Performance of large open-top chambers for long-term field investigations of tree response to elevated carbon dioxide concentration

In preparation for an investigation of the effects of elevated carbon dioxide (CO2) concentration on the two tree species Pinus radiata D. Don and Nothofagus fusca (Hook. f.) Oerst, the environmental conditions inside sixteen open-top chambers, of the design described by Heagle et al. (1989), were measured and compared with those outside. During a period in late summer, both air temperature and air saturation deficit were greater inside the chambers, with mean increases of 0.3 degrees C and 0.1 kPa, respectively. The increases were closely related to solar irradiance, reaching maximum differences for temperature and air saturation deficit of 4.3 degrees C and 0.8 kPa, respectively, when solar irradiance was greater than 1600 mu mol m(-2)s(-1). The mean (+/- standard deviation) CO2 concentrations for the ambient and elevated treatments were 362+/-37 and 654+/-69 mu mol mol(-1), respectively. However, the CO2 concentration in the elevated treatment decreased as windspeed increased, owing to incursions of ambient air into the chambers. Transmittance of visible solar irradiance (400-700 nm) through the plastic wall material decreased by 7% after 1 year of exposure at the site. In cloudy conditions the mean transmittance of solar irradiance into the chambers was 81% and on clear days this decreased from 80% to 74% with increasing solar zenith angle. The ratio of diffuse to total solar irradiance in the chambers was 13% and 21% greater than that outside for cloudy and clear conditions, respectively. The implications of these differences on water use efficiency for the trees growing inside and outside the chamber are discussed. A cost effective system, built to separate the CO2 required for the experiment from waste biogas, is described. This project is contributing to the Global Change and Terrestrial Ecosystems (GCTE) Core Research Programme by providing data on the long-term effects of elevated CO2 concentration on the above and below-ground carbon balance for the two tree species.