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

The portion of intercepted precipitation by forest canopies that drains from leaves and branches and is channeled to the stem and reaches the forest floor is defined as stemflow. Some studies have shown that when stemflow infiltrates into the soil, it is funneled preferentially along tree roots and macropores, bypassing the soil matrix and delivering large volumes of water to the deeper soil layers and to the aquifers. This bypass flow promoted by stemflow is of great hydrological and biogeochemical importance. In this context, the main aim of this study was to investigate the stemflow infiltration dynamics and patterns. With this objective we simulated stemflow at a flow rate of 7 L/h around a Scots pine tree with water labelled with deuterium and brilliant blue dye. The stemflow volume simulated (45L) corresponds to a rainfall event of about 50 mm rainfall. During the experiment, soil moisture and shallow piezometric levels, around the tree, were continuously monitored. In addition, 30cm depth small piezometers, located around the bole, were sampled to detect the labeled water arrival. To visualize the dye-stained areas and sample the soil for subsequent isotopic analysis and dye concentration determination, a pit was dig in each cardinal direction of the tree as near as possible of the tree bole. Almost no labeled water was found in the soil matrix, instead blue-dye was visualized near coarse roots and stones. In addition, the labeled water reached the small piezometers, indicating the presence of macropore flow. These results confirm the stemflow funneling effect, which infiltrates as preferential flow, and give some light on the complexity of the hydrological processes in the vadose zone.