(277 words)1 2 Resolving ecophysiological processes in elevated atmospheric CO 2 (C a ) at scales larger than 3 single leaves poses significant challenges. Here, we describe a field-based experimental system 4 designed to grow trees up to 9 m tall in elevated C a with the capacity to control air temperature 5 and simultaneously measure whole-tree gas exchange. In western Sydney, Australia, we 6 established the Hawkesbury Forest Experiment (HFE) where we built whole-tree chambers 7 (WTC) to measure whole-tree CO 2 and water fluxes of an evergreen broadleaf tree, Eucalyptus

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The water potential {ip) in twigs from four clones of Scots pine {Pinus sylvestris L.) growing in close proximity to each other was measured in a pressure chamber during five selected periods between May 1972 and March 1973. Diurnal and seasonal patterns of ip are presented in relation to irradiance, air temperature and vapour pressure deficit (VPD) for one cloudy and one clear day in each experimental period. Significant differences in 0 were found amongst most of the clones. Larger amplitudes and earlier day-time minima in 0 were found during the summer than during the winter. Noon values of 0 of less than -12 bars were regularly found during the summer months.Plots of 0 against irradiance or VPD during a day, showed marked hysteresis as the result of the simultaneous influence on 0 of several environmental factors. Close linear relations resulted when 0 was plotted against potential evaporation rate (calculated from the Penman-Monteith formula). The slopes of these regression lines, essentially the flow resistance, showed marked seasonal variations. At both high and low evaporation rates, the clone that grew the fastest had the lowest values of 0, and the clone that grew the slowest had the highest values of 0, while the other two clones had intermediate values. Differences in solute potential or in stomatal, plant or soil resistance are discussed as possible explanations of the more or less constant differences between the clones during the year of observations.

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The water potential (β) in ten‐year‐old Scots pine trees (Pinus sylvestris L.) from four different latitudinal provenances ranging from 57° N to 67° N, growing in close proximity to each other, was measured in a pressure chamber during five selected periods between May 1972 and March 1973. Diurnal and seasonal patterns of φ are presented in relation to irradiance, air temperature and vapour pressure deficit (D) for one cloudy and one clear day in each experimental period. The largest daily amplitude in φ was found at the beginning and end of summer, indicating a larger resistance to water flow from soil to needles. As the soil water potentials (measured as pre‐dawn values) were, at the same time, the highest for the year, it is suggested that these changes in resistance from period to period mainly take place in the trees. Plots of φ against D during clear days, showed marked hysteresis as the result of the simultaneous influence on φ of several environmental factors. Close linear relations resulted when φ was plotted against potential evaporation rate (calculated from the Pennman‐Monteith formula). The slopes of these regression lines, essentially the flow resistance, showed marked seasonal variations, with the largest resistance found at the beginning and end of summer. In most periods the water relations of the trees from different provenances were strikingly similar. A continuous change in the water relations of the remote provenances towards the situation for the “home‐provenance” is indicated by the experimental results. It is concluded that trees from different latitudes after ten years of growth have about the same chance as the home provenance to survive periods during which their water balance might become critical.