How do increased CO2 levels affect plant growth?

Answer

Plants grow better with higher CO2. In fact, most botanists agree that CO₂ is the single most limiting factor in plant growth and reproduction. Researchers have artificially increased CO₂ levels in greenhouses up to 10 times the normal atmospheric levels, with no diminishing returns in plant growth.

A different view

Higher CO₂ levels help some plants in controlled environments, such as greenhouses. In natural environments, the growth is temporary with plants either returning to previous levels or decreasing. See Related Links for source.

Rebuttal

Higher CO₂ levels help all plants, in all environments. Growth increases are temporary only when the CO₂ increase is temporary.

Your "source" contains a lot of "ifs" and "maybes" - truth is, all it does is raise conjectures about how growth MAY be limited in some cases.

C₄ Plants - True, they have a mechanism for "fixing" carbon dioxide. But the more carbon dioxide in the atmosphere, the more they can "fix". Furthermore, the vast majority of the world's plant biomass is C₃, not C₄. The only crops that are C₄ are corn, sorghum, sugarcane, and millet (with only corn being a major crop). C₃ crops include soybeans, all small grains (wheat, barley, oats, rye), and the most important food crop in the world, rice. Additionally, even the C₄ plants can be bio-engineered to be C₃ plants.

Tropics - While it MAY be true that increased CO₂ will be less beneficial in the tropics than in temperate regions, the vast majority of crops are grown in the temperate regions. So this is not an issue.

Overall, your source is a very poor attempt to counter the established and accepted scientific knowledge that higher carbon dioxide levels are beneficial to crops.