TEMPERATURE-CONTROLLED OPEN-TOP CHAMBERS FOR GLOBAL CHANGE RESEARCH

To enable experiments on the interactive effects of elevated atmospher ic CO2 and increased air temperature on physiological processes in tre es to be carried out, we altered the standard design of open-top chamb ers by replacing blowers with evaporative coolers and in-line heaters, with a feedback control system to maintain ambient or elevated air te mperatures within the chambers. Ambient and elevated (+ 4 degrees C) t emperature regimes were attained consistently and reliably throughout the growing season, with high reproducibility between chambers. From M ay through December the average of nearly 300,000 temperature measurem ents was 18.5 degrees C in ambient air, 18.9 +/- 0.6 degrees C in six ambient chambers, and 22.4 +/- 0.9 degrees C in six elevated temperatu re chambers. The difference in soil temperature between ambient and el evated chambers was 1.2 degrees C. Absolute humidity (vapour pressure) in the chambers was higher than that of ambient air, but it was gener ally similar between temperature treatments. Vapour pressure deficit t herefore was higher in elevated temperature chambers than in ambient c hambers, and this difference is considered an inseparable part of the temperature treatment. The addition of a temperature control system to open-top chambers removes what has been an important flaw in this imp ortant tool for global change research.