yatir forest, page-7

"high CO2 is not likely to limit plant growth"

There are numerous studies, especially in the past few years, that indicate that CO2 will not yield as significant an increase in plant growth as originally thought (although it would obviously be beneficial to plants). Largely due to the influence CO2 has on increasing other limiters to veg growth (primarily water and nitrogen).

http://www.sciencedaily.com/releases/2006/04/060412204831.htm