Interactive effects of elevated CO2 and drought on nocturnal water fluxes in Eucalyptus saligna

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.