Precipitation in a forest is intercepted by the canopy and partitioned into throughfall and stemflow, leading to heterogeneous water inputs that affect soil water dynamics. To clarify the effects of a tree stand on rainfall infiltration processes on a steep forested hillslope, we conducted detailed observations of throughfall, stemflow, soil water content, and pore water pressure at high spatial resolution for many storm events. The results 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 >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 more rapidly and to a greater degree in the region downslope from the tree stem than in the upslope region. For a heavy storm event, the cumulative stemflow per infiltration area along the downslope sides of the tree trunk was 18.9 times the cumulative open-area rainfall. Locally concentrated rainwater input attributable to the 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 downslope region, resulting in the development of an asymmetric saturated zone around the tree.