What was done
One-year-old loblolly pine seedlings (Pinus taeda) were grown for about four months in pots placed within growth chambers that received atmospheric CO2 concentrations of 360 and 660 ppm and adequate and inadequate levels of soil moisture.  In addition, the pots were seeded with a variety of C3 and C4 weeds to determine the effects of elevated CO2 and soil moisture on seedling growth, weed growth, and their competition for limiting resources.

What was learned
Total seedling biomass was always greater under well-watered as opposed to water-stressed conditions.  Elevated CO2 increased total seedling biomass by 22%, but did not interact significantly with water in its influence on this parameter.  However, under elevated CO2 and water-stressed conditions, seedling root-to-shoot ratios were about 80% greater than they were in the elevated CO2 and well-watered conditions, due to a 63% increase in root biomass.

Similarly, total weed biomass was always greater under well-watered compared to water-stressed conditions.  Surprisingly, however, elevated CO2 did not significantly affect total weed biomass, which was actually 22% less in elevated, as opposed to ambient, CO2 growth conditions.  There were no significant interactions between elevated CO2 and water treatments with respect to this parameter.  However, elevated CO2 caused a non-significant shift in the percentage of weed biomass composed of C4 weeds.  These weeds accounted for 53% of total weed biomass under ambient conditions but reduced their contribution to 35% under elevated CO2 conditions.

What it means
As the atmospheric CO2 concentration continues to rise, it is likely that loblolly pine seedlings will exhibit increases in biomass production, regardless of soil moisture and the presence of herbaceous weeds --which did not increase their total biomass in response to atmospheric CO2 enrichment.  Indeed, in assessing the effects of elevated CO2 on competition between seedlings and weeds, the authors report that "elevated CO2 still resulted in a significantly greater growth response for loblolly pine seedlings compared to ambient CO2."  Moreover, the CO2-induced increase in root-to-shoot ratio under water stressed conditions may, in their words, "contribute to an improved ability of loblolly pine to compete against weeds on dry sites under elevated CO2 levels."