What was done
Working with loblolly pine stands at the Duke Forest FACE facility, the authors describe new belowground data they obtained there, after which they present a synthesis of these and other results obtained over the period running from 1996 through 2008, seeking to determine "which, if any, variables show evidence for a decrease in their response to atmospheric CO2 during that time frame."

What was learned
Among many other things, Jackson et al. report that "on average, in elevated CO2, fine-root biomass in the top 15 cm of soil increased by 24%," and that in recent years the fine-root biomass increase "grew stronger, averaging ~30% at high CO2," while in terms of coarse roots having diameters greater than 2 mm and extending to a soil depth of 32 cm, they report that "biomass sampled in 2008 was "twice as great in elevated CO2." In fact, we calculate from the graphical representation of their results that the coarse-root biomass was fully 130% greater, which is really astounding, particularly in light of the fact that the extra 200 ppm of CO2 supplied to the air surrounding the CO2-enriched trees represented an enhancement of only about 55% above ambient conditions. And in the concluding sentence of their paper's abstract, they say that "overall, the effect of elevated CO2 belowground shows no sign of diminishing."

What it means
The four researchers state that "if progressive nitrogen limitation were occurring in this system, we would expect differences in productivity to diminish for trees in the elevated vs. ambient CO2 plots," but they say that "in fact there is little evidence from estimates of aboveground or total net primary productivity in the replicated Duke experiment that progressive nitrogen limitation is occurring there or at other forest FACE experiments [italics added]," even "after more than a decade of manipulation" of the air's CO2 content, citing in this regard -- with respect to the latter portion of their statement -- the report of Finzi et al. (2007). Consequently, there is good reason to believe that the "aerial fertilization effect" of atmospheric CO2 enrichment will continue to significantly benefit earth's forests for as long as the atmosphere's CO2 concentration continues to rise.

Reference
Finzi, A.C., Norby, R.J., Calfapietra, C., Gallet-Budynek, A., Gielen, B., Holmes, W.E., Hoosbeek, M.R., Iversen, C.M., Jackson, R.B., Kubiske, M.E., Ledford, J., Liberloo, M., Oren, R., Polle, A., Pritchard, S., Zak, D.R., Schlesinger, W.H. and Ceulemans, R. 2007. Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2. Proceedings of the National Academy of Sciences, USA 104: 14,014-14,019.