Continuous Stem Water Potential Measurements of a Diffuse-Porous Tree Species Offer New Insights Into Tree Water Relations

Abstract

Water potential is a crucial parameter for assessing tree water status and hydraulic strategies. However, methods for measuring water potential, such as the Scholander pressure chamber, are destructive, discontinuous and difficult to perform in tall forests. Consequently, important dynamics in water potentials, particularly during short-term drought, are difficult to capture. Recent advancements have introduced low-maintenance sensors capable of measuring continuous, high-resolution stem water potentials.

We evaluated these sensors in a temperate, diffuse-porous species (Carpinus betulus) over a growing season marked by dry-down periods and heat. Measurements of leaf water potential, sap flow and environmental factors (air temperature, vapour pressure deficit and soil water content) were conducted. Midday stem water potentials of C. betulus reached minimum values of −3.39 ± 0.10 MPa and exhibited pronounced seasonal fluctuations, mirroring changes in environmental conditions and sap flow.

Stem water potentials correlated well with Scholander-type measurements during predawn (R² = 0.98) but demonstrated an offset in absolute values during midday (R² = 0.71) and diurnal measurements. Minimum stem water potentials and maximum sap flow in the stem expressed a time lag and showed a distinct hysteresis. In this first assessment, the agreement with Scholander-type measurements, sap flow and environmental parameters suggests the tested water potential sensors yield reliable data, especially during predawn, but need further validation during midday conditions. If applicable to other tree species, these sensors could significantly advance our understanding of tree water relations and their role in forest drought responses.