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In the process of developing an ocean world setting for my fiction, I have often pondered whether to give the sapient species of the world access to some form of wood or other resource from underwater trees.

I would like to know you thoughts on how plausible underwater trees would be on an earth-like ocean planet, what would they be like and what features, resources and threats would they have?

Consider expanding your definition of trees for this question, I simply mean some large roughly vertical plants that fulfill the role of trees, even if you can picture them looking and acting nothing like terrestrial trees.

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  • $\begingroup$ You could try corals and fungi. I'll see if I can elaborate this further later. $\endgroup$ Jan 13 at 21:35
  • $\begingroup$ There are many large seaweeds growing just about vertically underwater. The problem is that things which live in water do not need a lot of strength to stay vertical, because water is so much denser than air that apparent weight is very small (or even negative). $\endgroup$
    – AlexP
    Jan 13 at 21:46
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    $\begingroup$ @FrankRebin Site etiquette is to give 24 hours before accepting an answer so people from around the world can have a chance to answer (or folks like me with day jobs). I love the question, but some folks might not give an answer if there is no hope of their answer being picked. $\endgroup$
    – DWKraus
    Jan 13 at 22:34
  • $\begingroup$ @DWKraus My apologies, I should have thought of that, it makes perfect sense. I have unticked The Square-Cube Law's answer for now. $\endgroup$
    – FrankRebin
    Jan 13 at 22:40
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    $\begingroup$ Adding to what @AlexP said, the differences between water and air also mean that air bladders are a more energy efficient way to stay upright than a thick trunk, which is more brittle and could be smashed in storm currents. Air bladders and lack of woody tissues are what most large seaweed like kelp do. $\endgroup$ Jan 14 at 4:29
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Mangrove Madness & Floating Islands:

Is your entire planet Ocean, or are there islands? Mangroves are a source of wood that is intimately tied to the sea. While it is not DEEP sea, many species are dependent on mangrove salt-water "swamps" for critical parts of their life cycles. It would likely fill the role you are looking for, allowing a sea-faring species access to wood, and might also provide an intermediary environment where sea and land species might come together in either competition or cooperation.

The problem with wood is that most aquatic plants don't find it necessary to support great weight against gravity, or have a reason to anchor resistant structures against the power of the currents, so buoyancy is the simple solution to keep a plant up. If you don't have islands, perhaps there were once islands, and there is wood preserved under the sea. My understanding is that wood is sometimes stockpiled in deep cold water to preserve it until needed. So some might be legacy wood.

A fun alternative might be to have a floating tree ecology. Ancient trees got around extinction by forming a floating platform of interconnected trees, almost like a grown ship. Perhaps some land animals or bird species take advantage of these structures to live on or lay eggs. There are certainly species of birds today that essentially never land and fly over the sea for months at a time unless laying eggs. The guano from the birds provides nutrients for the trees and carries seeds for sub-species of plants on the island. I imagine a species of Jelly fish that lives by hanging off the bottom, pulling up fish and other creatures to eat amidst the roots and providing a nutrient-rich supply of remains for the trees (plus protection from predators who might eat their roots), while the trees provide a home and safe haven for the jelly fish. The trees might have a distillation method to produce fresh water if desired (although over time they would likely tolerate salt better and better, but this could help support the floating island biome).

The jelly fish and surface environment would mean your people could get wood, but you can make that as challenging for them as you want. It could be common, or a rare and valuable commodity.

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  • $\begingroup$ This is a great concept! I have pondered the idea of floating lily pad ecologies to add some surface interactions to what I want to be an almost completely oceanic world (perhaps less than 1% surface is land) and these mangrove nest ecologies fit in perfectly to add variety. My only concern is how long the wood of surface trees, be they mangrove or your conceptual ones, would last when taken underwater by the alien species to use. $\endgroup$
    – FrankRebin
    Jan 13 at 22:49
  • $\begingroup$ @FrankRebin the colder/higher pressure, the better. Most seem to last a long time unless they dry out at some point. But wood is a renewable resource, so it's not uber-critical. This article says trees might last up to 10,000 years in the ocean. smithsonianmag.com/smart-news/… $\endgroup$
    – DWKraus
    Jan 13 at 22:55
  • $\begingroup$ I should have clarified in my last comment that this alien species, being marine mammals, has developed airbubble habitats to live in mini pockets of 'surface' deep underwater where the most/best resources lie. So any wood they incorporate into their civilisation would be exposed to repeated soaking and drying. $\endgroup$
    – FrankRebin
    Jan 13 at 22:59
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    $\begingroup$ I always thought materials like bamboo and rubber would be useful to an aquatic civilization. Bamboo for its wood-like rigidness and tubular structure, which could be made into floats, framework, pipes or storage vessels. Rubber would be an excellent flexible and waterproof adhesive and sealant. Both bamboo and plants that produce latex rubber like to grow near areas with water, so you might incorporate some of those useful aspects into your floating trees. Overall, it's a good answer, I like it a lot. $\endgroup$
    – Redbud201
    Jan 14 at 6:46
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Corals are not exactly plants, but they are alive and can be grown with enough patience. They have a variety of shapes, textures, densities and toughness'es.

You might also consider some sort of fungi. Prototaxites were prehistoric phalic fungi that could grow 1m (~3.3 feet) wide and 8m (~26.2 feet) tall. They were also very rigid too (they were quite vertical, which required some rigidity). In your world there could be even larger, underwater variants. Your aliens could harvest these for their equivalent of wood.

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  • $\begingroup$ Ah, prototaxites seem perfect. Does any reason come to mind why they absolutely could not evolve underwater? $\endgroup$
    – FrankRebin
    Jan 13 at 22:15
  • $\begingroup$ @FrankRebin The exact prototaxites we had on Earth were surely land-based lifeforms, but there is nothing keeping an alien lifeform on an alien planet from evolving a different way. They might need to be in a place where there aren't strong currents, though - water is 800 more viscous and 1,000 denser than air, and they would break under a strong current. $\endgroup$ Jan 13 at 22:18
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    $\begingroup$ Thank you for your insights, you've given me lots to think about! $\endgroup$
    – FrankRebin
    Jan 13 at 22:30

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