Free-Air CO₂-Enrichment May Not Be All It's Cracked Up to Be

Introducing the rationale for his study, Bunce (2012) - 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 P_N [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, Bunce proceeded to further explore the subject in greater detail, testing to see "whether the long-term growth and P_N of cotton and wheat plants were affected by 1-minute cycles of CO₂ concentration."

To accomplish his objective, Bunce "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.

In describing his findings 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."

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 the more optimistic assessments have suggested it will be.

Additional References
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.