Elongated chambers for field studies across atmospheric CO2 gradients

1, We describe a field facility that controls CO2 along continuous gradient s from superambient to subambient concentrations. The facility consists of two transparent, tunnel-shaped chambers, each 1-m wide and tall and 60-m lo ng. Pure CO2 is injected into one chamber during daylight to initiate a sup erambient CO2 gradient (550-350 mu mol mol(-1)). Ambient air is introduced to the second chamber to initiate a subambient CO2 gradient (350-200 mu mol mol(-1)). CO2 concentrations at night are regulated at 150 mu mol mol(-1) above daytime values along each gradient. The CO2 gradients are maintained by varying the rate and direction (day/night) of air flow. 2, Air temperature and vapour pressure deficit are regulated near ambient v alues by cooling and dehumidifying air at 5-m intervals along chambers. 3, Desired CO2 gradients were regulated on grassland for virtually the enti re 9-month growing season in 1998, including a 6-month drought. Consistent CO2 concentrations were maintained along gradients despite seasonal variati on in species composition, leaf area, and temperature. 4, Daytime temperatures in chambers tracked the seasonal pattern in 1998. T he polyethylene covering on chambers transmitted 90% of incident light, but usually increased the ratio of diffuse to direct light. 5, By enabling the study of trends in plant and ecosystem responses to CO2 over both subambient and superambient concentrations. elongated chambers fi ll a void in CO2 research facilities.