Metrics of Plant Response to CO2 Enrichment

Authors: ALLEN, LEON - Retired ARS Employee; KIMBALL, BRUCE - Retired ARS Employee; BUNCE, JAMES - Retired ARS Employee; BOOTHE, KENNETH - University Of Florida; WHITE, JEFFREY - University Of Florida

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/8/2025
Publication Date: 5/2/2025
Citation: Allen, L.H., Kimball, B.A., Bunce, J.A., Boothe, K.J., White, J.W. 2025. Metrics of Plant Response to CO2 Enrichment. Agricultural and Forest Meteorology. https://doi.org/10.1016/j.agrformet.2025.110557.
DOI: https://doi.org/10.1016/j.agrformet.2025.110557

Interpretive Summary: In a previous paper, three ARS researchers (now retired) published a paper comparing the responses of crops to fluctuating levels of high CO2 concentrations, such as found in free-air CO2 enrichment experiments (FACE), versus the corresponding responses to a steady elevated CO2 concentration equal to the long-term average of the fluctuating level. That comparison suggested the yields in FACE experiments were only about 0.67 of those expected had the CO2 been steady instead of fluctuating. However, the metric used to determine this ratio was with respect to ambient CO2 levels, which are increasing every year. In this manuscript, the same authors propose another metric that does not reference ambient CO2 but is simply the ratio between the response in fluctuating versus that in steady CO2. The ratio was 0.85, which indicates that crop responses in FACE experiments are closer to responses in steady CO2 than suggested by by the previous metric, so therefore, we can have more confidence that FACE experiments conducted under free-air open-field conditions are providing results like what we can expect at future elevated atmospheric CO2 levels. This research benefits all consumers of food and fiber.

Technical Abstract: This LETTER discusses metrics used to quantify plant response to fluctuating CO2 compared to that at constant CO2. The concept of “Reduction in CO2-stimulated uptake rate in oscillating elevated CO2” (Holtum and Winter, 2003), the FIRST METRIC of plant response in oscillating CO2 enrichment compared to steady CO2 enrichment at the mean of the CO2 concentration of the oscillating CO2 with a value of 0.33 does not describe the actual response of plants grown in fluctuating CO2, and neither does the associated SECOND METRIC “Relative Response Ratio” of Allen et al. (2020) with a value of 0.67. A THIRD METRIC of plant response to CO2 enrichment, “Plant response in fluctuating elevated CO2 / Plant response in constant elevated CO2”, which has an average value of 0.85, is much more relevant and better represents the response in Free Air CO2 Enrichment (FACE). This third metric does not rreference ambient CO2 concentration.