Using Water Stable Isotopes to Trace Water Sources of Three Typical Japanese Tree Species under Heavy Rainfall Conditions

Stable isotopes of xylem water (18O and D) have been successfully used to determine sources of soil water for plant transpiration, but mainly in drought-prone environments. The water uptake strategies of three representative tree species in Japan, namely cedar (Cryptomeria japonica), larch (Larix kaempferi) and beech (Fagus crenata), were investigated using δ18O and δD of water (precipitation, soil and xylem), together with wood α-cellulose δ13C and δ18O, along one growing season. The study was carried out in the research forest of Yamagata University (Shonai region), a high precipitation area in Japan, which exceeds 3000 mm per year. Precipitation water δ18O and δD increased along the summer growing season, but oxygen and hydrogen isotopic composition of soil water remained essentially unchanged. In general, xylem water isotopes of cedar and larch followed the local meteoric water line, but beech xylem water was decoupled from soil and precipitation values in July and August. For this tree species, the xylem water isotopic records were more depleted than cedar and larch xylem water isotopic values and the precipitation water isotopic records, indicating that beech used more water from soil layers during July-August than the other two species, which mainly used newly-fallen precipitation. Wood δ18O showed an opposite seasonal trend to the one found for xylem water, likely because of leaf water isotope enrichment, which was in turn controlled by seasonal transpiration rate. The higher δ13C values of cedar during summer suggested that this species had enhanced water-use efficiency during the growing season compared with the deciduous species larch and beech. Our results highlight different water use strategies among forest tree species even in areas where the annual water balance is far from limiting plant performance.