Monitoring root water uptake for understanding tree water use dynamics during dry periods

Sufficient water supply to the roots is needed to sustain transpiration demand.  However, detailed measurements of water fluxes into the roots and the gradients in water potential driving these fluxes are rare, particularly in the field, mainly due to the difficulty in accessing roots and performing measurements on them. As a result, field monitoring of tree water use often neglects the root system, and the water fluxes from the soil to the shoot of trees remain a frontier in soil plant water relations.  This measurement gap and lack of understanding of water fluxes in roots makes it difficult to properly determine what drives root water uptake and how it might vary depending on rooting depth, soil matric potential and transpiration rate in the field.

During dry summer 2022, we equipped beech and spruce trees with sap flow sensors and dendrometers on the stem and on roots accessing different soil depths. This allowed us to monitor water fluxes and potentials in the different plant parts, from the integrated fluxes in the stem to the water uptake of single roots. We conducted the measurements at the “Waldlabor” ecoydrological monitoring forest in Zurich, where we comprehensively monitor the soil-plant-atmosphere continuum of beech and spruce with dendrometers, sap flow sensors and frequently measured stem & leaf water potential as well as stomatal conductance. To uncover the roots with minimal disturbance we removed the soil with a special air pressurizer and vacuum pump that allowed soil removal without damaging the roots, installed sensors at roots accessing different soil depths and afterwards covered the roots with soil again. Soil matric potential was measured at 10, 20, 40 and 80 cm depth in proximity to selected roots. At the canopy level, we measured stomatal conductance and leaf water potential throughout the summer.

Here, we demonstrate how the collected data help to understand to which extent trees diversify their water uptake depending on water availability at different soil depths. Because of the scarce precipitation and the limiting soil water availability during the summer, pre-dawn leaf water potential, stomatal conductance as well as sap flow decreased, indicating a reduction in transpiration. Beech trees reduced their stomatal conductance more dramatically than spruce trees, thereby using soil water more quickly. The comparison of sap flow in the roots and the integrated signal measured along the stem reveals differences between roots, with roots accessing deeper soil upholding higher sap flow velocities than roots accessing shallow soil in both species.

The results allow to assess the interplay between aboveground tree hydraulics and water status and belowground water uptake for beech and spruce under drying conditions.