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 CO 2 in Free-Air CO 2 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 CO 2 , 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 CO 2 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 CO 2 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 CO 2 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. CO 2 enriched air is emitted from the pipes and the CO 2 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 CO 2 from the pipes, thus countering swings in CO 2 that might arise from changes in wind speed and direction.

In general, OTC studies yield plant CO 2 -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 CO 2 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 CO 2 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 CO 2 fluctuations in FACE experimental systems were found to be "more than 10-fold greater than in nature," adding that "exposures to elevated CO 2 in FACE are not representative of exposure to atmospheric elevated CO 2 (with natural fluctuations) projected to occur in the future."

But does such atypical fluctuation in CO 2 significantly impact plant growth and yield?

In a word, yes! According to Allen et al., "oscillations and fluctuations of elevated CO 2 decrease observed leaf photosynthetic rates compared to steady levels of elevated CO 2 ." And those reductions in photosynthesis do impact and reduce the CO 2 fertilization effect to the extent that the scientists say "prior conclusions of the response of plants to elevated CO 2 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 CO 2 (as determined via FACE studies) will be much greater than anticipated, which is good news for humanity and nature!