Plant production on the decline - is it now true that more CO2 is worse for plants?

While CO2 is good for some plants, such as weeds, there are other aspects to take into account as well, such as an increase in desertification, droughts and a greater abundance of pests. Stating such things as "CO2 is plant food" is a very simplistic child-like way of looking at it, such as I have heard deniers say in the past. I think that we've crossed many thresholds for many different species. Each reacting differently to different amounts of CO2 and water availability.