What would a thicker atmosphere do to plant growth/evolution?

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.

posted over 4 years ago

DehLeprechaun‭

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