Predawn disequilibrium between soil and plant water potentials in light of climate trends predicted for northern Europe

Abstract

The study was performed on coppice shoots and 9-year-old saplings of hybrid aspen (Populus tremula L. × Populus tremuloides Michx.) growing at the Free Air Humidity Manipulation site in 2013 and 2014. The main aims of our study were to determine the differences in water potentials between soil and leaves before dawn and the major environmental factors preventing the overnight equilibration of water potential. Secondly, we investigated the impact of increasing relative air humidity—a climate trend predicted for northern Europe—on the equilibration. A relatively small predawn disequilibrium (PDD) was recorded both for coppice shoots and saplings: on average 0.042 and 0.066 MPa, respectively. We found a significant correlation between PDD and the vapour pressure deficit of ambient air (R 2 = 0.80 and 0.66, respectively). The experiment with bagged leaves revealed that 76% (coppice shoots) and 47% (saplings) of the PDD are attributable to nocturnal transpiration. In coppice shoots, the magnitude of PDD is best explained by soil water availability. After cool and moist nights, when leaves were usually dew-wetted, the PDD was close to zero, supporting the idea of foliar uptake of water through the cuticle. After drier nights, predawn leaf water potential does not represent a reliable estimate of soil water potential even in moderately humid climates. Increasing atmospheric humidity does not affect the magnitude of PDD in hybrid aspen. However, the stomata of hybrid aspen remain open at night, which enables nocturnal transpiration and water loss from the foliage.