In a provocative paper they entitled "Food for Thought: Lower-Than-Expected Crop Yield Stimulation with Rising CO2 Concentrations," Long et al. (2006) suggested that future increases in crop production caused by the fertilization effect of the atmosphere's rising CO2 concentration may be only half as large as what had long been believed would be the case, due to confounding influences they claimed were inherent in all experimental assessments of the growth-promoting effects of atmospheric CO2 enrichment except those employing Free-Air CO2-Enrichment or FACE technology. Quite to the contrary, however, there is a strong possibility that just the opposite could well be true, i.e., that future increases in crop production caused by the aerial fertilization effect of the atmosphere's rising CO2 concentration may well be twice as large as what FACE experiments suggest.
The starting point for this analysis is the same for both propositions; and that is the fact that elevated CO2 enhances the yields of the crops Long et al. selected for study by about twice as much in enclosure studies that employed greenhouses, controlled-environment chambers and transparent open- or closed-top field chambers, as compared to what it does in FACE studies. But whereas Long et al. proclaimed FACE technology to be superior, implying that the FACE-derived results were the more realistic of the two sets of findings, others made a strong case for just the opposite conclusion.
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 likely grossly in error is suggested by an 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.
Similar findings were reported a decade later by Bunce (2014), who in a FACE study of two soybean cultivars with either daytime-only or 24-hour CO2 enrichment, found that "the seed yield of both elevated CO2 treatments exceeded that of the ambient CO2 controls, averaging 30% higher than the controls with daytime CO2 elevation, and about 40% higher with continuous CO2 elevation." Consequently, Bunce (2014) concluded that "FACE systems which only elevate CO2 during the daytime could underestimate crop responses to future CO2 concentrations."
Another problem associated with FACE technology was 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.
And 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 in both species."
In a similarly designed experiment, Bunce (2012) "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. And this work revealed 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."
Subsequent work by Bunce (2013) on wheat and rice also substantiated the much lower growth enhancement response of those plants to atmospheric CO2 enrichment via FACE techniques than the enhancement produced by non-FACE means.
The implications of the findings discussed above are exceptionally pure and simple. As Bunce (2012) straightforwardly put it: "the results suggest that treatments with fluctuating elevated CO2 concentrations [such as are characteristic of all FACE experiments, see Hendrey et al. (1999) and Okada et al. (2001)] could underestimate plant growth at projected future atmospheric CO2 concentrations." And, therefore, earth's plant life could well be far better off in a high-CO2 world of the future than what most FACE studies have indicated.
In summation of all the material presented above, with one common misjudgment (not enriching the air with extra CO2 at night), and with one hard-to-avoid problem (rapidly fluctuating CO2 concentrations), it is not surprising that the FACE experiments analyzed by Long et al. produced CO2-induced growth enhancements that were much lower than those produced in the Non-FACE studies they analyzed.
So why were these apparently "inconvenient truths" ignored for so long a time? The likely answer is that the "free-air" CO2 enrichment approach seemed more natural than enclosure studies; and, therefore, it seemed that FACE studies would be more correct in what they revealed.
In discussing some of the "unnatural" aspects of enclosure studies, for example, Long et al. noted that many such studies had used "plants grown in pots, which are now known to alter the response of plants to elevated CO2." 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.
Therefore, one would logically 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 likely 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, but in the opposite direction to what was originally supposed. And this fact suggests that many of the FACE-derived plant growth responses to atmospheric CO2 enrichment that are tabulated in our Plant Growth Database may well be considerably less than what they are in reality.
Bunce, J.A. 2005. Seed yield of soybeans with daytime or continuous elevation of carbon dioxide under field conditions. Photosynthetica 43: 435-438.
Bunce, J.A. 2011. Performance characteristics of an area distributed free air carbon dioxide enrichment (FACE) system. Agricultural and Forest Meteorology 151: 1152-1157.
Bunce, J.A. 2012. Responses of cotton and wheat photosynthesis and growth to cyclic variation in carbon dioxide concentration. Photosynthetica 50: 395-400.
Bunce, J.A. 2013. Effects of pulses of elevated carbon dioxide concentration on stomatal conductance and photosynthesis in wheat and rice. Physiologia Plantarum 149: 214-221.
Bunce, J.A. 2014. Limitations to soybean photosynthesis at elevated carbon dioxide in free-air enrichment and open top chamber systems. Plant Science 226: 131-135.
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.
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.
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.Last updated 23 February 2015