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I ask the question, not as a subjective "well, Dale, what do ya reckon?" but as a perfectly factual question about tree growth here on earth provided a finite set of physical (natural, real) conditions that are all, perfectly, verifiable.

Criteria or conditions that come to mind for "biggest possible tree on earth" (sorry for the trite superlatives, I put it in quotes facetiously actually) are

  • Gravity
  • Nutrient-rich soil
  • Lack of outside interference that stunts the tree (harsh winters, humans, etc)
  • Weather conditions
  • Ecosystem

I listed gravity because if for example, gravity were twice as intense as it is right now, tree growth would be affected in much the same way that all kinds of animals and insects would be affected. In the past, as yet another example of the importance of gravity, scientists used to think that dinosaurs couldn't have been as big as they are presently held to be because the gravity of the earth would have made it so their bones could not support their own weight. Such reasoning was meant to explain the then-accepted expanding-world theory, in which scientists hypothesized that the earth, therefore, must have been smaller, which seemed to reconcile evidence of massive dinosaurs and moving continents. Of course, the theory was later falsified by the theory of plate tectonics, but that's beside the point.

When the dinosaurs roamed the earth, forests could expand as much as natural conditions permitted and, again to my example, if gravity were less than it is presently by a factor of however much, one should think that vegetation would flourish accordingly. Additionally, there were no human industries or activities and hence no pollution. Even today, trees that are left uncut as monuments of the past, expand to utterly unbelievable girths, and can sometimes reach mind-boggling heights.

All that said, a tree as imagined in movies like Avatar would seem ludicrously impossible here on earth, which is where one starts to wonder: what are the natural limits of trees in the best possible environment. Perhaps with something like nitrogen or say a kind of steroid-equivalent for plants, trees could attain far greater heights?

Of course, I emphasize the absolute best conditions or environment because that is the means by which the question remains answerable and yet adapted to personal curiosity. If yet another example should help, consider a tree that had all the water it needed (but not too much, i.e. a Lebanese cedar tree versus a mangrove tree), all the ecological conditions necessary to reach its maximum growth, (endless nutrient-rich soil, insects that didn't kill it, no diseases, no other trees whose roots poison it; no trees that grow over it and suffocate it--yes, some trees suffocate other trees), the best possible conditions of gravity, etc. Additionally, one can't forget the genus of the tree in question, since for example a dwarf willow is considerably different (and smaller) from say a giant sequoia: Clearly you would agree, I think, genius is a serious criterion if to answer the question.

On a side note: I admit that gravity might be a hard condition to humor, but in as much as less gravity would allow for bigger trees, it should also seem a crucial criterion in answering the question. That said, I recognize that I'm only the questioner and not the responder, so take it away. :)

Nearly last but not least: While it may seem foolish to leave out the fact of evolution as a primary criterion, the problem with including it as one--at least to my mind--is that one could misuse it to create false or unsupported assumptions. Consider the following truism and its conclusion: If you follow any evolutionary timeline of any given plant (say a fern plant), you see that various subclasses appeared (evolved) and disappeared in different geologic time periods of that plant; therefore, a subclass of giant sequoia could appear (evolve) to ten times the size of their present ancestors. As you'll no doubt agree, such would be far too easy. Hence, while evolution is no doubt crucial point to consider, hopefully, it'll be used advisedly.

One final point: If you think the question can't be answered for some other reason, please share why you think such so that I can edit the question. In other words: please show how it doesn't meet some standard versus just citing the standard. It might be clear to you why it isn't right, but "down vote" doesn't always tell the questioner why their question isn't right for the community.

Thanks very much.

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  • $\begingroup$ One thing that's interesting to note is the size of insects millions of years ago. The predecessors of modern dragon flies had wingspans close to a meter. The reason for this was the extra oxygen in the atmosphere. It's possible that a similarly CO2-rich atmosphere might lead to gigantic flora. $\endgroup$ – Jonathan Landrum Apr 26 '18 at 14:23
  • $\begingroup$ Thank you, @JonathanLandrum, that is an interesting point and would certainly make sense. $\endgroup$ – Private Name Apr 26 '18 at 14:53
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    $\begingroup$ There are a lot of words here and I did not read all of them. But how do you define "Big" is it tallest, widest, heaviest, largest root system, If the roots of two are interconnected do you only count one or both/all $\endgroup$ – James Jenkins Apr 26 '18 at 15:51
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    $\begingroup$ worldbuilding.stackexchange is not about terraforming. It is about creating fictional worlds, even if they are variations of Earth. I think it is safe to say that Earth does not have any trees that are currently at the maximum possible height for a tree on Earth. This question is asking what the maximum could be, and asking about conditions which do not exist on Earth, especially the alternate gravity, therefore it is a hypothetical alternate Earth. This question fits great for worldbuilding.stackexchange, even if OP is not world building. $\endgroup$ – Loduwijk May 14 '18 at 14:51
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    $\begingroup$ @PrivateName But all of that is just arguing semantics anyway and is barely relevant. Maybe it's not off topic for Outdoors - that is debatable. It is, however, not the type of question Outdoors generally fields (that doesn't automatically disqualify it, but it does make it less likely to fit). This is perfectly well on topic for Worldbuilding, and Worldbuilding gets questions of this exact nature all the time, so it is a near perfect fit here. Do you want arguable fit or near-perfect? Also, you now have twice the exposure. $\endgroup$ – Loduwijk May 16 '18 at 0:31
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So if you want to know the tallest a tree could physically grow to, it would depend on the compressive strength of the wood and the density of the wood.

