# Tall trees : atmospheric pressure

In my previous post, I asked about how tall trees could grow in my world. To be honest, I really need those trees to be huge for my storie, because a big part of the plot will be linked to those big forests, so every characteristic of my world are settled for those trees to grow as big as possible (between 250 and 300 meters would be perfect).

But I want this world to be believable. A lot of factors influence the growth of a tree, and I have deduced (I am surely wrong) from answers to one of my post In what ways a higher level of oxygen (23 to 25%) and higher pressure (1,7 time the earth air density) would affect winds and ocean currents? that pressure seems to be a factor to think about (for example higher pressure slows down winds, which affect winds/ocean currents and precipitations, which could, I don't know, allow trees to grow without being bothered/destroyed by a hurricane for example). I decided to think about those trees step by step, so right now, it seems to me that definitely settling atmosphere is the first step.

Know that I have a map of my world with plate tectonic, ocean and winds currents and moutain range.

Concerning the planet characteristics : what is already settled is gravity (0,8g). But from some research, it seems that gravity influences pressure, because it keeps everything near the planet surface. So I don't know if I can play a lot with pressure.

My question is : considering what is influenced by atmospheric pressure (ocean and winds currents, climates - which both influence trees growth - ... etc), how much pressure (kg/m3 or atm) would I need to maximize my trees height ?

Thanks !

• Can you reference your source for atmospheric pressure as a determining factor of tree height? My first guess would be the energy required to make water overcome gravity to the height you specify, considering the vascular pressure of the tubules that transport water up. You get some help from Van Der Waals force, but beyond a certain height with respect to gravity, the fluidic pressure overcomes capillary action. Beyond that the plant is expending energy to actively transport water "against the current" via cellular mechanisms. The energy available for that is a limiting factor.
– Nolo
Nov 10, 2019 at 12:23
• How precise are you trying to be, exactly? Maybe I'm a bit dull, but the idea of 250 or 300 meter trees on another planet seems plausible, especially if you hand-waive some exposition about gravity, atmospheric pressure, and humidity. Or maybe I'm just gullible? Meh... Nov 10, 2019 at 12:54
• Many plants do have ways of absorbing water and nutrients other than through their roots. A rain forest type environment would provide water, and desert winds, as it turns out, are major contributors to the dispersal of nutrients on the wind. So these factors could well constitute a solution. I saw something I think in one of the Blue Planet? episodes which describes how dust from the Sahara causes algae blooms in the oceans. Some similar process could do the same for your giant trees.
– Nolo
Nov 10, 2019 at 13:09
• There is no direct relationship between surface gravity and atmospheric pressure. The most important factor in determining atmospheric pressure is simply how much atmosphere there is. Venus has the same surface gravity as Earth, yet atmospheric pressure is waaaay higher. Nov 10, 2019 at 14:31
• @Emie I'm referring to gravity wells. A planet twice the mass of Earth (for example) can have the same gravity at the surface as Earth, and an atmosphere many times the height (ie. thickness when viewed from space or the ground) of Earth's atmosphere - merely by virtue of being much larger in diameter than Earth because gravity tails off more slowly as you go up than from Earth by virtue of the planet being larger, so more gas is trapped in what we refer to as an atmosphere. The maths for gravitation are weird like that. Nov 11, 2019 at 19:58

I think the trees can be as tall as you'd like, but they'll start to look pretty different from Earth trees. I spent quite some time developing a world that sounds a lot like what you're working with and have since then taken chunks of that setting and made it more extreme for roleplaying game environments. Here's some of the stuff I did before I added any magic:

The trees are all interconnected. At multiple layers. The branches in each layer of the canopy grow into each other and allow forces to be distributed throughout the whole biomass. This shifts wind from being something that might blow the trees over to something which requires much needed ventilation to lower layers. The trees, flora and fauna will grow to optimize this (See also: How termite mounds work.) Think of it almost more like designing a reef than a forest. The trees get big as a team and build their own ecosystem.

The trees eat rocks. This one of the ways you can get around the compression and tensile strength of wood. Have it self-petrify. If you're feeling especially serious about it have it grow literal veins of iron throughout and make it natural rebar-reinforced concrete. You could also play with and optimize the grain structure. The root structure is designed to distribute weight at "ground-level" and to penetrate whatever bedrock the forest is growing over. The roots don't primarily absorb water any more, they're there for heavy minerals and structure.

