Seasonal and diel variations of electrical resistivity in a beech tree stem using time-lapse electrical resistivity imaging

Trees are thought to use internal water stored within the stem as a key buffer against short-term drought. Together with deep root water uptake and stomatal regulation, this mechanism helps trees to maintain critical physiological functions alive. Stem moisture content and water potential is generally observed using electrical conductivity (EC), moisture sensors, or microtensiometers installed in the sap wood of living trees. While these sensors offer precise information at the point scale, applying imaging techniques such as Electrical Resistivity Tomography (ERT) can help the investigation of spatial patterns and internal changes in electrical conductivity across larger portions of the trunk. Although ERT monitoring was mainly developed for hydrogeological applications to track soil moisture or groundwater dynamics at scales of tens to hundreds of meters, it has more recently been adopted in forest ecohydrological research for smaller-scale applications. ERT imaging of tree stems has proven its ability to inform on internal structures of trunks, while time-lapse ERT has shown promise to inform on stem water content variations.

Here, we present results from a field experiment conducted on a mature beech tree (Fagus sylvatica L.) at the Weierbach Experimental Catchment (WEC) in Luxembourg. The tree has been equipped with 4 rings of 30 stainless-steel screw electrodes each, with 5 cm electrode spacing and 50 cm vertical spacing between rings. ERT data was acquired during the growing season, from March to November, at a 4-hour temporal resolution. Additional tree sensors installed on the same tree, including sap flux, radial growth, moisture, water potential and temperature, provide complementary measurements for comparison with the ERT results.

At the seasonal scale, observations indicate spatially consistent changes in resistivity, with progressive resistivity decrease in the sap wood during the growing season. At the diel scale, pronounced daily variations in electrical resistivity are also observed, which seem to follow physiological processes also picked up by sap flow sensors and dendrometers. We will discuss challenges linked with the downscaling of the time-lapse ERT technique, both in time and space. These include: (i) accounting for strong temperature effects within the stem that influence reconstructed resistivity models and require advanced correction methods, and (ii) accurately determining electrode geometry at high resolution, including electrode orientation and seasonal changes in stem diameter. Finally, we address the interpretation of resistivity changes in terms of wood moisture dynamics and potential variations in sapwood chemical composition.