Whole-tree chambers for elevated atmospheric CO₂ experimentation and tree scale flux measurements in south-eastern Australia: The Hawkesbury Forest Experiment

Resolving ecophysiological processes in elevated atmospheric CO₂ (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 9m 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 CO₂ 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 +240μmolmol^-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°C, 0.3kPa, and 15μmolmol^-1 of the set-points for 90% of the time, respectively. Diurnal responses of whole-tree CO₂ 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 CO₂ uptake for entire trees showed a clear diurnal hysteresis, attributed to stomatal closure at high D_air. Tree scale CO₂ 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.