Variation in nocturnal stomatal conductance and development of predawn disequilibrium between soil and leaf water potentials in nine temperate deciduous tree species

It is widely acknowledged that many plant species can keep stomata open during night. We examined how nocturnal stomatal conductance differs among potted saplings of nine temperate tree species from diverse native habitats in wet and dry soil conditions, and how it affects plant predawn water status. Nocturnal stomatal conductance in dry soil conditions was low in all the species (with a maximum value of 14.6 mmol m(-2) s(-1)); in wet conditions, it was the highest in Populus tremula L., a fast-growing and anisohydric pioneer species, and the lowest in Quercus robur L., a late-successional and isohydric species. Relatively high nocturnal stomatal conductance in wet conditions in P. tremula compared with the other species resulted in the highest difference in water potential values between the leaves and soil at predawn. As drought progressed, different species tended to keep stomata almost closed at night, and the observed differences between anisohydric and isohydric species disappeared. At an ample soil water supply, nocturnal stomatal behaviour was species dependent and varied according to both the water-use and the life strategies of the species. Keeping that in mind, one should therefore be careful when using predawn leaf water potential as a proxy for soil water potential, sampling different species.