Whole tree chambers for elevated atmospheric CO2 experimentation and tree scale flux measurements in south eastern Australia: the Hawkesbury Forest Experiment.
Abstract Resolving ecophysiological processes in elevated atmospheric CO 2 C a at scales larger than single leaves poses significant challenges. Here, we describe a field based experimental system designed to grow trees up to 9 m tall in elevated C a with the capacity to control air temperature and simultaneously measure whole tree gas exchange. In western Sydney, Australia, we established the Hawkesbury Forest Experiment (HFE) where we built whole tree chambers (WTC) to measure whole tree CO 2 and water fluxes of an evergreen broadleaf tree, Eucalyptus saligna A single E. saligna tree was grown from seedling to small tree within each of 12 WTCs; six WTCs were maintained at ambient C a and six WTCs were maintained at elevated C a targeted at ambient C a +240 mmol mol 1 All 12 WTCs were controlled to track ambient outside air temperature T air and air water vapour deficit D air During the experimental period, T air D air and C a in the WTCs were within 0.5 degC, 0.3 kPa, and 15 mmol mol 1 of the set points for 90% of the time, respectively. Diurnal responses of whole tree CO 2 and water vapour fluxes are analysed, demonstrating the ability of the tree chamber system to measure rapid environmental responses of these fluxes of entire trees. The light response of CO 2 uptake for entire trees showed a clear diurnal hysteresis, attributed to stomatal closure at high D air Tree scale CO 2 fluxes confirm the hypothesised deleterious effect of chilling night time temperatures on whole tree carbon gain in this subtropical Eucalyptus The whole tree chamber flux data add an invaluable scale to measurements in both ambient and elevated C a and allow us to elucidate the mechanisms driving tree productivity responses to elevated C a in interaction with water availability and temperature.