In my fantasy world I want fantastically large trees, particularly in the height category. The tallest trees on earth grow just over 100 meters tall. Why can't they get taller? What circumstances would allow a tree to grow taller?

Currently I'm growing my trees in micro-gravity, in a sterile environment with no diseases to kill the trees, and with little to no wind to knock them over. How tall could my trees get? What else do I need to take into account if I want my trees to grow to several kilometers in height?

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    $\begingroup$ I can sense the square-cube law getting ready to crush your dreams under their own weight... $\endgroup$ – DaaaahWhoosh Feb 24 '15 at 15:25
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    $\begingroup$ Why would the square-cube law be limiting to tree height? The limiting factor is generally assumed to be energy required to draw water up the trunk, which is linearly proportional to height. $\endgroup$ – ckersch Feb 24 '15 at 16:14
  • $\begingroup$ Interestingly enough, "no wind" may not actually be advantageous for tree growth: en.wikipedia.org/wiki/Biosphere_2#First_mission $\endgroup$ – Mansfield Feb 24 '15 at 21:01
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    $\begingroup$ Derek Muller from Veritasium goes into some details about how and why trees grow so tall, and their limits - here, here and here $\endgroup$ – Möoz Feb 24 '15 at 23:36
  • $\begingroup$ Much of what has been said is based on gravity to limit the transport of water. We have all seen trees that have fallen over when the soil became to water laddened. Many of these trees still live but are now horizontal rather then vertical. Would this not all but eliminate the problem of gravity keeping water from the top of the tree? $\endgroup$ – Skip Riffle Jan 1 '17 at 14:58

Water movement appears to be one of the big limiting factors in tree growth. It appears on earth that 120-130m is about the maximum height of a tree. It gets harder and harder to get water up to the top.

So the easier thing to solve would be a smaller gravity. The other would be for a stronger 'pump' action maybe a symbiotic microbe that helps move the water up. Or maybe the tree has reservoirs to collect rain water, though those voids would make the tree susceptible to breaking.

After that then it is a matter of having enough nutrients to continue feeding such a giant, as well as a base to support it. The larger a tree the greater the forces are for wind (longer lever) etc.

Lesser gravity would also help reduce problems with the squared-cubed law...

Oh, and I'm not sure how well they'd do in micro-gravity, Some gravity would be better, Mars or the Moon (WAG-IMO)

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    $\begingroup$ I don't know enough physics to be sure about that, but if water movement is induced by a pressure gradient, then reducing the air pressure at the leaf-level and increasing it at the ground level may be a good idea. Or maybe having hotter air at the leaf level so that water evaporates more quickly. $\endgroup$ – Maxime Lucas Feb 24 '15 at 16:12
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    $\begingroup$ or a way to harvest water from the air (storing rain or something) $\endgroup$ – ratchet freak Feb 24 '15 at 16:20
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    $\begingroup$ For obtaining water, why not have a tree that relies on heavy fog to provide moisture to the leaves? This eliminates the potential energy issue without needing internal water storage. Some Earth trees can already do this: newphytologist.org/news/view/12 $\endgroup$ – ckersch Feb 24 '15 at 16:21
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    $\begingroup$ If you get around the issue of water transport, the new limiting factor is likely to be the tree's ability to support its own weight. The base of the tree must be able to withstand the pressure from all the material above it without collapsing. (Again, lower gravity would enable taller trees.) $\endgroup$ – Royal Canadian Bandit Feb 24 '15 at 16:49
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    $\begingroup$ @ckersch However the tree still has to bring nutrients to the leaves. But maybe if you put nutrients in the rain it could work. $\endgroup$ – Maxime Lucas Feb 24 '15 at 16:52

I think a lot could be learned from the way very tall trees make it work here on earth.

Scientists are making some pretty interesting discoveries about how the redwoods are reaching their impressive heights. It appears that the redwoods are collecting water from the top and the bottom by drinking water from the ground as well as from the canopy.

Some redwoods have lived since the days of Jesus Christ. With time, their immense, complex canopies trap needles, dust and seeds, creating peaty soil mats a yard thick and as big as a bus that grow plants, sustain animals and absorb water hundreds of feet above the ground.

"Eventually, you get this huge sponge that builds up," said Steve Sillett, a Humboldt State professor who began studying the phenomena in redwoods in 1996. "During most of the year, it's an aquatic environment up there" fed by rain and fog.

He's discovered mollusks, crustaceans and other animals ordinarily found in stream beds -- even the wandering salamander, which lacks lungs and must stay moist to absorb oxygen through its skin.

Like trees in the Pacific Northwest and other temperate rain forests and cloud forests, the redwoods sprout canopy roots from their branches that Sillett believes take in water and nutrients from the hidden gardens.

Effectively the trees create their own little gardens high up in the canopy and sprout canopy roots or adventitious roots to collect water and nutrients.

Another path or even a combined approach might be the use of aerial roots:

Aerial roots: roots entirely above the ground, such as in ivy (Hedera) or in epiphytic orchids. Many aerial roots, are used to receive water and nutrient intake directly from the air - from fogs, dew or humidity in the air.

Scaling this up to kilometers may be a bit of a stretch, but it does solve the water issue.

Just had a thought on a way to potentially work around the kilometer issue...

What if your trees were partially petrified? As in the inner core of the trees turned to stone by absorbing minerals from the soil. You may think this would mean that your trees would have to be millions of years old, but that may not necessarily be the case:

The rate of petrifaction is not exactly known. In some cases it may be fairly rapid. For example, mine timbers have been partly petrified after a few years' exposure to mineral-laden water. Most petrified wood was formed long ago. For instance, stone logs in Petrified Forest National Park, Arizona, are of the Triassic Period and more than 160,000,000 years old.

Again this is probably stretching a little far, but it may give your trees the added strength they need to grow to the heights your looking for.

Another option may be to have your trees form an interconnected network, like The Great Banyan having branches roots and trunks interconnected may offer some really significant strength advantages.

enter image description here

"Acharya Jagadish Chandra Bose Indian Botanic Garden - Howrah 2011-01-08 9724" by Biswarup Ganguly - Own work. Licensed under CC BY 3.0 via Wikimedia Commons.

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  • $\begingroup$ I also have a need to create a large tree, and found your answer intriguing. Assuming a gigantic tree had its base made out of aerial roots like the Banyan, would those roots be able to support the tree? It seems to me that a lot of thin supports wouldn't be the same as one thick support. Would I be wrong? $\endgroup$ – Thomas Reinstate Monica Myron May 27 '15 at 21:58
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    $\begingroup$ Petrification would be unlikely to be a good thing, part of a tree's strength is in flexibility, a petrified core would shatter and become dead weight. $\endgroup$ – Separatrix Nov 24 '17 at 8:43

The Long Cosmos by Pratchett/Baxter actually contains chapters (+/- ch 40) where hero climbs a miles high tree in a "forest" of such gargantuan trees.

Trees on that world evolved to embed hydrogen gas in their wood/structure, so their wood is nearly weightless. These trees also use hydrogen gas - filled structures to transport water upward.

Needless to say, fire has rather spectacular consequences on this world.

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