Geophysical Monitoring of Oak Trees in a Marsh-Forest Upland Transect

Flooding and subsequent saltwater intrusion from rising sea levels pose significant concerns for shore-front areas, particularly agricultural lowlands and coastal forests. Changes in soil salinity, driven by both vertical infiltration from high tides and lateral seawater intrusion from storm surges, are documented to produce ghost forests and render agricultural soils unsuitable for cultivation. This study explores the sensitivity of geophysical methods to the exchange of water between trees and soil in white oak (Quercus alba) trees on the upper Delmarva Peninsula. High-frequency ground-penetrating radar (GPR) was employed to image root structures, identifying the depth of highest root density at approximately 0.3 m. This data provided critical geometry for Hydrus-1D evapotranspiration models. Small-scale 3D time-lapse electrical resistivity tomography (ERT) can be used to image the root zone and base of the trees. Initial work with ERT shows variability in the rainfall infiltration patterns but there are issues regarding sensitivity at the base of the tree. To assess tree physiology, vertical arrays of true self-potential (SP) non-polarizing electrodes were installed on tree trunks; SP measurements are a passive electrical method typically used in soil to measure streaming potential which is voltage values which are generated by the movement of water through porous media or capillary action. There have been attempts to use SP electrodes on trees before but the information has been ambiguous as a result of polarization effects as proper SP electrodes were not used, leading to misinterpreted data. The SP electrodes are deployed alongside more traditional sap flow sensors for validation of the collected data. In instances where in-situ sap flux data were unavailable, transpiration rates modeled using Hydrus-1D were utilized as a proxy of sap flux values. Wavelet analysis of the SP data revealed distinct diurnal cycles with strong 24-hour peaks that correlated with both the available sap flow measurements and the modeled transpiration. These results confirm that SP is a viable proxy for monitoring soil-tree moisture dynamics. This strategy may offer a novel framework for monitoring tree health and verifying subsurface water dynamics in coastal ecosystems threatened by saltwater intrusion.