Plant Community Interactions under Elevated CO2
A number of experiments have found that some plant species that respond positively to elevated CO2 when grown alone experience decreased growth under elevated CO2 when grown in mixed plant communities (Poorter & Navas 2003). This effect likely results because the direct positive effects of elevated CO2 are outweighed by negative effects due to stimulation of the growth of competitors. Rising atmospheric concentrations of CO2 may therefore lead to changes in the composition of plant communities, as some species reap more of an advantage from the increased CO2 than do others. In mixed-species experiments under high fertility conditions, C4 plants decrease as a proportion of the biomass of plant communities under elevated CO2. Similarly, under low fertility conditions, legumes increase as a proportion of the biomass of plant communities under elevated CO2 (Poorter & Navas 2003).
Summary

Current evidence suggests that that the concentrations of atmospheric CO2 predicted for the year 2100 will have major implications for plant physiology and growth. Under elevated CO2 most plant species show higher rates of photosynthesis, increased growth, decreased water use and lowered tissue concentrations of nitrogen and protein. Rising CO2 over the next century is likely to affect both agricultural production and food quality. The effects of elevated CO2 are not uniform; some species, particularly those that utilize the C4 variant of photosynthesis, show less of a response to elevated CO2 than do other types of plants. Rising CO2 is therefore likely to have complex effects on the growth and composition of natural plant communities.

FK – Don’t know much about biology, but I know we can’t blindly trust either ‘side’ in this debate.