Routing stemflow water through the soil: a dual labelling approach with artificial tracers

Abstract. Few studies have explored the stemflow double-funnelling phenomenon, although subsurface flow along roots and macropores plays a significant role in determining hydrological responses in forested catchments. In this study, a stemflow experiment on Pinus sylvestris L. (Scots pine) used artificial tracers to view and quantify the preferential flow of stemflow water after infiltration into the soil. Forty-one litres of water labelled with enriched deuterium and Brilliant Blue FCF were applied at a flow rate of 7 L h-1 to the stem of a pine tree, which corresponds to stemflow caused by about 50 mm rainfall. TDR probes were distributed around the tree trunk and in depth profiles to measure high-resolution volumetric water content. One day after the stemflow discharge, soil pits were dug in the different cardinal directions and at varying distances from the tree. Photographs were taken for imaging analysis to quantify preferential flow metrics. Soil samples were taken from the different profiles to analyse dye concentrations and isotopic compositions. We found that stemflow infiltrated through an annulus-shaped area around the tree base. We observed a heterogenous spatiotemporal soil moisture response to stemflow and the occurrence of shallow perched water tables around the tree trunk. Dye staining demonstrated that stemflow infiltrated primarily along the surface of coarse roots and also through macropores. The dye coverage was less extensive close to the soil surface and increased with depth and with proximity to the tree trunk. Lateral flow was also observed, mainly in the shallow soil layers. A set of metrics demonstrated the prevalence of preferential flow. Deuterium and Brilliant Blue FCF concentrations correlated with each other significantly. The concentrations decreased at increasing distance from the tree trunk, indicating dilution and mixing with residual soil water. Macropores, coarse roots (living or decayed) and perched water tables produced a complex network regulating the preferential flow. Our results suggest that stemflow heavily affects soil moisture distribution, and thus also groundwater recharge and surface runoff. Our study provides insights into the physical processes controlling stemflow belowground funnelling and improves our understanding of forest-water interactions.