Pairing geophysical techniques improves understanding of the near-surface Critical Zone: Visualization of preferential routing of stemflow along coarse roots

There is compelling evidence from aboveground observations that trees considerably affect precipitation partitioning through the Critical Zone. However, due to the lack of appropriate methods, the role of root systems (the hidden half of trees) on redistributing precipitation and infiltration into and routing through the soil remains inadequately visualized and understood. Here, we designed a novel experiment to pair two non-invasive geophysical techniques, ground-penetrating radar (GPR) and electrical resistivity tomography (ERT), to trace stemflow through the soil in a forested hillslope after water was released on the trunk of an American beech (Fagus grandifolia Ehrh.) to induce stemflow. We used GPR to locate lateral coarse roots and GPR and ERT together to confirm the wetting areas in response to the non-uniform transport of stemflow. Signal changes between time-lapse geophysical images were used to reveal both the response time and location of stemflow infiltration into and redistribution through the soil. This first known study to investigate the subsurface routing of stemflow by combining GPR and ERT revealed that the belowground funneling of stemflow along laterally oriented coarse roots transported water 2.8 m downslope from the study tree in 30 min after stemflow was initiated. In situ excavation validated the distribution of lateral roots and lateral root-derived preferential flow paths identified in geophysical images, confirming the utility of pairing GPR and ERT to gain insights into the temporal dynamics and spatial distribution of subsurface routing of stemflow. The proposed method visualized and confirmed the funneling effect of roots on belowground water redistribution that contributed to subsurface lateral flow. Pairing GPR and ERT provides a useful combination of geophysical methods to advance our understanding of the complex interactions between plant and soil, such as the role of tree roots in soil hydrological process by revealing areas of funneling in the hidden part of the Critical Zone.