Hillslope subsurface flow is driven by vegetation more than soil properties in colonized valley moraines along a humid mountain elevation

Abstract

Valley moraines along an elevation gradient are colonized by different climax vegetation; here, preferential flow paths (PFPs) and ground layers, as important hillslope structures, critically influence hillslope flow. However, the roles of these hillslope structures in flow dynamics, and their mechanisms in contrasting forest types within moraines, remain enigmatic. To this end, we conceptualized PFPs and ground layers as explicit elements in HYDRUS 2D, constructing set-ups of hillslope internal structures that incorporated an optional ground layer and varying intensities of PFPs using a random placement method to represent the shallow root zone (0-50 cm) of vegetated moraines. The results showed that, out of the 50 set-ups for each forest type, only 3 set-ups for the coniferous forest and 2 for the broadleaf forest successfully predicted the flow dynamics and water balance of the hydrological response at the event scale. Notably, all 5 successful set-ups featured below-average vertically connected PFPs that covered only 5 % of the total spatial area in both forests, following the principle of maximum free energy dissipation, which is achieved when flow passes through a network composed of partial PFPs and a steepened soil matrix gradient. The similar intensity of PFPs across forest types is attributed to the occurrence of similar coarse-textured soils, resulting from frequent precipitation and clay washout, as well as comparable fine root biomass, both contributing to the connectivity of vertical PFPs, which governs subsurface flow in shallow soil hillslopes. In addition, a linear relationship between vertically connected PFPs and subsurface flow was observed in both forests, with the coniferous forest being more sensitive to changes in PFPs due to its lower soil organic matter content. The presence of the ground layer caused the PFPs to be buried and trigged rapid lateral flow within the ground layer towards downslope positions, reducing the spatial homogeneity of the water exchange between PFPs and the soil matrix, leading to earlier peak flow timing and increased peak flow magnitude. This study highlights the role of vegetation-related processes in influencing subsurface flow, advancing our understanding of hillslope structures and runoff evolution over time in humid valley moraines.