So for example, lets take the the Bubinga Tree.

http://workshopcompanion.com/KnowHow/Design/Nature_of_Wood/3_Wood_Strength/3_Wood_Strength.htm Says its psi is 10,500psi which is 7382217.215488 Kg/m^2 according to this conversion tool http://www.endmemo.com/sconvert/kgf_m2psi.php.

So a square cross section of 1m would support 7382217 KG or 16275002.598 pounds.

Now the Bubinga Tree has a density of 890 kg/m3 from http://www.wood-database.com/bubinga/

So a 1m^2 cross section would be able to support 7382217/890 which is roughly 8295m.

So the tallest a Bubinga Tree could possibly reach would be roughly 8295m Tall before it would collapse due to its own weight.

Now this is a super simplified answer and I'll point out why:

  • The maths uses the dry weight of the wood and doesn't include water in a living tree
  • The maths also assumes the tree is the same thickness the entire way up. Tree's usually aren't (or so I believe) and hence your limit would be a bit higher
  • I don't know if a tree would be able to physically get water and nutrient up that high
  • Gravity will affect the height as it affects the weight, but it should also affect how the tree moves around nutrients and water
  • A slight breeze will probably knock the tree over. It would have to grow perfectly straight
  • It will take forever to grow and I don't know how extensive a root network it would need to support itself
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    $\begingroup$ The strength of the wood isn't the limiting factor for height, it's the ability to transport water & nutrients from the roots to the top of the tree. That limits the max height to about 400-426 ft /(122-130 m): livescience.com/14667-tall-trees-grow.html $\endgroup$ – jamesqf May 16 '18 at 4:02
  • $\begingroup$ Furthermore, you are just including the tallest tree without weitht from branches or leaves, and no wind. However, your aswer it's insteresting because it shows that the weight of the trunk itself is not a constraint for practical purposes. $\endgroup$ – Pere Jan 30 at 11:20
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The limiting factor to tree height is the ability of the tree to pump water up from the soil to the leaves. That limit is about 130m / 426 feet.

Normally the limit to pump water is about 10m - that's the highest that a vacuum can lift water. However, trees play all sorts of fancy games with surface tension and capillary action to dramatically increase that limit.

There's a cool video describing the effects here: https://www.youtube.com/watch?v=qiiFMRYUEQM

Scientific studies have put the absolute limit at about 130m. Here's one such study, but it's behind a paywall: https://www.nature.com/articles/nature02417

Here's a popular science press discussion of the topic: https://science.howstuffworks.com/life/botany/tree-grow.htm

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The main factor determining where and how fast plants can grow is the amount of CO2 in the atmosphere. Optimum is somewhere between 1000-2000 ppm of CO2 in atmosphere - depending on type of plant. In that case the plants grow about 2-5 times as fast compared to current conditions. Tree growth is guided by few rather simple rules, but depending on conditions there will be a lot of permutations, so it it really depends on tree species, climate, soil and location.

It would also impact the range and size of the biome on Earth, as the more CO2 is available in atmosphere, the less water plant requires to live (at optimum CO2 level it's about a third compared to today). This in turn can impact the maximum height for tree growth. But again: how high tree grows is determined by number of factors, of which gravity impacts theoretical maximum, which is rarely, if ever, achieved by any tree on earth. This is density of the forest, type of the soil, geographical location, and so on. But lets theorize a bit: since less water needs to be transported within the tree (due to high CO2 concentration), so let's add 50% (mine, arbitrary) to current theoretical maximum of 140m and we get maybe 200m tall trees. Possibly.

As for biome range - theoretically one could expect almost complete coverage of Earth's surface with forest of all types (with obvious exception of water bodies, alpine and polar regions), within several centuries or less.

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  • $\begingroup$ Can the downvoter be bothered to leave a reasoning? I hate when that happens. $\endgroup$ – AcePL Jul 11 '18 at 14:11
  • $\begingroup$ I am not the downvoter, but maybe it would help to add some sources for your statement that the optimium CO2 composition for example. $\endgroup$ – Sec SE - clear Monica's name Jul 13 '18 at 13:41
  • $\begingroup$ @Secespitus On one hand I can understand that. On the other one does not have to link - for example - a relevant section of the traffic code to argue that in UK motor traffic is left-side... 1500 PPM is the reference value in greenhouse gardening. It is well established fact. It's a value that any food producing specialist will rattle off when asked after being woken up in the middle of the night. With all due respect (and I really mean it). $\endgroup$ – AcePL Jul 13 '18 at 15:50
  • $\begingroup$ Well, most people aren't food producing specialists, which is why to most people this probably looks like a random number that you could just as well have made up. And pointing to a traffic code might be relevant if the side is important to your answer. Not every country drives on the left side. But obviously it's up to you. I just saw your comment and thought I might as well inform you of something that might be the reason. Whether you act upon that suggestion or not is up to you. $\endgroup$ – Sec SE - clear Monica's name Jul 13 '18 at 15:54
  • $\begingroup$ @Secespitus Most people aren't also professional drivers, yet know which side to drive on. I understand that they have somewhat more evidence to that than "I told you it's left", but still. Some things are really "mainstream". But I get your meaning. Editing accordingly. Hopefully no one will object to "quality" of the sources. $\endgroup$ – AcePL Jul 13 '18 at 16:12

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