Night-time transpiration, predawn hydraulic conductance and water potential disequilibrium in hybrid aspen coppice

Abstract

Predawn water potential disequilibrium in hybrid aspen coppice growing in humid environment was mostly determined by hydraulic conductance and the ratio of nocturnal to daily water loss was relatively small. Water relations are of crucial importance for biomass accumulation in trees and forest productivity, yet the significance of nocturnal water use is still debated. We investigated which environmental factors influence nocturnal transpiration, development of the equilibrium between soil and plant water potentials and plant hydraulic conductance in hybrid aspen (Populus tremula L. × P. tremuloides Michx.) coppice. Predawn leaf water potential and sap flow were measured simultaneously with relevant environmental factors on 2-year-old coppice shoots during the growing season of 2015. Nocturnal water loss constituted on average 4–5% of daily total transpiration and almost 2/3 of nocturnal sap flow. The nocturnal water loss was mainly driven by atmospheric evaporative demand, while maximum sap flux densities could extend 35% of the daytime values on dry nights. Predawn leaf water potential was constantly more negative than soil water potential, resulting in predawn disequlibrium between leaf and soil water potentials (PDD). Total soil-to-leaf hydraulic conductance was the primary factor explaining the variation in PDD. Our results suggest that in humid conditions plant hydraulic capacity is more substantial determining plant predawn water status than stomatal behaviour with respect to current environmental drivers.