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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 CO 2  (elevated [CO 2 ]) 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 [CO 2 ] on nocturnal water flux of field-grown  Eucalyptus saligna  trees using sap flux through the tree stem expressed on a sapwood area ( J  s ) and leaf area ( E  t ) 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  J  s  and  E  t  were observed during the severe drought period in the dry treatment under elevated [CO 2 ], but not during moderate- and post-drought periods. Elevated [CO 2 ] affected night-time sap flux density which included the stem recharge period, called ‘total night flux’ (19:00 to 05:00,  J  s,r ), but not during the post-recharge period, which primarily consisted of canopy transpiration (23:00 to 05:00 , J  s,c ). Elevated [CO 2 ] wet (EW) trees exhibited higher  J  s,r  than ambient [CO 2 ] wet trees (AW) indicating greater water flux in elevated [CO 2 ] under well-watered conditions. However, under drought conditions, elevated [CO 2 ] dry (ED) trees exhibited significantly lower  J  s,r  than ambient [CO 2 ] dry trees (AD), indicating less water flux during stem recharge under elevated [CO 2 ].  J  s,c  did not differ between ambient and elevated [CO 2 ]. Vapour pressure deficit ( D ) was clearly the major influence on night-time sap flux.  D  was positively correlated with  J  s,r  and had its greatest impact on  J  s,r  at high  D  in ambient [CO 2 ]. Our results suggest that elevated [CO 2 ] may reduce night-time water flux in  E. saligna  when soil water content is low and  D  is high. While elevated [CO 2 ] affected  J  s,r , it did not affect day-time water flux in wet soil, suggesting that the responses of  J  s,r  to environmental factors cannot be directly inferred from day-time patterns. Changes in  J  s,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.

Interactive effects of elevated CO 2 and drought on nocturnal water fluxes in Eucalyptus saligna