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Rising atmospheric concentrations of CO2 ( Ca) can reduce stomatal conductance and transpiration rate in trees, but the magnitude of this effect varies considerably among experiments. The theory of optimal stomatal behaviour predicts that the ratio of photosynthesis to transpiration (instantaneous transpiration efficiency, ITE) should increase in proportion to Ca. We hypothesized that plants regulate stomatal conductance optimally in response to rising Ca. We tested this hypothesis with data from young Eucalyptus saligna Sm. trees grown in 12 climate-controlled whole-tree chambers for 2 years at ambient and elevated Ca. Elevated Ca was ambient + 240 ppm, 60% higher than ambient Ca. Leaf-scale gas exchange was measured throughout the second year of the study and leaf-scale ITE increased by 60% under elevated Ca, as predicted. Values of leaf-scale ITE depended strongly on vapour pressure deficit ( D) in both CO2 treatments. Whole-canopy CO2 and H2O fluxes were also monitored continuously for each chamber throughout the second year. There were small differences in D between Ca treatments, which had important effects on values of canopy-scale ITE. However, when Ca treatments were compared at the same D, canopy-scale ITE was consistently increased by 60%, again as predicted. Importantly, leaf and canopy-scale ITE were not significantly different, indicating that ITE was not scale-dependent. Observed changes in transpiration rate could be explained on the basis that ITE increased in proportion to Ca. The effect of elevated Ca on photosynthesis increased with rising D. At high D, Ca had a large effect on photosynthesis and a small effect on transpiration rate. At low D, in contrast, there was a small effect of Ca on photosynthesis, but a much larger effect on transpiration rate. If shown to be a general response, the proportionality of ITE with Ca will allow us to predict the effects of Ca on transpiration rate. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

• Rising atmospheric CO 2 may stimulate stem respiration (R S) because of greater assimilation and carbohydrate supply. • R S in mature Eucalyptus trees was not significantly increased in elevated compared to ambient CO 2. • Elevated CO 2 increased maintenance respiration in the cool season, decreasing the temperature sensitivity of R S. The CO 2 efflux from tree stem surfaces to atmosphere (R S) is an important component in the carbon (C) balance of forest ecosystems. Rising atmospheric carbon dioxide concentrations [CO 2 ] are expected to stimulate R S , because of greater C assimilation and carbohydrate supply to stems under rising [CO 2 ]. Growth respiration (R g) and maintenance respiration (R m) during the warm growing season may respond differently to rising [CO 2 ] due to different metabolic demands. To test the effect of elevated [CO 2 ] (eCO 2 , ambient +150 ppm) on R S , we examined R S in mature Eucalyptus trees on a monthly basis for an entire year during the seventh year of exposure to eCO 2. R S varied seasonally and mirrored seasonal variation in temperature. While R S was not significantly increased under eCO 2 compared to ambient CO 2 (aCO 2), its temperature sensitivity was significantly decreased (Q 10 of 1.92 for aCO 2 and 1.56 for eCO 2). The estimated annual R g accounted for approximately 7–8% of annual total R S , 419 ± 103 g C m−2 yr-1, indicating that R m contributes substantially to total R S in this mature woodland. Monthly mean R S was correlated with monthly mean soil temperature, soil moisture and monthly stem growth rate in this dry year, but soil moisture levels may have been insufficient to observe the impacts of eCO 2 on stem growth in this droughted and phosphorous limited site. Our results highlight that eCO 2 tends to increase R m at low temperatures during the non-growing season, thus decreasing the temperature sensitivity of R S , despite a neutral effect of eCO 2 on R S rates on a yearly basis. [ABSTRACT FROM AUTHOR]