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Effects of Atmospheric CO2 Enrichment on Vegetative Productivity
Reference
Ziska, L.H. and Bunce, J.A. 2007. Predicting the impact of changing CO2 on crop yields: some thoughts on food. New Phytologist 175: 607-618.

Background
The authors write that "in a recent hypothesis first published in the Philosophical Transactions of the Royal Society (Long et al., 2005), and later reiterated in Science (Long et al., 2006), Long and colleagues argued that current modeling efforts overestimate the impact of increasing CO2 on future crop yields, because the models used are parameterized with data obtained from earlier 'enclosure studies' and not from the more sophisticated FACE [free-air CO2-enrichment] systems."

What was done
Ziska and Bunce methodically evaluated this claim for three crops of global importance -- rice (Oryza sativa), soybean (Glycine max) and wheat (Triticum aestivum) -- based upon a critical review of the relevant scientific literature.

What was learned
The two researchers from the USDA's Agricultural Research Service report they "could find no support for a consistent, large (~2x) overestimation of relative yield response to elevated CO2 in rice, soybean or wheat in (field) enclosure methodologies relative to FACE systems as reported by Long et al. (2005, 2006)," and they state that in the only study they could identify in which there was a side-by-side comparison of different methodologies, i.e. Kimball et al. (1997), "the relative responses of the aboveground biomass and absolute growth relative to elevated CO2 were nearly identical in the two systems."

Noting that in the earlier study of Kimball et al. (2002) it was also concluded that, "for the most part, the FACE- and chamber-based results have been consistent, which gives confidence that conclusions drawn from both types of data are accurate," Ziska and Bunce go on to say that "this conclusion is in line with the observations reported here," which, we might add, are also consistent with the prior independent conclusions of Tubiello et al. (2006).

Last of all, and providing even more support for the conclusion that the pronouncements of Long et al. are overly pessimistic with respect to the likely future strength of the aerial fertilization effect of atmospheric CO2 enrichment, Ziska and Bunce remind us that "there are over 100,000 rice cultivars (and thousands of soybean and wheat cultivars)," and that a thorough evaluation of their characteristics could identify, in their words, "those cultivars that could maintain, or improve, yields in response to changes in CO2/climate," giving us yet another means of making the most of the positive biological benefits of this amazing phenomenon.

What it means
The analysis of Ziska and Bunce, which upholds the long-known strength of the aerial fertilization effect of atmospheric CO2 enrichment, plus the demonstration of the hollowness of the progressive nitrogen limitation hypothesis provided by Finzi et al. (2007), give us added confidence that earth's terrestrial biosphere -- which Ziska and Bunce remind us "evolved at a time of high atmospheric CO2 (4-5 times present values)" -- is likely headed towards ever more verdant times in response to the ongoing rise in the air's CO2 content.

References
Finzi, A.C., Norby, R.J., Calfapietra, C., Gallet-Budynek, A., Gielen, B., Holmes, W.E., Hoosbeek, M.R., Iversen, C.M., Jackson, R.B., Kubiske, M.E., Ledford, J., Liberloo, M., Oren, R., Polle, A., Pritchard, S., Zak, D.R., Schlesinger, W.H. and Ceulemans, R. 2007. Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2. Proceedings of the National Academy of Sciences, USA 104: 14,014-14,019.

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

Kimball, B.A., Pinter Jr., P.J., Wall, G.W., Garcia, R.L., LaMorte, R.L., Jak, P.M.C., Fruman, K.F.A. and Vugts, H.F. 1977. Comparisons of responses of vegetation to elevated carbon dioxide in free-air and open-top chamber facilities. In: Allen Jr., L.H., Kirkham, M.B., Olszyk, D.M. and Whitman, C.E., Eds. Advances in Carbon Dioxide Research. American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Madison, Wisconsin, USA, pp. 1113-130.

Long, S.P., Ainsworth, E.A., Leakey, A.D.B. and Morgan, P.B. 2005. Global food insecurity treatment of major food crops with elevated carbon dioxide or ozone under large-scale fully open-air conditions suggests recent models may have overestimated future yields. Philosophical Transactions of the Royal Society B 360: 2011-2020.

Long, S.P., Ainsworth, E.A., Leakey, A.D.B., Nosberger, J. and Ort, D.R. 2006. Food for thought: Lower-than-expected crop yield stimulation with rising CO2 concentrations. Science 312: 1918-1921.

Tubiello, F.N., Amthor, J.S., Boote, K.J., Donatelli, M., Easterling, W., Fischer, G., Gifford, R.M., Howden, M., Reilly, J. and Rosenzweig, C. 2006. Crop response to elevated CO2 and world food supply: A comment on "Food for Thought ..." by Long et al., Science 312: 1918-1921. European Journal of Agronomy: 10.1016/j.eja.2006.10.002.

Reviewed 7 November 2007