Lessons From FACE: CO2 Effects and Interactions with H2O, N, and Temperature.

Over the past two decades, CO₂-FACE (free-air CO₂ enrichment or free-air controlled enhancement) experiments have been conducted on several agricultural crops: wheat, perennial ryegrass, rice, and barley, which are C₃ grasses; sorghum, a C₄ grass; white clover and soybean, which are C₃ legumes; potato, a C₃ forb with tuber storage; and cotton and grape, which are C₃ woody perennials. Generally, the magnitude of these responses varied with the functional type of plant and with interacting variables like H₂O, and N. As expected, with adequate H₂O and N, elevated CO₂ (about 200 ppm increase above ambient) increased photosynthesis and biomass production and yield substantially (about 12 to 40% for yield) in C₃ species, but little in C₄, and it decreased stomatal conductance (about 20%) and transpiration in both C₃ and C₄ species and greatly improved water-use efficiency in all the crops. Growth stimulations were as large or larger under water-stress compared to well-watered conditions. Growth stimulations of non-legumes were reduced by about half at low soil nitrogen, whereas elevated CO₂ strongly stimulated (about 25%) the growth of the clover legume both at ample and under low N conditions. Roots were generally stimulated more than shoots. Woody perennials had larger growth responses to elevated CO₂, while at the same time, their reductions in stomatal conductance were smaller. Tissue nitrogen concentrations went down (about 7-30%) while carbohydrate and some other carbon-based compounds went up due to elevated CO₂, with leaves and foliage affected more than other organs. Although the CO₂ by temperature interaction will likely be very important in determining the ultimate response of crops to global change, until recently the methodology for conducting free-air controlled warming experiments in open-field plots was problematic. However, deployment of infrared heater arrays with computer-controlled warming looks promising, and soon results from such combined CO₂-FACE and T-FACE experiments should be available.