What was done
The authors grew one-year-old longleaf pine seedlings in large pots placed within open-top chambers receiving atmospheric CO2 concentrations of 365 and 730 ppm for nearly two years.  In addition to the CO2 treatment, seedlings were also subjected to combinations of high and low soil nitrogen and soil moisture levels to study the interactive effects of these variables on photosynthesis.

What was learned
Overall, atmospheric CO2 enrichment tended to increase photosynthesis, decrease stomatal conductance, and increase water-use efficiency.  Seedlings grown in elevated CO2, for example, exhibited rates of net photosynthesis that were nearly 50% greater than those observed in seedlings grown at ambient CO2 concentrations.  Although soil nitrogen and water availability influenced the photosynthetic response of seedlings to elevated CO2, they generally did not affect the magnitude or direction of the response.

It is interesting to note that when subjected to high soil nitrogen, seedlings grown in elevated CO2 increased whole-plant water-use (due to greater biomass, despite an increase in water-use efficiency), thus causing greater water stress when subjected to low soil moisture availability.  However, when grown at low nitrogen levels, CO2-enriched seedlings (of smaller biomass) decreased whole-plant water-use and exhibited decreased water stress when grown under the low soil moisture regimes.

What it means
As the atmospheric CO2 content rises, longleaf pine forests, which currently exist in the southeastern USA on coastal plain soils that typically are low in nitrogen and water content, will likely display increases in photosynthetic prowess.  As a result, such forests should increase the biomass of standing timber and may actually expand into adjacent regions also characterized by soils of low nitrogen and moisture availability.