Crop Responses to Elevated CO2 and Its Interactions with H2O, N, and Temperature: An Updated Review.

About two decades ago, free-air CO₂ enrichment (FACE) technology was developed that enabled the air above open-field plots to be enriched with CO₂ for entire growing seasons. Since then, FACE experiments have been conducted on several agricultural crops. Generally, elevated CO₂ from FACE (enrichment to 550 ppm from an ambient concentration of about 353 ppm in 1990) stimulated biomass and yield in C₃ crops 12-30%. Yields of most C₃ grain crops were increased on average about 12% by the FACE treatments, although some rice varieties have been stimulated by more than 30%, suggesting potential for future crop improvement by selection under elevated CO₂. Growth stimulations were often but not always reduced by low soil N. Elevated CO₂ caused partial stomatal closure resulting is some soil water conservation, so growth stimulations were generally as large or larger under water-stressed conditions. Yields of the C₄ crops, sorghum and maize, were not stimulated by elevated CO₂ except under water-stress conditions, with stimulations of 22-40% due to the water conservation. Woody perennials had larger growth stimulations than the average for herbaceous crops, while their reductions in stomatal conductance were smaller. Increasing temperature alone can stimulate or decrease plant growth depending on whether a plant is currently below or above its temperature optimum for growth. Therefore, elevated CO₂ plus warming can result in greater growth stimulations below the temperature optimum, or on the other hand, the beneficial effects of elevated CO₂ can be negated if the warming occurs above the temperature optimum.