Paper Reviewed
Allen, L.H., Kimball, B.A., Bunce, J.A., Yoshimoto, M., Harazono, Y., Baker, J.T., Boote, K.J. and White, J.W. 2020. Fluctuations of CO2 in Free-Air CO2 Enrichment (FACE) depress plant photosynthesis, growth, and yield. Agricultural and Forest Meteorology 284: 107899.

Although numerous laboratory and field-based studies have confirmed plant growth and biomass are generally enhanced at higher levels of CO2, the magnitude of such enhancement remains a topic of debate. This is, in part, due to different experimental methods employed in those studies that have yielded different results.

As a brief history on the topic, early CO2 enrichment studies were typically performed in laboratories under controlled-environment settings. Following criticism that such conditions were not representative of the real world, scientists shifted their work to examine plant responses to elevated CO2 in open-top chambers (OTCs) in outdoor environments, where the plants could be exposed to the elements under mostly natural growing conditions. Not long thereafter the desire to remove certain confounding artifacts from OTCs led to the next (and current) generation of studies employing what is referred to as Free-air CO2 Enrichment (FACE) technology.

FACE experiments today are constructed without walls. Their typical setup consists of several horizontal or vertical pipes that are placed around an experimental plot area that ranges from 1 to 30 m in diameter. CO2 enriched air is emitted from the pipes and the CO2 concentration is kept at the desired level via a system of strategically placed sensors that provide feedback to a computer that adjusts the flow of CO2 from the pipes, thus countering swings in CO2 that might arise from changes in wind speed and direction.

In general, OTC studies yield plant CO2-induced growth response increases that are significantly higher than those found in FACE studies. But because some (including the IPCC) consider FACE technology to be closer to the real world than the OTC setup, the benefits of atmospheric CO2 enrichment on plant growth have been downplayed. Others, in contrast, have countered it is FACE that is the inferior technology (see, for example, a discussion on this topic here).

The latest group of researchers to contribute to this debate is that of Allen et al. (2020), who provide a detailed review and critical analysis of the CO2 fluctuations inherent in experiments employing FACE technology and the effects of those fluctuations on plant growth and yield. In discussing their findings, the scientists report that CO2 fluctuations in FACE experimental systems were found to be "more than 10-fold greater than in nature," adding that "exposures to elevated CO2 in FACE are not representative of exposure to atmospheric elevated CO2 (with natural fluctuations) projected to occur in the future."

But does such atypical fluctuation in CO2 significantly impact plant growth and yield?

In a word, yes! According to Allen et al., "oscillations and fluctuations of elevated CO2 decrease observed leaf photosynthetic rates compared to steady levels of elevated CO2." And those reductions in photosynthesis do impact and reduce the CO2 fertilization effect to the extent that the scientists say "prior conclusions of the response of plants to elevated CO2 under FACE conditions may need to be reassessed by applying an adjustment factor (about 1.5) for photosynthesis, biomass, and yield to conclusions reached in experimental and modeling studies."

In light of the above, it is clear that FACE studies are not as realistic of future growing conditions as the IPCC and others make them out to be. They are much more conservative in projecting future biomass and yield responses of plants and crops, considering that a correction factor of about 1.5 must be applied to their findings. And that means that the likely impact of elevated CO2 (as determined via FACE studies) will be much greater than anticipated, which is good news for humanity and nature!