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I'm working on a concept for a habitable world with venus-like atmospheric pressures, where humans have settled onto huge tabletop mountains where the atmosphere isn't as crushing. This is a terraformed/life-seeded world, and presumably whatever systems went to work on it was able to balance things out so that temperatures and chemical levels (at least at the mountaintop elevations) are similar to Earth-like biomes.

I'd like to develop some interesting flora and fauna that have adapted to the extreme pressures of the areas below the tabletop mountains. Things like giant trees and lighter-than-air drifters that could float by taking advantage of the higher density of the atmosphere. I've yet to determine what kinds of flora and fauna would be introduced and then evolve in this environment, but they'll have plenty of time to adapt and spread and evolve across the world as it's terraformed.

I presume light would be somewhat dimmer, depending on the weather like a perpetual cloud layer but more likely the diffusion of light through the thicker atmosphere. Winds below the mountaintops would also be very slow, but have a higher drag - and winds on top of the mountains would be very fast.

In this context, what changes would plants need to undergo to thrive under such atmospheric conditions?

While I'd also like to know how such conditions would affect algae, fungi and anaerobic fermentors, they're secondary in this question and I'll ask about them later.

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    $\begingroup$ I do not believe higher pressures = slower wind speeds. Or at least, I am pretty sure it does not make the wind hit any less harder as it blows. $\endgroup$
    – DKNguyen
    Commented May 17, 2020 at 1:09
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    $\begingroup$ Atmosphere has same mix of gases as on earth? I forsee fire... $\endgroup$
    – Willk
    Commented May 17, 2020 at 2:01
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    $\begingroup$ A back-of-the-envelope estimates suggest that with Venus pressures at the baseline, and Earth-like gravity, the tolerable pressure would be at the altitude of 35 kilometers, and having a mountain that high is very unlikely. What's worse, to have reasonable temperatures at such altitude, the temperatures at the baseline would be in hundreds of centigrages. I seriously doubt the existens of life there. $\endgroup$
    – user58697
    Commented May 19, 2020 at 1:04
  • $\begingroup$ I hadn't thought of the temperature at the surface, that's an interesting variable. And don't worry about the existance of the mountains, the probability of their formation is not a part of the question. $\endgroup$
    – DehLeprechaun
    Commented May 20, 2020 at 4:40
  • $\begingroup$ @DKNguyen Most definitely "higher pressures = slower wind speeds.". The surface winds of Venus average only about 1.4m/s, peak of about 2.2m/s. However, they do "push" muchharder than Earth's average wind of 3.8m/s. This due to the surface air density being so very much higher. More dust is blown than on Earth or Mars, for example. $\endgroup$
    – PcMan
    Commented Jan 3, 2021 at 14:01

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Well, lets ignore those details not mentioned in the question (existence of those mountains, extreme temperature down in the valleys and such) for one moment, then the changes the plants would have to undergo evolutionary in difference to earthly plants are not as big as it seems. As mentioned in this article from 2012 some smaller plants like moss or trifolium are able to survive pressures up to 75.000 times those we have here on earth without any visible damage (testet for at least half an hour). The main problem could be the degrading efficiency of chlorophyl under high pressures stated here. Just 100 times our pressure significantly reduces the abilities of the chlorophyl. Even more, the leaf tissues of both tested plant-types were damaged. Another problem will be the cooling of the plants which usually works by vaporising water out of the leaves. Higher air pressure results in lower transpiration of the leaves and by that lower cooling effect (as you can see here ).

Following this analysis of the problems I would conclude that plants which developed in a habitat with these pressures will grow lower and 'denser', having a more robust cell wall and be possibly less flexible so the cells are able to hold up to a higher inner pressure which is needed to compete with the higher pressure from the outside. Better cooling could be provided by thinner leaves with a higher surface. There will be some changes in the genetics of the chlorophyl too, but my knowledge in biology is too low by far to guess what would be needed here.

(Sorry for my english, its not my native language.)

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