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Free-Air CO2-Enrichment May Not Be All It's Cracked Up to Be
Bunce, J.A. 2012. Responses of cotton and wheat photosynthesis and growth to cyclic variation in carbon dioxide concentration. Photosynthetica 50: 395-400.

The author - a research scientist at the Crop Systems and Global Change Laboratory of the U.S. Department of Agriculture - notes that large rapid fluctuations in CO2 concentration often occur in free-air CO2-enrichment or FACE studies, citing Hendrey et al. (1999), Okada et al. (2001) and Bunce (2011), further noting that "the importance of these rapid fluctuations in CO2 concentration to plant function remains uncertain." However, he also notes that "Holtum and Winter (2003) measured PN [net photosynthesis] and found significantly lower mean rates when the CO2 concentration varied with a period of 40 seconds compared to rates measured at a constant mean CO2 concentration." And, therefore, he proceeded to further explore the subject in greater detail, testing to see "whether the long-term growth and PN of cotton and wheat plants were affected by 1-minute cycles of CO2 concentration."

What was done
As Bunce describes it, he "used open-top chambers to expose cotton and wheat plants to either a constant elevated CO2 concentration of 180 ppm above that of outside ambient air, or to the same mean CO2 concentration, but with the CO2 enrichment cycling between about 30 and 330 ppm above the concentration of outside ambient air, with a period of one minute." These procedures were followed for three short-term (27-day) periods of cotton over two summers, plus one winter wheat crop that was grown from sowing to maturity.

What was learned
The USDA researcher reports that "total shoot biomass of the vegetative cotton plants in the fluctuating CO2 concentration [FACE] treatment averaged 30% less than in the constantly elevated CO2 concentration treatment at 27 days after planting," while "wheat grain yields were 12% less in the fluctuating CO2 concentration treatment compared with the constant elevated CO2 concentration treatment."

What it means
The meaning of Bunce's findings is pure and simple. As he straightforwardly puts it: "the results suggest that treatments with fluctuating elevated CO2 concentrations [such as are characteristic of all FACE experiments] could underestimate plant growth at projected future atmospheric CO2 concentrations." Thus, earth's plant life could well be far better off in a high-CO2 world of the future than even we have long contended it will be.

Bunce, J.A. 2011. Performance characteristics of an area distributed free air carbon dioxide enrichment (FACE) system. Agricultural and Forest Meteorology 151: 1152-1157.

Hendrey, G.R., Ellsworth, D.S., Lewin, K.F. and nagy, J. 1999. A free-air enrichment system for exposing tall forest vegetation to elevated atmospheric CO2. Global Change Biology 5: 293-309.

Holtum, J.A.M. and Winter, K. 2003. Photosynthetic CO2 uptake in seedlings of two tropical tree species exposed to oscillating elevated concentrations of CO2. Planta 218: 152-158.

Okada, M., Lieffering, H., Nakamura, H., Yoshimoto, M., Kim, H.Y. and Kobayashi, K. 2001. Free-air CO2 enrichment (FACE) using pure CO2 injection: system description. New Phytologist 150: 251-260.

Reviewed 9 January 2013