Observations of soil water dynamics around a tree on a hillslope before and after intercepting stemflow

In rainfall redistribution processes in forest stands, a part of rainfall is intercepted by the canopy, while other parts are partitioned into throughfall and stemflow through the canopy as diffuse and point inputs to the forest floor, respectively. Therefore, the rainwater amount reaching the forest floor is considerably heterogeneous. To clarify the effects of stemflow on soil water dynamics, we conducted detailed observations of soil water content and pore water pressure around a tree on a forested hillslope which were separated into two periods of with or without stemflow. Observations were conducted from in April 2007 through April 2009 on a hillslope at the Kamigamo experimental station of Kyoto University, located in southern Kyoto Prefecture, central Japan. The hillslope has a mean gradient of 28 degrees, which is predominantly covered with “tall stewartia” (Stewartia monadelpha). To monitor the soil water dynamics around a tree, we selected a tall stewartia and delineated a longitudinal observation line from upslope to downslope of this tree. We installed a capacitance meter (Sentek, EasyAG-5p) at each of 10 points upslope and downslope from the tree stem. Each capacitance meter consisted of five sensors to measure soil water content at depths of 10, 20, 30, 40, and 50 cm. Additionally, we installed tensiometers at the soil-bedrock interface at the same 10 points to measure pore water pressure. In the second year (April 2008-April 2009), we intercepted stemflow of the tree and maintained observations above. We used two tubes cut longitudinally and wrapped spirally around the upslope and downslope sides of the trunk to collect separately the stemflow along the upslope and downslope sides of the trunk of the tree, and measured those using tipping-bucket gauges. The results before intercepting stemflow showed that the soil water content increased rapidly and greatly in the region downslope from the tree stem, especially at points close to the tree stem. At these points, maximal soil water storage was more than 100 to 200% of the cumulative open-area rainfall, and occurrences of bypass flow were recognized. Moreover, the pore water pressure at the soil-bedrock interface increased rapidly and greatly in the region downslope of the tree stem, especially at the points close to the tree stem. Locally concentrated rainwater inputs attributable to stemflow on the downslope side of the tree trunk probably caused the large and rapid increases in water content and pore water pressure in the region downslope of the tree stem, resulting in the development of an asymmetric saturated zone around the tree. On the other hand, the results after intercepting stemflow showed no clear differences in soil water dynamics between the regions upslope and downslope of the tree stem, which were dominated by a slow expansion of the wetting front. The frequency of saturated zone occurrence observed in the without-stemflow period was much smaller than in the with-stemflow period. It indicated the characteristics of stemflow infiltration process, in which water flowed rapidly through the deeper layer, causing an irregular distribution of vertical soil water content changes and saturated zone occurrence on the soil-bedrock interface.