Comparisons of Responses to Elevated CO2 in Free-Air and Open-Top Chamber Systems.

With an anticipated increased demand for agricultural products, there is considerable interest in estimating how much the yields of crop plants may increase with increases in atmospheric CO2 concentration.  Two common methods of exposing crops in the field to elevated concentrations of CO2 are free-air CO2 enrichment systems (FACE), and open top chambers (OTC).  There has discussion in the literature about whether these systems indicate similar crop responses to elevated CO2, but only one side-by-side comparison has been reported previously.  I compared leaf gas exchange and yield responses of soybean and winter wheat cultivars grown in FACE and OTC at Beltsville for two years.  In both species the reduction in stomatal conductance at elevated CO2 averaged significantly less in FACE than in OTC, with steeper Ball-Berry slopes in the elevated FACE treatment than in all other treatments.  Neither species differed consistently between FACE and OTC in the stimulation in photosynthesis by elevated CO2.  In both species, yields were less in OTC than in FACE, but the relative CO2 effect on yield was larger in OTC than in FACE.