What was done
The authors studied the effects of atmospheric CO2 enrichment to 200 ppm above ambient in a FACE experiment conducted in the field at Maricopa, Arizona, USA, in which they evaluated the growth of C3 cotton (Gossypium hirsutum L.) and C4 sorghum (Sorghum bicolor (L.) Moench) in monocultures and low and high density mixtures under both optimum and less than optimum soil water conditions.

What was learned
Surprisingly, there were no significant CO2 by soil water interactions in this experiment.  Hence, the authors reported the plant growth responses to atmospheric CO2 enrichment as averages of the wet and dry treatment results.

When grown in monoculture for two months, the aboveground biomass of cotton was 85% greater in the CO2-enriched treatment than in the ambient-air treatment, while the aboveground biomass of sorghum was 2% lower in the CO2-enriched air than in the ambient air.  In the low density plant mixture, these responses were exacerbated: the aboveground biomass of sorghum was reduced by 14% in the CO2-enriched air, while that of cotton was increased by 154%.  In the high density mixture, this trend continued: sorghum biomass was reduced by 16% in the CO2-enriched air, while that of cotton was increased by a whopping 276%.

What it means
When grown in monoculture, in this particular experiment, C3 cotton was significantly stimulated by atmospheric CO2 enrichment, while C4 sorghum experienced but the tiniest of (non-significant) growth reductions.  When grown in competition with each other, these positive and negative tendencies were both amplified, and increasingly so as the crowding of the plants increased.  Do various pairs of plants that occur together in nature respond similarly?

Obviously, one cannot answer this question on the basis of what was learned in this specific experiment.  In fact, one cannot be very confident about its implications for cotton and sorghum themselves, because of certain of the study's peculiar findings.  For one thing, soil water status did not impact the growth response of either species to atmospheric CO2 enrichment, in contradiction of what has been observed in many other experiments (Idso and Idso, 1994).  Also, the negative response of sorghum to elevated CO2 clashes with the results of several other studies of C4 plants, where double-digit percentage increases in growth have been the rule (Poorter, 1993; Wand et al., 1999).  Hence, we concur wholeheartedly with the authors when they say "there remains a critical need to address the role that a CO2-mediated increase in C3 growth may have under field conditions in natural ecosystems to more fully understand CO2 effects on vegetation change."

References
Idso, K.E. and Idso, S.B.  1994.  Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research.  Agricultural and Forest Meteorology 69: 153-203.

Poorter, H.  1993.  Interspecific variation in the growth response of plants to an elevated ambient CO2 concentration.  Vegetatio 104/105: 77-97.

Wand, S.J.E., Midgley, G.F., Jones, M.H. and Curtis, P.S.  1999.  Responses of wild C4 and C3 grass (Poaceae) species to elevated atmospheric CO2 concentration: a meta-analytic test of current theories and perceptions.  Global Change Biology 5: 723-741.