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Summary 1.Modifications and improvements in the hardware and software of a free-air CO 2 enrichment (FACE) system are described. The modifications were undertaken to minimize the amount of structure required and to improve software control of CO 2 set points. 2. A new low-cost CO 2 flow controller which provides increased reliability is described. 3. Analysis of system performance during a 79-day evaluation period of operation (13 h day -1 ) showed that for 1 min averages, the system is capable of maintaining a 550 p.p.m. ± 10% CO 2 set point during 78% of the operating time and a 550 p.p.m. ± 20% set point during 95% of the operating time. Ten-minute averages were within ± 10% and 20% during 87% and 96% of the operating time, respectively. 4. Continuous measurements of the spatial variation in CO 2 concentration inside the FACE ring over an 18-day period showed that of the total 16-m diameter treatment area, the central 11-m diameter portion remains within the 550 p.p.m. ± 10% set point. 5. Daily course gas-exchange measurements in matched individuals of the chaparral shrub Adenostoma fasciculatum just prior to and following a 6-week fumigation period at 550 p.p.m. CO 2 in the FACE treatment ring showed the FACE plants with reduced photosynthetic rates and higher (less stressful) water potentials compared with control A. fasciculatum plants measured at the same times and conditions, indicating the responsiveness of this species to elevated CO 2 conditions.

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