Stemflow was measured in a planted young stand of Japanese cypress for four years from ages 9 to 12. Canopy cover increased with growth from 55% to 94% during the measurement period. The ratio of stemflow SF to rainfall R, SF/R (the funneling ratio FR that represents the efficiency in collecting stemflow), was 5.9% (81.3) at age 9 on an annual basis; however, it abruptly fell to 2.8% (30.0) at age 10. Following the drop, SF/R recovered gradually with growth, reaching 3.8% at age 11 and 4.3% at 12, while FR remained almost constant at values of around 30. The relation between R and SF/R analyzed quarterly on a rain event basis revealed that changes in canopy structure and/or tree architecture caused the drop in SF/R in the April-June period at age 10. Saplings of the species must compete for light and water until canopy closure because their growth rate is slower than that of competitors. As stemflow effectively supplies rainwater into the soil around the root system, it can be hypothesized that large SF/R and FR at age 9, and probably younger ages, are a strategy to acquire water for juvenile unclosed stands in dry summers.