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The CO2 Fertilization Effect
Volume 8, Number 13: 30 March 2005

In a recent issue of New Phytologist, Australia's Roger Gifford (2004) reports on the proceedings of the International Free Air CO2 Enrichment (FACE) Workshop on Short- and Long-Term Effects of Elevated Atmospheric CO2 on Managed Ecosystems, which was held at Ascona, Switzerland in March 2004.  In his review, he concentrates on a few key aspects of the aerial fertilization effect of atmospheric CO2 enrichment and how it will be expressed in the real world of nature as the air's CO2 content continues to rise.

Gifford begins by noting that Kimball et al. (2002) compared what was learned about elevated CO2 effects on eleven different crops (including grass, cereal, C4 sorghum, tuber and woody crops) from recent FACE experiments with the things that had been learned from the large number of prior chamber experiments, including open-top chambers.  He reports that Kimball et al. determined that the FACE experiments did indeed confirm, under longer-term field conditions and with but a couple of exceptions, "all the prior quantitative chamber findings on crops grown and measured in elevated CO2 concentration compared with ambient CO2 concentration."  In addition, he notes that the subsequent study of Long et al. (2004) confirmed, "with greater statistical rigour and for a much wider range of species including crops, pasture species and trees, most of the conclusions of the evaluation by Kimball et al. (2002)."

On a related topic, Gifford reports what was discussed about photosynthetic acclimation or the down regulation of growth that is sometimes observed in long-term atmospheric CO2 enrichment experiments in conjunction with a decrease in foliage nitrogen content, which latter phenomenon has typically been found to be "less pronounced in FACE studies."  Of this acclimation process, he asks the question: should it "be seen as a mechanism of plants 'resisting' a positive response to elevated CO2?"  His answer is "probably not," as he reports that "Stephen Long presented an elegant exposition of how photosynthetic down-regulation involves an optimization of the deployment of N from photosynthetic machinery to growth organs such that a balance between C-source and C-sinks is maintained in the plant under elevated CO2 concentration - a response that generally increases [plant] nitrogen use efficiency (Wolfe et al., 1998)."  What is more, he reports that several FACE studies demonstrate an increased abundance of legumes in CO2-enriched plots, and that this observation "is supportive of the notion that, in the long run, elevated CO2 concentration may cause N-fixation to entrain more atmospheric N2 into the ecosystem, leading ultimately to fuller expression of the increased growth and standing biomass potential that the elevated CO2 provides (Gifford, 1992)."

Next, in an update of the analysis of Hendry et al. (1997) that focuses on the effects of the rapidly fluctuating atmospheric CO2 concentrations that are an inherent characteristic of FACE experiments, the technique's primary developer (George Hendry) concluded, according to Gifford, that plant photosynthesis rates "can be decreased by 17% or more for the mean concentration reported when that mean is of large CO2 fluctuations on the order of half the mean, and the deviations from the mean occur over a minute or longer."  In light of this finding, both of them suggest, in Gifford's words, that "FACE technology might be systematically understating [our italics] the effect of globally elevated CO2 on ecosystem productivity."

In writing about the ultimate bottom-line consequences of long-term atmospheric CO2 enrichment in natural situations, Gifford reports the results of one of the longest open-top-chamber CO2-enrichment studies ever to be conducted: that of the Smithsonian Environmental Research Center on a salt marsh ecosystem on Chesapeake Bay.  Bert Drake, the scientific director of the project, told the FACE meeting attendees that "after 17 years," in the words of Gifford, "the elevated CO2 concentration had increased the marsh shoot density by > 100% compared with ambient air control chambers." This response is similar to that recorded by Idso and Kimball in their equally long open-top-chamber study of sour orange trees (see our Editorial of 5 Mar 2003).

In concluding his review of the 2004 international FACE meeting, Gifford sums up the consensus of the participants with respect to "the CO2 fertilizing effect," stating that "the evidence for its existence in the real world continues to consolidate."  And so it does ... year after year after year ... to where it is now about as certain as death and taxes.

Sherwood, Keith and Craig Idso

Gifford, R.M.  1992.  Interaction of carbon dioxide with growth-limiting environmental factors in vegetation productivity: Implications for the global carbon cycle.  Advances in Bioclimatology 1: 25-58.

Gifford, R.M.  2004.  The CO2 fertilising effect - does it occur in the real world?  New Phytologist 163: 221-225.

Hendrey, G.R., Long, S.P., McKee, I.F. and Baker, N.R.  1997.  Can photosynthesis respond to short term fluctuations in atmospheric carbon dioxide?  Photosynthesis Research 51: 170-184.

Kimball, B.A., Kobayashi, K. and Bindi, M.  2002.  Responses of agricultural crops to free-air CO2 enrichment.  Advances in Agronomy 77: 293-368.

Long, S.P., Ainsworth, E.A., Rogers, A., Ort, D.R.  2004.  Rising atmospheric carbon dioxide: Plants FACE the future.  Annual Review of Plant Biology 55: 591-628

Wolfe, D.W., Gifford, R.M., Hilbert, D. and Luo, Y.  1998.  Integration of photosynthetic acclimation to CO2 at the whole plant level.  Global Change Biology 4: 879-893.