CO2 helps plants grow faster, but it is complex in that there are often other limiting factors. As you eliminate some, others become more prominent. A limiting factor for trees is often water or a shortage of one or more of the big three fertilizers: Nitrogen, Phosphorus and Potassium. Humans have added CO2 to the air, but also added Nitrogen, Phosphorus and Potassium, so much that they are sometimes pollutants. It might be that we have lucked onto something good out of potentially bad if increased CO2 levels allow faster growth to suck up some of that excess NPK. This is unlikely to apply as much to tropical forests as temperate ones, however.