What was done
Circular FACE plots (30 m diameter) receiving 365 and 565 ppm CO2 were established in a 13-year-old loblolly pine plantation in North Carolina, USA, to determine the long-term effects of elevated CO2 on primary productivity, growth and the nitrogen budget of this important coniferous species.  This paper describes results obtained after four full years of differential CO2 exposure.  It is interesting to note that soils in the study region are characteristically low in available nitrogen and phosphorus.  Hence, any CO2-induced responses of trees in the FACE plots occurred in the face of these nutrient limitations.

What was learned
On average, the trees in the elevated CO2 plots maintained rates of net primary productivity that were 25% greater than those displayed by ambiently-growing trees.  Moreover, the extra CO2 increased the average yearly dry matter production of the CO2-enriched trees by 32%.  Along with this CO2-induced increase in biomass, the elevated CO2 also increased the total amount of nitrogen present in the trees' biomass.  In fact, the average annual requirement for nitrogen rose by 16% for the trees growing in the air enriched with CO2.  In order to compensate for this increased nitrogen demand, the average uptake of nitrogen from the soil was enhanced by 28% in the CO2-enriched plots.  In addition, average nitrogen-use efficiency rose by approximately 10% with atmospheric CO2 enrichment.

What it means
As the air's CO2 content rises, it is likely that loblolly pine forests will display persistent increases in net primary productivity that will lead to greater dry matter production; and if more nitrogen is needed to sustain greater biomass production, the trees will likely extract the extra nitrogen they require from the soil and use it more efficiently to enable the CO2-enhanced rates of growth to persist.  Consequently, carbon sequestration within natural loblolly pine forests and managed plantations will likely increase with future increases in atmospheric CO2 concentration.