What was done
The authors report what they have learned from the Duke Forest FACE Experiment over a period of four years.  This ecosystem - a predominantly loblolly pine (Pinus taeda L.) forest with sweetgum (Liquidambar styraciflua L.) and yellow poplar (Liriodendron tulipifera L.) trees as sub-dominants, together with numerous other trees, shrubs and vines - was established in 1983 following the clear-cutting of a regenerating forest in 1979.  The experiment began in August of 1996, when three 30-meter-diameter FACE plots were enriched with CO2 to atmospheric concentrations 200 ppm above ambient, while three similar plots were maintained at ambient conditions as controls.

What was learned
Based on the standing pool of ecosystem biomass in 1998 and more recent measurements of various carbon fluxes, the authors calculated a complete carbon budget for the forest for that particular year.  This exercise revealed that the extra CO2 supplied to the FACE plots stimulated net ecosystem productivity (NEP) by 41%.  For a 300 ppm increase in atmospheric CO2 concentration - which is the most common increment of CO2 enrichment that has been employed in such experiments over the years - this result translates into a CO2-induced NEP increase on the order of 60%, which represents a significant stimulation of biological carbon sequestration, especially for trees growing on a soil the authors describe as being of "low nitrogen and phosphorus availability."

What it means
As the air's CO2 content continues to rise, we can expect to see ever greater amounts of CO2 removed from the atmosphere by young and productive forests - such as would result from the planting of trees for this purpose - even when growing on nutrient-poor soils.  We can also expect that increases in the air's CO2 content may significantly stimulate the growth rates of preexisting forests, such as was observed in this experiment.  What is needed now is to see how the degree of CO2-induced growth stimulation of various tree species behaves over much longer time periods, i.e., decades to centuries, in order to determine the ultimate ability of this phenomenon to reduce the rate of rise of the air's CO2 content and mitigate global warming.