Trees control hillslope subsurface flow: Insights from stemflow and throughfall experiments, geophysical surveys, and numerical modeling

Trees play a key role in subsurface water dynamics through stemflow and throughfall, affecting runoff and soil infiltration. However, a comprehensive understanding of their role in hydraulic responses across spatial scales requires integrated approaches and further research. In this study, we investigated the effects of rainfall parti­ tioning on subsurface water dynamics across multiple spatial scales on a forested hillslope (30◦ slope) in the Re della Pietra catchment of Central Italy. Four hierarchical spatial scales were considered to capture subsurface water dynamics: the point scale (via single-ring infiltration tests at three depths), the single-tree scale (through artificial stemflow events), the plot scale (covering a 100 m2 area, assessed either by an artificial throughfall event alone or in conjunction with multiple artificial stemflow events), and the hillslope scale (through pie­ zometers data). Geophysical surveys were also conducted at the plot and single-tree spatial scales using both Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT) to improve interpretation of hydrological functioning. Our results showed that the soil exhibits dual-permeability behavior, with throughfall promoting infiltration primarily into the matrix, while stemflow enhances fast-flow infiltration. Infiltrometer tests indicated lower infiltrability between tree stems (mean Ks values ranging from 177.8 to 237.4 mm h‒1, depending on depth), consistent with the artificial throughfall events. In contrast, at tree bases, artificial stemflow produced rapid infiltration through macropores and fractures, reaching a mean steady-state rate of 1031.9 mm h‒1. Numerical model inversions yielded consistent hydraulic parameters, and dual-permeability modeling showed that, at the plot scale, the stemflow-driven fast-flow region had higher Ks than the matrix (1047.4 vs. 217.5 mm h‒1). Geophysical surveys confirmed these dynamics: ERT detected vertical wetting beneath stems, while GPR revealed lateral subsurface flow at depth. Piezometer data further identified fast-flow pathways at the hillslope scale, where rapid water table rises could be explained by flow connectivity between vertical and deeper lateral pathways.