A large closed canopy chamber for measuring CO2 and water vapour exchange of whole trees

A transient-state chamber was developed to measure canopy gas exchange of single trees in the field. The chamber, with a volume of 41.6 m3, is designed to enclose a medium-size orchard tree; chamber top and windows can be left open, causing minimum disturbance to the tree environment. Transitory closures allow simultaneous measurement of CO2 exchange and transpiration of the enclosed tree. The chamber was tested during a 2-year study in an olive orchard submitted to different irrigation treatments: control with no water stress (CI) and regulated deficit irrigation (RDI). Leakage had a minimal impact on flux calculations (0.8% min−1); adsorption was not detectable. Maximum increases in canopy temperature of 0.58 °C min−1 for CI and 1.3 °C min−1 for RDI generated very small effects on fluxes. Changes in the transpiration rate induced by the chamber's modification of the canopy environment were evaluated by continuous sap flow measurements with heat pulse gauges inserted in the trunk of two trees enclosed by chambers. Results showed a sap flow decrease of about 8% after 180 s of chamber closure. The artificial turbulence generated by fans into the chamber to facilitate air mixing did not alter the transpiration rate. The enclosure had a very small impact on the tree canopy conductance (Gc). The initial lag and mixing time was estimated as 30 s; the optimal duration of the calculation window was 70 s. Hourly carbon assimilation (A), transpiration (E), and water use efficiency (WUE) for two olive trees in the field subjected to different levels of water stress were measured.