Vertical Variation in Branchflow and Stemflow Cycling of Radiocesium between Coniferous and Deciduous Forests in Japan

Thursday, 17 December 2020: 04:24
Zul Hilmi Saidin1, Yuichi Onda1, Hiroaki Kato1, Janice Elaine Hudson1, Kazuki Nanko2 and Delphis F Levia Jr3, (1)University of Tsukuba, Center for Research in Isotopes and Environmental Dynamics, Tsukuba, Japan, (2)Forestry & Forest Products Research Institute, Department of Disaster Prevention, Meteorology and Hydrology, Tsukuba, Japan, (3)University of Delaware, Departments of Geography and Spatial Sciences & Plant and Soil Sciences, Newark, DE, United States
The objective of this study was to better understand the vertical variation of radiocesium cycling via branchflow and stemflow between the tree canopy and trunk compartments. The work was conducted in a coniferous forest (Cryptomeria japonica (L. f.) D. Don (Japanese cedar)) with a mean height of 10.9 m and mean DBH of 0.560 m, and a mixed deciduous broadleaved forest dominated by Quercus serrata Murray (Japanese oak) with a mean height of 14.3 m and mean DBH of 0.789 m. Stemflow was measured from upper portion of trunk (1 m below the canopy layer) and lower portion at trunk base (1.37 m from forest floor), whereas branchflow was collected from the base of primary branches for pre-selected canopy layers (C. japonica: younger foliage, mixed foliage, and dead foliage; Q. serrata: upper and middle canopy). Our results showed that stemflow in cedar stand was largely generated in the canopy, with 83% compared to 59% in the oak stand, even though the cedar stand has a larger bark water storage capacity. We also found that the 137Cs concentration was the largest within the trunk compartment for both cedar and oak stands, possibly due to the increased residence time of stemflow on the lower reaches of the trunk. The 137Cs depositional flux varied between the tree compartment for both tree stands, depending on tree phenology and seasonality of precipitation. In addition, branchflow was generally enriched in δ18O and δD compared to the stemflow. This study provides more detailed insights into the vertical variation of radiocesium cycling at the tree scale.