If you increase the CO2 level you increase the growth rate of plants. If you increase the growth rate of plants all year, you should expect more growth than if you increase it for only a few months. Therefore, a diverse ecology with plants maturing all year should show more total growth than if you remove some of the species from the plot and so have maturation for a shorter period. You could take a plot of every species of oak known to man and the entire lot would grow slower than a homogeneous plot of white pine. We have spent much time and effort changing the corn DNA to make it convert as much CO2 as possible to seed.
Diversity is like a box of chocolates.