The four multi-year experiments Norby et al. analyzed were: (1) the Duke-FACE study near Durham, North Carolina, USA, which was initiated in an established monoculture plantation of evergreen loblolly pine (Pinus taeda) trees, (2) the ORNL-FACE study near Oak Ridge, Tennessee, USA, which was initiated in an established monoculture of deciduous sweetgum (Liquidambar styraciflua) trees, (3) the Aspen-FACE study near Rhinelander, Wisconsin, USA, which was initiated on bare ground but was ultimately comprised of multi-tree assemblages dominated by Populus species, and (4) the POP-EUROFACE study near Tuscania (Viterbo), Italy, which was also initiated on bare ground and ultimately comprised of multi-tree assemblages dominated by Populus species. To be compatible with the first two experiments in terms of the trees' state of development, no data were used from the latter two experiments until the trees had grown to the point where their canopies were completely closed. Under these conditions, and across all appropriate years of all experiments (6 years in the Duke-FACE study, 5 years in the ORNL-FACE study, 1 and 3 years in different portions of the Aspen-FACE study, and 2 years in the POP-EUROFACE study), the average atmospheric CO2 concentration in the ambient-air control plots was 376 ppm, while the average concentration in the CO2-enriched plots was 550 ppm, yielding an average CO2 concentration differential of 174 ppm between the two CO2 treatments.

So what was learned from this massive experimental enterprise? In what the four groups of researchers describe as a "surprising consistency of response across diverse sites," they found that forest NPP was enhanced by 23 ± 2% at the median NPP of their combined data set in response to the 174-ppm increase in the air's CO2 concentration. This NPP stimulation is substantial, considering that most of the CO2 stimulation figures one sees are for a 300-ppm increase in atmospheric CO2 concentration; and linearly extrapolating Norby et al.'s median result to correspond to that greater CO2 concentration differential yields an NPP stimulation of approximately 40%, or just slightly less, due to the fact that as the air's CO2 content rises, the NPP stimulation provided by extra CO2 rises ever more slowly.

In further commenting on their findings, Norby et al. say "the data in our analysis all come from fast-growing, early successional stands, and there has been no evidence to date for a negative feedback on NPP through nitrogen availability in these stands," as some have suggested would occur, but as we have consistently maintained will not occur (see Nitrogen (Cycling) in our Subject Index). As a result, Norby et al. confidently conclude that "the effect of CO2 fertilization on forest NPP is now firmly established, at least for young stands in the temperate zone."

Nevertheless, nitrogen availability does play a role in this phenomenon. In the Duke-FACE study, for example, where Norby et al. say "a wide range of response to CO2 enrichment across replicate plots correlated with differences in soil nitrogen availability," it was observed that "under low nitrogen availability, CO2 enrichment increased NPP by 19%, whereas under intermediate and high nitrogen availability the percent CO2 stimulation was 27%," or 42% greater (27%/19% = 1.42). This observation is very important, for it is "almost certain," in the words of Shaw et al. (2002), that significant nitrogen deposition originating from anthropogenic activities will continue to accompany the ongoing rise in the atmosphere's CO2 concentration throughout the foreseeable future; and this phenomenon should further boost forest NPP. But by how much?

Looking to the past, Lloyd (1999) calculated that from 1730 to the early 1980s the increase in temperate deciduous forest NPP due solely to the historical increase in the atmosphere's CO2 concentration was approximately 7%, and that the increase in NPP due to a modest proportional increase in nitrogen deposition over the same time period would have been about 25%. However, when CO2 and nitrogen increased together in the model employed by Lloyd, the NPP stimulation was 40%, which is more than the sum of the individual contributions of the extra CO2 and nitrogen. Although this exercise does not allow us to make a precise prediction of the percentage stimulation of forest NPP in response to future concomitant increases in atmospheric CO2 content and nitrogen deposition, it strongly suggests that the increase will be much larger than what is suggested by the analysis of Norby et al., which deals solely with the effects of increasing CO2.

Sherwood, Keith and Craig Idso

References
Lloyd, J. 1999. The CO2 dependence of photosynthesis, plant growth responses to elevated CO2 concentrations and their interaction with soil nutrient status, II. Temperate and boreal forest productivity and the combined effects of increasing CO2 concentrations and increased nitrogen deposition at a global scale. Functional Ecology 13: 439-459.

Norby, R.J., DeLucia, E.H., Gielen, B., Calfapietra, C., Giardina, C.P., King, S.J., Ledford, J., McCarthy, H.R., Moore, D.J.P., Ceulemans, R., De Angelis, P., Finzi, A.C., Karnosky, D.F., Kubiske, M.E., Lukac, M., Pregitzer, K.S., Scarasci-Mugnozza, G.E., Schlesinger, W.H. and Oren, R. 2005. Forest response to elevated CO2 is conserved across a broad range of productivity. Proceedings of the National Academy of Sciences 102: 10.1073/pnas.0509478102.

Shaw, M.R., Zavaleta, E.S., Chiariello, N.R., Cleland, E.E., Mooney, H.A. and Field, C.B. 2002. Grassland responses to global environmental changes suppressed by elevated CO2. Science 298: 1987-1990.