[M-AG44] Dynamics of radionuclides emitted from Fukushima Dai-ichi Nuclear Power Plant in the environment

[MAG44-04] Vertical variation in the depositional flux and biomass inventory of radiocesium through stemflow partitioning between tree canopy and trunk

*Zul Hilmi Saidin^1,2, Yuichi Onda², Hiroaki Kato², Janice E. Hudson², Kazuki Nanko³, Delphis F. Levia^4,5 (1.Graduate School of Life and Environmental Sciences, University of Tsukuba, Japan, 2.Center for Research on Isotopes and Environmental Dynamics, University of Tsukuba, Japan, 3.Department of Disaster Prevention, Meteorology and Hydrology, Forestry and Forest Products Research Institute, Japan, 4.Department of Geography and Spatial Sciences, University of Delaware, Newark, Delaware, USA, 5.Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware, USA)

Keywords:Forest hydrology, Cedar stand, Oak stand, Cs-137 depositional flux, Cs-137 biomass inventory

Understanding hydrological depositional flux through stemflow that transports radiocesium to the base of trees is crucial in estimating radiocesium sources and inventory in the forested ecosystems affected by the Fukushima Dai-ichi Nuclear Power Plant accident. Therefore, this study seeks to better understand the vertical variation in the depositional flux and biomass inventory of radiocesium through stemflow partitioning between tree canopy and trunk compartments. Working in a coniferous forest (Cryptomeria japonica, young Japanese cedar, with a mean height of 10.9m, mean DBH of 0.560m, and n=3) and a mixed deciduous broadleaved forest (Quercus serrata, Japanese oak, with a mean height of 14.3m, mean DBH of 0.789m, and n=2), we investigated precipitation partitioning via generation of stemflow and branchflow by collecting stemflow from upper portion of trunk (1m below the canopy layer) and lower portion at trunk base (1.37m from forest floor), and branchflow from the base of primary branches at the various canopy layers (C. japonica: younger foliage, mixed foliage, and dead foliage; Q. serrata: upper and middle canopy). We determined partitioning between canopy-trunk compartments via ¹³⁷Cs depositional flux by multiplying a volume-weighted mean of ¹³⁷Cs (Bq L^-1) with branchflow and stemflow depth of basal area (L m^-2) and a ¹³⁷Cs inventory by multiplying a volume-weighted mean of ¹³⁷Cs (Bq L^-1) with biomass density in each tree compartment (kg m^-2). Furthermore, we characterized seasonal changes of the ¹³⁷Cs depositional flux and ¹³⁷Cs inventory between canopy and trunk compartments to clarify the sources and dynamics distribution of leachable ¹³⁷Cs in both forest stands.


Results revealed that the magnitude of the total ¹³⁷Cs depositional flux for the oak stand was higher than that of the cedar stand by ten times, with 0.95 and 10.34 Bq m^-2 stand^-1 day^-1 for cedar stand and oak stand, respectively. Such a difference could be attributed to higher branchflow and stemflow generated by oak stands. However, the distribution of ¹³⁷Cs sources was the largest within the canopy compartment of the cedar stand, with 71% compared to 47% in the oak stand. The result implies that ¹³⁷Cs leached from the canopy of cedar stands corresponded to initially intercepted radiocesium that tightly bonded to the vegetative cover and then washed off during incident rainfall. Meanwhile, the total ¹³⁷Cs inventory of the oak stand is six times greater than the cedar stand, with 1.46×10^-2 and 8.31×10^-2 Bq m^-2 stand^-1 day^-1 for cedar and oak stands, respectively. Both forest stands have a similar contribution of leachable ¹³⁷Cs that interacted with tree biomass with 85% and 87% originating from the canopy of cedar and oak stands, respectively. This study concludes that the sources and dynamics of ¹³⁷Cs leached via stemflow from a forest stand can be distinguished by partitioning through canopy and trunk compartments and subsequently corresponding to hydrological flux and biomass density fraction.