Screen-aided CO2 control (SACC): a middle ground between FACE and open-top chambers

We have developed a novel CO2 exposure system for natural vegetation that is a middle ground between Free Air CO2 Enrichment (FACE) and traditional open-top chambers (OTC). Screen-Aided CO2 Control (SACC) technology uses much less CO2 per experiment and per replicate than FACE and is superior to OTCs in terms of its effects on microclimate. A SACC unit consists of a thin metal frame, a clear plastic ''screen'', and a pipe at the base of the screen through which CO2 enriched jets of air are directed into the unit. There is a gap between the ground and the bottom of the pipe and the screen is relatively short in comparison to the maximum height of the vegetation. Our SACC units are hexagonal and enclose a ground area of 1.27 m(2). SACC works in the following way: 1) the screen breaks the wind and creates turbulent mixing within the unit, 2) the mixing of the outside air with the CO2 enriched jets of air,generates relatively uniform CO2 concentrations within the screened-in vegetation, and 3) a fully automated system monitors CO2 concentrations and adjusts CO2 injection rates for each unit every ca. IO minutes to maintain preset CO2 concentrations. Twenty-four hour means of CO2 concentrations in the middle of a unit are typically maintained within 1 mu l l(-1) of their set points. Spatial variation and short-term fluctuations in CO2 concentration are similar to those in OTCs and FACE. CO2 consumption at our site is 5 kg CO2 day(-1) replicate(-1) for a total of ca. 30 tons per year for 20 elevated CO2 SACC units. Compared to OTCs, SACC units have reduced temperature peaks at full sunlight, minimal effects on solar radiation, reduced rainfall interception by chamber walls, and freer access of small animals to experimental plots. We believe that SACC is the best method for exposing short stature vegetation to elevated CO2 when financial constraints do not allow for a properly replicated FACE experiment.