Hmmm, separately of plant-types, I wonder if there may be a distinction here between how a surge in individual growth doesn't necessarily translate to a surge in the forest.
Imagine a higher CO2 concentration allows a tree to reach maturity a whole +25% faster, taking 16y instead of 20y. However its happening in an established forest, already bounded by mountains, rivers, etc, where mature trees sustain for another 100y before they finally die off and take 10y to decompose, opening the spot for a replacement.
In that case, the number of simultaneous trees doesn't go up very much, because the main effect is to reduce "downtime". The "duty-cycle" for a tree-sized patch of ground goes from having a mature tree ~77% of the time to ~79%.
Interesting theory. I imagine there would be a stratification of mature and immature trees that would be pretty striking if this is the case. It might not be hard to find out if it's true!