Direct impact of CO₂ on flora?

Plants survive with difficulty in low rainfall areas. Without thinking much one would assume that the plants that are there would do better if there was a bit more rainfall. Nothing could be further from the truth. The plants that are there are adapted to the harsh conditions. If the conditions got wetter they would be out-competed by plants suited to the new conditions.

Which suggests the question: What is the direct impact of rising CO₂ on plant communities? Presumably those best specialized for the normal 280ppm are finding those genes less useful, and are being outcompeted by plants which have other advantages but which had been held back by less effective adaptation to the previously prevailing low levels of CO2.

We do know one effect of rising CO2. Plants open pores to let in CO2, which lets out moisture. As CO2 levels rise plants can open their pores less, and thus survive in drier conditions than previously.