The trees get nutrients and water the whole way up.
-Lower levels are designed to trap falling debris and form ponds and small lakes with the last of the water that's managed to make it this far down. Roots grow into these to acidify them and break down any biologicals that filter down into usable nutrients for the trees. This layer is its own ecosystem, with fish analogues and very little light. The forest floor is effectively an acid-oozing desert with very little life.

-Lower-mid levels are very structural, dropping their leaves and extra branches to facilitate air movement and gas exchange. Depending on the size of the forest and length of day, temperature changes due to shifting sunlight could generate a fair amount of internal wind, which would make this area inhabited by flora and fauna able to hang directly from the branches and get their nutrients from the air (Things like Spanish Moss.) Sections of the tree could have lighter or darker leaves, or flip-able leaves light on one side and dark on the other to facilitate this process.

-Upper mid levels are where most of the leaves are. Branches grow more thickly here, full of leaves which trap rain and provide nutrients to the tree and the vast amount of flora and fauna living on this layer. Most of the light gets blocked by this layer, so if you want to have any "normal" Earth surface life you'd probably want to adapt it to live here. Due to the amount of biomass here this layer is where what we would consider soil is deposited. Something like 85% of the water works its way through this layer to be redirected and absorbed by the supporting branch networks. From here build a rain forest with understory and canopy levels stacking in such a way to keep permitting dappled light through.

-Upper levels are where the clouds are. The leaves here are for redirecting and taking in water. They are clear and are used for generating alcohols, not sugars. This keeps them from freezing and they sweat the alcohols to lower the leaf temp and encourage condensation. The forest makes its own rain when it needs to. You can get around a lot of the fluid dynamics issues (or at least trade them for different ones...) this way.

This is my first answer, I hope I did everything right and it helps give you some ideas. Thanks for building what sounds like a really cool world!

• Nice first post. +1 Welcome to the site. Nov 10, 2019 at 22:14
• Thank you so much ! Actually, the fact that you speak of connection between trees made me think about the Great Banyan tree (allthatsinteresting.com/great-banyan-tree). I though having one massive tree that has the size of a forest would be perfect for the storie I have in mind, but maybe I am being too crazy. It would bring strengh I guess, but I maybe I would have less light coming bellow for example.
– Emie
Nov 10, 2019 at 23:03

There are a number of issues to overcome with giant trees. The first is the force of the wind. Any storms will have a disproportionately larger effect on taller trees. This might be aided by a less dense atmosphere. Additionally if the day length was longer than on Earth it might help as one of the major drivers for storms is the difference in atmospheric speed between the poles (0 miles/hour) and the equator (1000+ miles/hour). Just to be on the safe side I would also plan your forest to be in a low wind area free from the worst effects of whatever storms there are.

Another problem is getting water and nutrients to the top of the tree. I suggest having a traditional tree structure with plenty of deep and well spread roots. But the tree will eventually struggle so I suggest having additional sources of water and nutrients delivered at high level.

Water could be collected from rain into bowl shaped structures on branches and on the trunk especially where branches met the trunk (for stability). If there was enough rain these would regularly be topped up and would become colonized by plants, insects and animals. The influx of dead leaves, decaying vegetation and animal/bird waste would make an ideal place for the tree to send out high level roots for added sustenance.

One further aid would be strong evolutionary pressure to grow tall. Whilst this would be natural for trees, indeed its competition for light that forces them higher, but perhaps the animals/birds prefer the very tallest trees for some reason. In this way the tallest trees would get the most nutrients.

Finally gravity is problematic. You have helped by reducing gravity to 0.8 and by having low level winds, but some structural changes would also help. Very large buttress root supports would be expected so the trees would be wide and have an irregular cross section at the base. Stronger natural materials might also help. Perhaps the wood might incorporate proteins like spider silk to greatly improve strength.

• Thank you for your answer ! To be sure, by « This might be aided by a less dense atmosphere », do you meen that I should have a smaller pressure ?
– Emie
Nov 10, 2019 at 20:31
• NP. Yes I think a lower pressure would be better as there would be less mass in the atmosphere so less energy in storms. You might need to increase the partial pressure of oxygen to compensate. Nov 10, 2019 at 23:12
• But the winds would be faster, so storms could have the same effect than on Earth, no ?
– Emie
Nov 11, 2019 at 19:28
• @Emie why would the winds be faster with a longer day, lower gravity and lower pressure atmosphere? Nov 11, 2019 at 21:52