Species distribution and composition change over time and water competition could be one of major reasons for those changes. Our 15-year survey on tree species abundance and diversity, which was conducted at 880 plots in Mt. Baegun and Mt. Jiri of Korea, showed the declining tendency of coniferous species including Korean red pine (Pinus densiflora). To inspect what caused the decline of conifer species, we compared water use schemes of two neighboring species by measuring stable isotopic composition of 18O, 2H from soil, xylem, and rain water. Six study plots were selected based on their species composition in Mt. Baegun. For each plot, tension lysimeter was installed at 5 soil depths (10, 30, 50, 100, and 120 cm) to extract soil water. This sampling was done for Apr. 2016 to Nov. 2016 with about 2-month interval. Xylem water from the branches was extracted by azeotropic distillation method. The isotopic composition of sampled water was measured to get the proportional contribution of each source (soil water) to xylem water by a Bayesian isotope mixing model (MixSIAR 3.1).
The hydrogen and oxygen isotope value of each soil depth showed decreasing pattern except for the first to second layers (10 cm to 30 cm). Currently, the water use scheme of two neighboring species (Chamaecyparis obtusa and Styrax obassia) was inspected. Both species used water mainly from 10 cm to 50 cm depth (about 80%). However, the fractional distribution within the layer of the 2 species was largely different. C. obtusa had relatively uniform distribution among the 3 depths (20% ~ 34%). On the other hand, S. obassia showed a biased distribution to 50 cm depth (about 59%). Our results showed the late successional understory species (S. obassia) was relying more water from deeper layers than the early successional dominant species (C. obtusa). In addition, this also can be used to see how the neighboring tree species partition their hydrological niche and the seasonal pattern of this partitioning.