Background
The productivity of earth's temperate forests is often limited by insufficient soil nitrogen. Especially is this so in the southeastern United States, where the growth of pine and hardwood forests often removes so much nitrogen from the soils on which the forests grow that it induces what Finzi and Schlesinger (2003) have described as "a state of acute nutrient deficiency that can only be reversed with fertilization." It would seem only natural, therefore, to presume (as was hypothesized in the early stages of the Duke Forest FACE study) that long-term stimulation of forest productivity by rising atmospheric CO2 concentrations would be limited by soil nitrogen shortages.

What was done
Working at the Duke Forest FACE facility, the authors employed minirhizotrons to characterize "the influence of free-air-CO2-enrichment (ambient + 200 ppm) on fine roots for a period of 6 years (Autumn 1998 through Autumn 2004) in an 18-year-old loblolly pine (Pinus taeda) plantation near Durham, North Carolina, USA."

What was learned
Averaged over all six years of the study, Pritchard et al. found that the extra 200 ppm of CO2 increased average fine-root standing crop by 23%, which is to be compared to an overall stimulation of forest net primary productivity of 18-24% observed over the period 1996-2002.

What it means
In light of the researchers' finding that "the positive effects of CO2 enrichment on fine root growth persisted 6 years following minirhizotron tube installation (8 years following initiation of the CO2 fumigation)," there is once again no hint of any progressive nitrogen limitation to the stimulatory effect of atmospheric CO2 enrichment in a situation where one might have been expected to be encountered. In partial explanation of this important positive finding, Pritchard et al. conclude their report by stating that the distal tips of fine roots are "the primary site for initiation of mycorrhizal partnerships which are critical for resource acquisition and could also influence whether or not forests can sustain higher productivity in a CO2-enriched world." Nearly all evidence obtained to date continues to suggest that earth's forests can indeed do so, and the reason may well reside in the CO2-induced stimulation of the growth of their trees' important fine-root tips.

Reference
Finzi, A.C. and Schlesinger, W.H. 2003. Soil-nitrogen cycling in a pine forest exposed to 5 years of elevated carbon dioxide. Ecosystems 6: 444-456.