What was done
A FACE study was established within a ten-year-old stand of sweetgum (Liquidambar styraciflua L.) trees growing in a forest plantation on nutrient-rich soils in Tennessee, USA.  The trees, which were in a linear growth phase at the onset of the experiment, were exposed to atmospheric CO2 concentrations of 360 and 550 ppm.  This paper reports on photosynthetic rates and carbon allocation within the trees following three years of treatment exposure.

What was learned
Elevated CO2 increased ecosystem net primary productivity by 21% in all three study years.  As a result, aboveground woody biomass increased in the CO2-enriched plots by 33% in the first year of the study.  However, the increase dropped to 15% and 7% in the second and third study years, respectively, while net primary productivity remained unchanged.  The reason for the drop was that an increasing amount of newly-fixed carbon in the CO2-enriched trees was being utilized to increase fine root and leaf production in each progressive study year.  Nonetheless, over the three-year period of the study, 77% of the additional fixed carbon was allocated to aboveground woody biomass.

What it means
As concluded by the authors, "this experiment has provided the first evidence that CO2 enrichment can increase productivity in a closed-canopy deciduous forest."  Indeed, their data showed a CO2-induced enhancement of net primary productivity in mature sweetgum stands that persisted for three years without exhibiting any signs of depression.  In contrast, CO2-induced increases in aboveground woody biomass decreased over time, while fine root and leaf production increased.  Thus, in terms of carbon sequestration, it is possible that carbon accumulation in woody biomass will initially be greater than that occurring within soils.  However, if carbon allocation progressively increases in fine roots and leaves, which readily add carbon to soils, then carbon accumulation and sequestration within soils should ultimately be greater than that in woody biomass.