We begin our exploration of this possibility by accepting Long et al.'s quantitative evaluations of the CO2-induced growth responses derived from FACE and Non-FACE assessment techniques, i.e., those employing enclosures such as greenhouses, laboratory controlled-environment chambers, and transparent open- or closed-top field chambers. That is to say, we accept the fact that elevated CO2 enhances the yields of the crops they selected for study by about twice as much in enclosure studies as it does in FACE studies.

Although we agree with them on this point, we are not convinced of the validity of Long et al.'s claim that FACE experiments provide the "best simulations of the future elevated CO2 environment." Quite to the contrary, one huge procedural failure of many FACE studies to replicate reality - including all of those conducted by Long and his associates on soybeans and maize - has been the decision of the studies' principal investigators to not enrich the air with CO2 at night, due in part to the large cost of doing so, plus the assumption that nighttime CO2 enrichment would have a negligible impact on plant growth and development.

That this assumption is possibly grossly in error is suggested by an important experiment conducted by Bunce (2005), who grew soybean plants from seed to maturity out-of-doors in open-top chambers exposed to normal precipitation while continuously fumigating them with either ambient air (AC) or with air enriched with an extra 350 ppm of CO2 either 24 hours per day (ECdn) or 14 hours per day centered on solar noon (ECd) for a total of four entire growing seasons. This sustained effort, in Bunce's words, revealed that "ECdn increased seed yield by an average of 62% over the four years compared with the ambient CO2 treatment, while ECd increased seed yield by 34%," indicative of the fact that the CO2-induced yield enhancement in the 24-hour CO2 enrichment treatment was almost twice as great as that of the daylight-only CO2 enrichment treatment.

Another potential problem associated with FACE technology is revealed by the work of Holtum and Winter (2003), who studied the physiological impacts of the rapidly fluctuating CO2 concentrations that occur in response to the continual over- and under-shooting of plot CO2 concentration targets as the FACE apparatus continually adjusts to counteract the concentration-perturbing effects of incessant variations in wind speed and direction. These researchers grew well watered and fertilized seedlings of two tropical tree species (Tectona grandis and Pseudobombax septenatum) in pots within controlled-environment chambers maintained at atmospheric CO2 concentrations of either 370 or 600 ppm, the latter of which concentrations was either held constant or achieved, in the mean, via symmetric CO2 oscillations around the 600-ppm target concentration.

So what did Holtum and Winter learn? In air of constant 600 ppm CO2 concentration, the net CO2 uptake rates of shoots and leaves of the T. grandis and P. septenatum seedlings rose by approximately 28 and 52%, respectively, while in the presence of atmospheric CO2 oscillations with a half-cycle of 20 seconds and an amplitude of 170 ppm about a mean of 600 ppm, they found that "the CO2 stimulation in photosynthesis associated with a change in exposure from 370 to 600 ppm CO2 was reduced by a third [our italics] in both species."

To briefly recapitulate, with one common misjudgment (not enriching the air with extra CO2 at night) leading to close to a 50% FACE-induced reduction in CO2-induced growth enhancement in one comparative study, and with one hard-to-avoid problem (rapidly fluctuating CO2 concentrations) leading to a FACE-induced reduction in CO2-induced growth enhancement of about a third in another comparative study, it is not surprising that the FACE experiments analyzed by Long et al. produced CO2-induced growth enhancements that were only about half as great as those produced in the Non-FACE studies they analyzed, as this result is just what would be expected from what was learned from the comparative studies of Holtum and Winter (2003) and Bunce (2005).

So why have we all ignored these apparently "inconvenient truths" for so long a time? The likely answer is that the "free-air" CO2 enrichment approach seemed more natural and less environmental-altering than enclosure studies; and, therefore, it seemed that FACE studies would be more correct in what they revealed. However, the environmental alterations associated with enclosure studies are imposed upon ambient and CO2-enriched treatments alike, and there is no a priori reason to suppose they would benefit CO2-enriched plants more than plants growing in ambient air. In fact, there was reason to suppose just the opposite.

In discussing some of the "unnatural" aspects of enclosure studies, for example, Long et al. note that many such studies have used "plants grown in pots, which are now known to alter the response of plants to elevated CO2 [our italics]." And how do pots and other containers alter the responses of plants to atmospheric CO2 enrichment? For one thing, pots or other soil containers can be more restrictive to the typically larger root systems of CO2-enriched plants, preventing their roots from exploring the larger soil volumes they would be capable of exploring if they were planted out-of-doors in the ground; and this container-induced root restriction impedes the ability of CO2-enriched plants to acquire the greater amounts of nutrients and water needed to support the greater growth potentials they possess compared to plants growing in ambient air. Consequently, one would expect that plants growing in pots or other containers in enclosure studies might well be less responsive to CO2 enrichment than they really are in nature or real-world agricultural situations, and that FACE studies would thus produce greater plant growth responses to elevated CO2 than enclosure studies. However, it now appears that the FACE problems discussed above may far outweigh the potential the FACE approach was thought to provide for improvement in this regard, ultimately leading to plant growth responses to atmospheric CO2 enrichment that are even more removed from reality in the opposite direction.

In conclusion, unless FACE studies employ 24-hour CO2 enrichment, there is now more reason than ever to believe they may significantly underestimate (by perhaps 50%) the crop-growth-promoting prowess of the ongoing rise in the air's CO2 concentration. In addition, even when 24-hour CO2 enrichment is provided, there is the nagging concern that rapidly fluctuating CO2 concentrations that occur in response to the continual over- and under-shooting of plot CO2 concentration targets may be similarly deflating CO2-induced growth enhancements. Consequently, with so much of importance "hanging in the balance," as it were, it is imperative that additional studies be conducted to clarify these issues to the satisfaction of everyone concerned about the effects of atmospheric CO2 enrichment on plant growth and development. In the interim, and until persuaded differently by definitive data, we are not confident that FACE studies are superior to enclosure studies when it comes to discovering the real-world response of crops to the ongoing rise in the air's CO2 content. In fact, they could be far worse.

Sherwood, Keith and Craig Idso

References
Bunce, J.A. 2005. Seed yield of soybeans with daytime or continuous elevation of carbon dioxide under field conditions. Photosynthetica 43: 435-438.

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.

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.