>>7619116You'd have to look at each of your conditions separately, but it's hard since our experimental knowledge of plants does not extend beyond Earth conditions -- so basic comparisons are a good starting point.
-At 3x Earth's gravity: plants would grow laterally and most of their surface area would be spread on the ground. Plants that can grow vertically would likely be very fibrous and slender at the top.
-8x Earth's atmospheric density: assuming there is still an inorganic carbon source (eg CO2) then I can't think of what would change in the plant morphology, beyond maybe the need for increased metabolism. If carbon turnover is relatively similar to Earth plants, maybe there is some mechanism for shutting off energy transformation periodically -- to avoid build up of metabolic waste-products, etc.
-Hydrogen rich atmosphere: not sure what you mean exactly. If you mean hydrogen compounds, like hydrogen sulfide, then you could assume in your time frame that plants evolved metabolic pathways to use the gas and enhance growth -- some Earth plants benefit from H2S already. Pure, diatomic hydrogen? I guess it might be inert (to the plant). If there is oxygen in the atmosphere, and diatomic hydrogen floating around... it might not last long if there is lightning, natural fires, etc.
-Dim star, 80 year orbit: can't think of what harm this would cause to a photosynthetic species... assuming they exist, they will have likely evolved the mechanisms to transform any amount of light into usable energy.
-Greenhouse effect with little temperature variation: this could only help as far as I can tell. And if it's true, modifies the previous condition a bit -- temperature would stay relatively stable throughout whole orbit.
-Circular orbit: probably inert given the time frame for the plants to evolve, can't think of how that would change morphology.
