How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic

Nitrogen Dynamics in the Duke Forest FACE Study
Finzi, A.C., DeLucia, E.H. and Schlesinger, W.H.  2004.  Canopy N and P dynamics of a southeastern US pine forest under elevated CO2Biogeochemistry 69: 363-378.

"Biogeochemical models," in the words of the authors, "predict a progressive decline in forest production under elevated CO2 in nutrient-limited ecosystems (Rastetter et al., 1997; Luo and Reynolds, 1999)."  Why?  "Because," as they continue, "rapid plant growth under elevated CO2 immobilizes nutrients in long-lived plant tissues and soils more rapidly than their mineralization from soil pools or weathering from soil minerals," or so, at least, the theory stipulates.

What was done
Finzi et al. "tested this idea by quantifying canopy biomass, foliar concentrations of N and P, and the total quantity of N and P in a loblolly pine (Pinus taeda) canopy subject to 4 years of free-air CO2 enrichment."  This pine stand, of course, is the home of the Duke Forest FACE facility in Orange County, North Carolina, USA, which has produced a number of important studies we have reviewed previously [see essentially everything listed under FACE Experiments (Trees - Pine) in our Subject Index].

What was learned
The authors state that "forest production is strongly nutrient limited throughout the southeastern US," and that there is a "clear limitation" of net primary production [NPP] at their site.  Nevertheless, they report "there were significant increases in canopy N and P contents under elevated CO2" (26 and 50%, respectively), and that "canopy biomass was significantly higher under elevated CO2 during the first 4 years of this experiment."

What it means
"Despite the clear limitation of NPP by N at this site," quoting the authors, "this limitation has not precluded a response to elevated CO2 during the first 4 years of this experiment."  Likewise, they say "there is no evidence that the mass of N in the canopy is declining through the first 4 years of CO2 fumigation."  Therefore, as they are forced to conclude, "whole-canopy C assimilation is strongly stimulated by elevated CO2 making this forest a larger net C sink under elevated CO2 than under ambient CO2," even in the face of conditions that theory says should disallow such a response.

In spite of these amazing observations, Finzi et al. continue to believe that "limited soil N availability will eventually curtail the initial increase in productivity under elevated CO2," noting, however, that "the time scale for such a down-regulation ... is not known."  And they could be right.  But they also could be wrong; and that is why it is so important to continue such experiments as the Duke Forest FACE study as long as we possibly can, as our Editorial of 5 Mar 2003 also demonstrates.  We must learn the facts about the long-term responses of trees to atmospheric CO2 enrichment; and the only way to do so (and know that we have found the truth, or at least a good approximation to it) is to continue such studies for as long as is humanly possible.

Luo, Y. and Reynolds, J.  1999.  Validity of extrapolating field CO2 experiments to predict carbon sequestration in natural ecosystems.  Ecology 80: 1568-1583.

Rastetter, E.B., Agren, G.I. and Shaver, G.R.  1997.  Responses of N-limited ecosystems to increased CO2: a balanced-nutrition, coupled-element-cycles model.  Ecological Applications 7: 444-460.

Reviewed 1 September 2004