Fluctuations of CO2 in Free-Air CO2 Enrichment (FACE) depress plant photosynthesis, growth, and yield

Free-Air CO2 Enrichment (FACE) was conceived as an experimental method to measure plant responses to elevated CO2 in natural environments rather than in chambered or controlled environments. However, due to the difficulty of controlling elevated CO2 concentrations in turbulent air, the range of fluctuations of CO2 in FACE experiments are more than 10-fold greater than plants experience in natural conditions. One early study reported that photosynthetic increases of leaves in 40- and 80-s periods of oscillating elevated CO2 were only about 68% of those in leaves exposed to constant elevated CO2 with the same mean CO2 concentration. Later whole-plant studies reported smaller increases of responses in 60-s periods of oscillating elevated CO2 compared to constant elevated CO2 with the same mean concentration. After eliminating problematic data from studies that predicted plant responses in FACE to be only 45% of responses in open top chambers, we calculated that yields increased 65% as much in fluctuating elevated CO2 of FACE as in constant elevated CO2. The smaller plant responses in fluctuating elevated CO2 can be attributed partially to the non-linear, convex-upward curved response of photosynthesis to CO2 concentration, but other unknown mechanisms must exist. Future leaf chamber studies and FACE studies should include fundamental photosynthetic physiologists who can focus on uncovering the mechanisms responsible for lower photosynthetic, biomass, and yield response in both regular waveform oscillating and irregular fluctuating elevated CO2. Because CO2 fluctuates in FACE and recent experiments indicate reduced photosynthesis and growth under fluctuating CO2, responses of plants in FACE are likely to underestimate the benefits of rising CO2. We found that a correction factor of about 1.5 needs to be applied to FACE results. While responses to elevated CO2 in FACE experiments are smaller than those in chamber experiments, FACE responses are obtained in natural conditions not available in chambers and thus are conservative regarding future projections of agricultural productivity.