Leaving aside the atmospheric composition (let's say there's the same amount of oxygen, despite the higher pressure), would the pressure itself have any effect? Would plants need to be stockier and sturdier? Would the size of their leaves change?
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$\begingroup$ Atmospheric pressure is only one factor of natural selection, the amount of wind is another. I expect that a current composition of various plants would roughly stand, as there is no exact need for them to be sturdier from mere double the pressure, as wind resistance is not required for the majority of the plants, and is sometimes beneficial to lose a part of a plant in order to let it grow into another one elsewhere. $\endgroup$– VesperCommented Sep 7, 2023 at 4:26
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$\begingroup$ What is to be understood from "the same amount of oxygen despite the higher pressure"? $\endgroup$– AlexPCommented Sep 7, 2023 at 5:59
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The main effect of a higher atmospheric pressure would be the higher partial pressure of oxygen and carbon dioxide, and the increased density of the atmosphere.
While plants would still be evolving to maximise photosynthetic area, the higer partial pressures of the relevant gases would mean that structures could be larger and thicker and chloroplasts would still have access to the necessary carbon dioxide.
However, higher atmospheric density would mean that a wind of a given speed would impose twice the force upon objects in its path. This would mean that if all else was equal, plants would evolve to be more robust in order to tolerate the higher wind forces, and because of the higher partial pressures of gases, wouldn't necessarily have to compensate for thicker, stronger structures as much as might have been the case if the partial pressures had remained the same.
answered Sep 7, 2023 at 4:29
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$\begingroup$ Does photosynthesis-as-we-know-it even work at double the partial pressure of oxygen? $\endgroup$– AlexPCommented Sep 7, 2023 at 5:59
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2$\begingroup$ @AlexP Yes. Higher pressure with higher CO2 partial pressure leads to faster photosynthesis: pubmed.ncbi.nlm.nih.gov/23994480 $\endgroup$– Monty Wild ♦Commented Sep 7, 2023 at 6:09
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$\begingroup$ But would wind have the same speed as on Earth? Exerting twice the force would mean having twice the energy, and I see no reason for the wind having twice the energy, so absent other factors, I would expect the wind to have half speed and thus exert the same force. $\endgroup$ Commented Sep 7, 2023 at 12:52
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$\begingroup$ @MatthieuM. We shouldn't assume that it won't be as fast... however that's another question. $\endgroup$– Monty Wild ♦Commented Sep 7, 2023 at 13:01