Background
The productivity of earth's temperate forests is often limited by the availability of soil nitrogen. Especially is this so in the southeastern United States, where the growth of pine-hardwood forests often removes so much nitrogen from the soils in which their trees grow that they induce what Finzi and Schlesinger (2003) describe as "a state of acute nutrient deficiency that can only be reversed with fertilization." It would seem only natural, therefore, to presume (as they originally hypothesized in the early stages of the Duke Forest FACE study, which is being conducted in the Piedmont region of North Carolina), that "the increase in carbon fluxes to the microbial community under elevated CO2 would increase the rate of N immobilization over mineralization," which would ultimately lead to a decline in -- and perhaps even the total negation of -- the significant CO2-induced stimulation of forest net primary production that developed over the first two years of the experiment, a phenomenon often referred to as the progressive nitrogen limitation (PNL) hypothesis.

What was done
The study of Moore et al. represents a test of the PNL hypothesis at the eight-year point of the Duke Forest FACE experiment of initially 13-year-old loblolly pine (Pinus taeda) trees, based on area assessments of the trees' trunks at approximately 1.4 m above ground level that have been made at monthly intervals since the inception of the experiment.

What was learned
In response to the 50% increase in atmospheric CO2 concentration employed in the Duke Forest FACE study, the researchers report there was "a sustained increase in basal area increment over the first 8 years of the experiment" that varied between 13 and 27% with variations in weather and the timing of growth. What is more, they say "there was no evidence of a decline in the relative enhancement of tree growth by elevated CO2 as might be expected if soil nutrients were becoming progressively more limiting," which is pretty amazing (to many people, but not us), considering the unfertilized state of the soil in which the experiment is being conducted.

What it means
In spite of the presumptions of many that the CO2-induced growth stimulation of long-lived woody plants will gradually (and drastically) decline with the passage of time, there is no evidence that such is occurring in the Duke Forest FACE study, where the trees just keep chugging along at a significantly elevated rate, even when nutrient limitations would "normally" be expected to keep them from doing so.

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.