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Summary: Photosynthetic water‐use efficiency (WUE) describes the link between terrestrial carbon (C) and water cycles. Estimates of intrinsic WUE (iWUE) from gas exchange and C isotopic composition (δ13C) differ due to an internal conductance in the leaf mesophyll (gm) that is variable and seldom computed.We present the first direct estimates of whole‐tree gm, together with iWUE from whole‐tree gas exchange and δ13C of the phloem (δ13Cph). We measured gas exchange, online 13C‐discrimination, and δ13Cph monthly throughout spring, summer, and autumn in Eucalyptus tereticornis grown in large whole‐tree chambers. Six trees were grown at ambient temperatures and six at a 3°C warmer air temperature; a late‐summer drought was also imposed.Drought reduced whole‐tree gm. Warming had few direct effects, but amplified drought‐induced reductions in whole‐tree gm. Whole‐tree gm was similar to leaf gm for these same trees. iWUE estimates from δ13Cph agreed with iWUE from gas exchange, but only after incorporating gm. δ13Cph was also correlated with whole‐tree 13C‐discrimination, but offset by −2.5 ± 0.7‰, presumably due to post‐photosynthetic fractionations.We conclude that δ13Cph is a good proxy for whole‐tree iWUE, with the caveats that post‐photosynthetic fractionations and intrinsic variability of gm should be incorporated to provide reliable estimates of this trait in response to abiotic stress. [ABSTRACT FROM AUTHOR]

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]

A correction to the article " Forest-Scale Sap Flux Responses to Rainfall in a Dryland Eucalyptus Plantation" that was published in the July 30, 2008 issue is presented.