Precipitation partitioning and related nutrient fluxes in a subtropical forest in Okinawa, Japan

Precipitation partitioning into throughfall and stemflow and related hydrochemical fluxes were examined during a 3 y period from January 1998 to December 2000 in a subtropical evergreen broad-leaved forest on Okinawa Island, Japan. Monthly water samples were collected to determine the concentrations and associated fluxes of bioelements. The mean annual precipitation during the study period was 3325 mm. Typhoons played a central role in the hydrology of the study forest with 11 typhoons contributing 29.1% of the total rainfall over the 3 y period. Throughfall and stemflow contributed 53.9% and 30.9% of the annual rainfall, respectively, implying a rainfall interception of 15.2%. The very high fraction of stemflow could be due to the crown morphology of the dominant species, Castanopsis sieboldii, that has inclined branches and concave shaped leaves. Mean pH in the precipitation was 6.22, and decreased significantly as the water passed through the canopy. Concentrations of total N, DOC, K, Na, Ca, and Mg showed a clear pattern of enrichment in both throughfall and stemflow compared to rainfall. The proximity to the Pacific Ocean strongly influenced the nutrient fluxes via rainfall at our site as illustrated by the extremely high Na fluxes via rainfall (213 kg Na ha -1 y -1 ) and throughfall plus stemflow (291 kg Na ha -1 y -1 ). The mean annual nutrient inputs (in kg ha -1 ) were: total N 43, P 2.6, K 76, Ca 49, Mg 30, Na 291, Al 1.8, Fe 1.0, and Mn 1.1. The input of DOC reached 361 kg C ha -1 y -1 . The high nutrient inputs via net precipitation (throughfall plus stemflow) especially for N at our site is thought to reflect the frequent occurrence of sea salt-induced stress and serious herbivory by insect.