While CO2 can acidify an environment, I am unaware of harm CO2 in potential concentrations does to plants. Rather than limiting the CO2 they uptake, I suspect the reduction in stomatal density observed as CO2 concentrations increase is a result of the plant simply needing fewer to gain necessary CO2 to feed.

There are other limits on plants, including biogenic, which restrict their growth. Ecosystems are complex tradeoffs, and in the Pacific Northwest the apex tree in the forest is the Hemlock, not the Douglas Fir, which grows faster and taller. By tolerating the shade of the Doug Fir, the Hemlock can wait for it's competitor for canopy space to senesce and die, and then assume it's place.

Thanks for an excellent article!