FACE stands for Free Air CO₂ Enrichment. FACE technology is capable of providing a means by which the environment around growing plants may be modified to realistically simulate future concentrations of atmospheric carbon dioxide (CO₂). Unlike growth chambers and greenhouses, no containment is required with FACE designs. Previously difficult-to-study natural conditions such as temperature, precipitation, pollination, wind, humidity, and sunlight are now possible. In addition, there are, in theory, no plant size or growth problems caused by the constraint of enclosures (although the system must be sized, or expandable, to accommodate the anticipated future size of the plants during the lifetime of the experiment). Therefore, long-term studies may be conducted. FACE field data represent plant and ecosystem responses to concentrations of atmospheric CO₂ in a natural setting possible during the next century.

The most sophisticated FACE system was designed by Brookhaven National Laboratory's FACE Group, although simpler systems [e.g., Mini-FACE (see Miglietta et al. 1996) and MidFACE] are also in use. The Brookhaven design, summarized here, consists of a CO₂ tank, vaporizers, high-volume blower, a plenum or wide ring-shaped pipe for air distribution, vertical standing vent pipes for emitting CO₂ into the exposure area, sensors to measure wind speed, wind direction, and CO₂ concentration, and finally a computer-control system to regulate and monitor CO₂ releases. Wind direction, wind speed, and CO₂ concentration are measured by sensors at the center of each ring. The computer-control system receives information from these sensors via fiber optics and uses the information to adjust the CO₂ flow rates to maintain the desired CO₂ concentration at the center of the FACE ring. The system then uses the wind direction information to turn on only those pipes upwind of the plots, so that CO₂ -enriched air flows across the plots, no matter which way the wind blows. The CO₂ flow rate is updated every second, and the choice of which vertical pipes to release from is updated every 4 seconds. This feature reduces CO₂ losses that would occur if it were released on the downwind side and reduces the potential for carryover of injected CO₂ to adjacent plots. The entire FACE system is designed to cause no significant changes to the plots' terrestrial environment or natural airflow.

FACE research technology creates a platform for multidisciplinary, ecosystem-scale research on the effects of elevated atmospheric CO₂ concentrations over extended periods of time. In doing so, a large amount of high-CO₂-grown plant material can be produced, enough to support the research of many cooperating scientists. This would encourage research by teams of investigators, who can study different aspects of an ecosystem's response to CO₂ enrichment. This concurrent use by numerous independent scientists provides economies of scale and the potential to gain new insights into ecosystem responses that are difficult or impossible to obtain with smaller scale studies.

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