Nocturnal stomatal conductance responses to rising [CO ₂], temperature and drought

• The response of nocturnal stomatal conductance (g _s,n) to rising atmospheric CO ₂ concentration ([CO ₂]) is currently unknown, and may differ from responses of daytime stomatal conductance (g _s,d). Because night-time water fluxes can have a significant impact on landscape water budgets, an understanding of the effects of [CO ₂] and temperature on g _s,n is crucial for predicting water fluxes under future climates. • Here, we examined the effects of [CO ₂] (280, 400 and 640μmolmol ^-1), temperature (ambient and ambient+4°C) and drought on g _s,n, and g _s,d in Eucalyptus sideroxylon saplings. • g _s,n was substantially higher than zero, averaging 34% of g _s,d. Before the onset of drought, g _s,n increased by 85% when [CO ₂] increased from 280 to 640μmolmol ^-1, averaged across both temperature treatments. g _s,n declined with drought, but an increase in [CO ₂] slowed this decline. Consequently, the soil water potential at which g _s,n was zero (Ψ ₀) was significantly more negative in elevated [CO ₂] and temperature treatments. g _s,d showed inconsistent responses to [CO ₂] and temperature. • g _s,n may be higher in future climates, potentially increasing nocturnal water loss and susceptibility to drought, but cannot be predicted easily from g _s,d. Therefore, predictive models using stomatal conductance must account for both g _s,n and g _s,d when estimating ecosystem water fluxes.