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This question is in relation to speculative biology/evolution: A genre of science fiction and hypothetical branch of science which seeks to explore "what if" questions about life using established principles of biology, ecology, evolution and related scientific fields.

Illustration of a pneumatized bone cross section

Illustration of pneumatized bone cross section, and hypothetically what a pneumatized tree trunk might look like.

Assuming ideal Earth-like conditions, just how large could a tree with a heavily pneumatized trunk and branches become?

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  • $\begingroup$ You'll also probably need to detail what you mean by pneumatic trees with words, a simple picture of a pneumatic bone side cut isn't likely... Well, to cut it 😋. (It's also kind to blind people!) $\endgroup$
    – Tortliena - inactive
    Commented Oct 4, 2023 at 10:51
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    $\begingroup$ @Tortliena I thought that the question already provided the necessary information for the topic? I will make some changes though, so thanks for the feedback. $\endgroup$
    – Inanis343
    Commented Oct 4, 2023 at 11:36
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    $\begingroup$ People build animals and plants on many worlds, some even considering non-carbon based life. We can assume that unless stated you talk about an Earth-like planet, but it's much better to explicitly state some core elements to make sure. Also, if I'm visually impaired, I cannot at all read your picture, and so I cannot read your question, either. $\endgroup$
    – Tortliena - inactive
    Commented Oct 4, 2023 at 12:55
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    $\begingroup$ (1) Large and tall are different things. (2) The limitation to tree height is the mechanism which pumps water up from the roots to the leaves, and not the strength of the wood. (And please note that the transpiration-cohesion pumping mechanism used by trees is already amazingly capable.) The tallest freestanding structure built entirely out of wood was the 190 meters (620 ft) tall Mühlacker radio transmitter antenna tower built in 1934. (3) The width of tree trunks is limited to about 10 meters (30 ft) only because why more? $\endgroup$
    – AlexP
    Commented Oct 4, 2023 at 13:00
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    $\begingroup$ Since you're new to worldbuilding you may find it useful to carefully read the tour and visit the help center there's a section on how to handle question closure I think you'll find useful. If you're unsure about how to interpret a policy regarding edge cases you can find discussions about that on Worldbuilding Meta. The sandbox is useful if you want feedback on a question before posting to the main site. $\endgroup$
    – sphennings
    Commented Nov 2, 2023 at 20:46

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With a strict botanical definition, 0m

If you use a commonly used botanical definition, trees are distinguishable from other plants in that they grow new rings each year. As soon as you only have one ring, it's technically not really a tree anymore. For instance, palm trees have a single disk, which is why we tell it's a "tree-like" plant rather than an actual tree.

And unfortunately, the same thing happens here with one single, hollow ring, so if they're pedantic, a botanist will tell you you cannot have trees with an hollow trunk.

But honestly, that's neither interesting for the common folk, nor as an answer to a question that just wants plants to grow. So we'll use the broader definition of "tree" : Something big, with a sturdy trunk going upwards that needs more than your fists to be cut down1.

With the day-to-day definition

Since your tree cannot actually really grow like a tree, I suggest you look at bamboos. Indeed, they have a very similar structure to your proposal : A single ring with some reinforcement inside. The main difference is that the reinforcement is set at regular intervals for bamboos in the shape of disks. For your hollow tree, it's more chaotic but likely more "filled" at the same time. While its growth is quite different from regular trees2, it can actually be an inspiration of how they can reach such heights.

A photo of a patch of Dendrocalamus giganteus, reaching up to the skies
A patch of Dendrocalamus Giganteus. By David Stang, from Wikimedia Commons

As far as I know, the tallest bamboos live in the Dendrocalamus genus3, 4, with height branching from 20 to 40 m and with a comparetively low 10 to 30 cm in diameter. So you can very reasonably set your expectations in those areas. If you add more internal structure, you'll likely be able to grow higher and -most importantly- wider. Higher fiber proportions and higher material density should also allow overall stronger trees5.

You likely won't get much wider trees because there's not much evolutionary advantage in that : The canopy of leaves won't extend as much as a regular tree which allows more developped, fatter branches. It's a trade-off between winning sunlight dominance (by faster vertical growth) and a stronger structure allowing more sunlight at the end. Also, a larger hollow trunk is more likely to get the attention of animals and insects. Whether they are looking for shelter or to parasite the tree, they don't need to carve all their house anymore, mainly the exterior wall now. It can be nice to think about that too when designing your ecosystem as a whole. This can be also an impeding factor in how big your tree is.

1 : Contrary to what a famous video game can tell you :p.
2 : The bamboo's shoots acts a bit like a telescopic tube : Everything is already inside, ready to expand and grow leaves. See this video for a more visual demonstration (in French, but having watched it without sound it should be understandable enough)
3 : Source : https://en.wikipedia.org/wiki/Dendrocalamus_giganteus
4 : Source : https://bambubatu.com/genus-dendrocalamus-the-biggest-bamboos-of-all/
5 : Source : https://jwoodscience.springeropen.com/articles/10.1007/s10086-017-1687-3

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