it into its own structure in high quantities, greatly strengthening the tree to keep it from essentially imploding.

Pools of water from interlocking branches and/or leaves above ground-level, drawing in critters and providing energy reserves if needs be. Microbes, insects, small mammals, and other organisms would be attracted to the relative safety/shelter of the tree, and with water, they would be even more likely to wish to take up refuge there. They could provide minerals and other nutrients in the form of their waste, and could even form a stronger symbiotic relationship of some kind (use your imagination). The pools would also work as energy stores, where the water could be saved and then used by the water-absorbent leaves.

Very thin channels in the tree. Water and other nutrients could travel throughout the tree based simply on the behaviour of fluids in very small channels (see Jesse Williams’ response for a somewhat more detailed description).

Large root system capable of bringing other nutrients and support/stability.

Multiple "trunks" winding together. This would divvy out the trees own pressure more evenly, among other things.

In my opinion, a world tree would be impossible to make if the trunk is only composed of one part. But if we take into consideration some trees like the Banyan, or much more known in our country as balete, I think it could grow to become a world tree, provided that it has sufficient nutrients to support it.

A banyan tree is really a small tree if it is alone. However, it makes multiple aerial roots. And these roots grow big until it becomes like it's mother trunk. Its aerial root system made it possible for it to become bigger and bigger without fearing of being toppled down by gravity.

So, that's it. If our world tree has multiple aerial roots, it can support itself and not fear the ferocious hand of gravity. And it will grow taller and taller, but it will expand more and more, taking up more space than we would like it to be.