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Elevated atmospheric [CO2] (eCa) often decreases stomatal conductance, which may delay the start of drought, as well as alleviate the effect of dry soil on plant water use and carbon uptake. We studied the interaction between drought and eCa in a whole-tree chamber experiment with Eucalyptus saligna. Trees were grown for 18 months in their Ca treatments before a 4-month dry-down. Trees grown in eCa were smaller than those grown in ambient Ca (aCa) due to an early growth setback that was maintained throughout the duration of the experiment. Pre-dawn leaf water potentials were not different between Ca treatments, but were lower in the drought treatment than the irrigated control. Counter to expectations, the drought treatment caused a larger reduction in canopy-average transpiration rates for trees in the eCa treatment compared with aCa. Total tree transpiration over the dry-down was positively correlated with the decrease in soil water storage, measured in the top 1.5 m, over the drying cycle; however, we could not close the water budget especially for the larger trees, suggesting soil water uptake below 1.5 m depth. Using neutron probe soil water measurements, we estimated fractional water uptake to a depth of 4.5 m and found that larger trees were able to extract more water from deep soil layers. These results highlight the interaction between rooting depth and response of tree water use to drought. The responses of tree water use to eCa involve interactions between tree size, root distribution and soil moisture availability that may override the expected direct effects of eCa. It is essential that these interactions be considered when interpreting experimental results. [ABSTRACT FROM AUTHOR]

Nocturnal water flux has been observed in trees under a variety of environmental conditions and can be a significant contributor to diel canopy water flux. Elevated atmospheric CO2 (elevated [CO2]) can have an important effect on day-time plant water fluxes, but it is not known whether it also affects nocturnal water fluxes. We examined the effects of elevated [CO2] on nocturnal water flux of field-grown Eucalyptus saligna trees using sap flux through the tree stem expressed on a sapwood area (Js) and leaf area (Et) basis. After 19 months growth under well-watered conditions, drought was imposed by withholding water for 5 months in the summer, ending with a rain event that restored soil moisture. Reductions in Js and Et were observed during the severe drought period in the dry treatment under elevated [CO2], but not during moderate- and post-drought periods. Elevated [CO2] affected night-time sap flux density which included the stem recharge period, called ‘total night flux’ (19:00 to 05:00, Js,r), but not during the post-recharge period, which primarily consisted of canopy transpiration (23:00 to 05:00, Js,c). Elevated [CO2] wet (EW) trees exhibited higher Js,r than ambient [CO2] wet trees (AW) indicating greater water flux in elevated [CO2] under well-watered conditions. However, under drought conditions, elevated [CO2] dry (ED) trees exhibited significantly lower Js,r than ambient [CO2] dry trees (AD), indicating less water flux during stem recharge under elevated [CO2]. Js,c did not differ between ambient and elevated [CO2]. Vapour pressure deficit (D) was clearly the major influence on night-time sap flux. D was positively correlated with Js,r and had its greatest impact on Js,r at high D in ambient [CO2]. Our results suggest that elevated [CO2] may reduce night-time water flux in E. saligna when soil water content is low and D is high. While elevated [CO2] affected Js,r, it did not affect day-time water flux in wet soil, suggesting that the responses of Js,r to environmental factors cannot be directly inferred from day-time patterns. Changes in Js,r are likely to influence pre-dawn leaf water potential, and plant responses to water stress. Nocturnal fluxes are clearly important for predicting effects of climate change on forest physiology and hydrology. [ABSTRACT FROM AUTHOR]