Cesium absorption through bark of Japanese cedar (Cryptomeria japonica)

Absorption of radiocesium (Cs-137 and Cs-134) through bark, and its subsequent translocation into wood and needles, has been suggested as a potential source of tree contamination, but the process is not well understood. Field experiments were conducted to confirm whether Cs could enter a Japanese cedar tree through the bark and how Cs moves within a tree. Stable Cs (Cs-133) was applied to the bark at 1.2-m height on 10- and 26-year-old Japanese cedars. The Cs-133 concentrations were determined in the bark, sapwood, and heartwood (for 26-year-old cedar only) of stem disks from several heights, as well as in current-year needles from the canopy. The Cs-133 concentrations were considerably higher in the sapwood and heartwood of stem disks from 1.2-m height in treated trees than in untreated trees, suggesting that Cs-133 penetrated the bark to enter the wood. The average Cs-133 concentrations were higher in the heartwood than the sapwood, indicating Cs-133 accumulation in the heartwood. High Cs-133 concentrations in the needles of treated trees implied acropetal movement of Cs-133 to actively growing organs. Our results demonstrate that Cs can enter Japanese cedar trees through the bark and that Cs is transported radially to the heartwood and vertically to the apex.