A through year behavior of 137Cs in a Japanese flowering cherry tree in relation to that of potassium.

To understand the transfer of radiocesium (137Cs) in inside of deciduous trees, changes in 137Cs activity concentrations, primarily derived from the Fukushima accident in March 2011, were observed in the upper parts of a Japanese flowering cherry tree (Prunus x yedoensis cv. Somei-Yoshino) between 2015 and 2018. The sampling of the foliar parts occurred over the entire leaf life span from winter bud to litterfall and those of the branches were distinguished based on emergence years (2017, 2016, 2015, 2014-2011, and 2010/before). First, every tissue demonstrated a clear seasonal variation in 137Cs activity concentration. Second, a synchrony of seasonal variations in 137Cs activity concentration with those in the biological analogue of K concentration was observed in foliar parts during their growth season, but not in branches nor during the other seasons. With respect to the timing of changes in each tissue with tree phenology, it is possible that K and 137Cs alternate between leaves and branches via the same translocation mechanisms. The resorption efficiencies (i.e., 1 - [the concentrations in the last litterfall]/[the maximum concentrations in green leaves]) of K and 137Cs were 76% and 46% in average, respectively. In addition, both leaf buds and branches played an important role as reservoirs during dormancy. The buds storage ratio before and after bud burst (i.e., [the inventories in buds at the end of defoliation]/[those before and after bud burst]) for K were 0.57 and 0.10 in median, respectively, and those for and 137Cs were 1.14 and 0.14 in median, respectively. Consequently, the transfer of 137Cs in inside of trees was still visible seven years after deposition, even though the annual reduction in 137Cs activity concentration was apparent in each tissue.

Radioactive and stable cesium isotope distributions and dynamics in Japanese cedar forests.

Dynamics of the Fukushima-derived radiocesium and distribution of the natural stable isotope 133Cs in Japanese cedar (Cryptomeria japonica D. Don) forest ecosystems were studied during 2014-2016. For the experimental site in Yamakiya, Fukushima Prefecture, we present the redistribution of radiocesium among ecosystem compartments during the entire observation period, while the results obtained at another two experimental site were used to demonstrate similarity of the main trends in the Japanese forest ecosystems. Our observations at the Yamakiya site revealed significant redistribution of radiocesium between the ecosystem compartments during 2014-2016. During this same period radionuclide inventories in the aboveground tree biomass were relatively stable, however, radiocesium in forest litter decreased from 20 ± 11% of the total deposition in 2014 to 4.6 ± 2.7% in 2016. Radiocesium in the soil profile accumulated in the 5-cm topsoil layers. In 2016, more than 80% of the total radionuclide deposition in the ecosystem resided in the 5-cm topsoil layer. The radiocesium distribution between the aboveground biomass compartments at Yamakiya during 2014-2016 was gradually approaching a quasi-equilibrium distribution with stable cesium. Strong correlations of radioactive and stable cesium isotope concentrations in all compartments of the ecosystem have not been reached yet. However, in some compartments the correlation is already strong. An increase of radiocesium concentrations in young foliage in 2016, compared to 2015, and an increase in 2015-2016 of the 137Cs/133Cs concentration ratio in the biomass compartments with strong correlations indicate an increase in root uptake of radiocesium from the soil profile. Mass balance of the radionuclide inventories, and accounting for radiocesium fluxes in litterfall, throughfall and stemflow, enabled a rough estimate of the annual radiocesium root uptake flux as 2 ± 1% of the total inventory in the ecosystem.

Radiocesium distribution and fluxes in the typical Cryptomeria japonica forest at the late stage after the accident at Fukushima Dai-Ichi Nuclear Power Plant.

The Fukushima-derived radiocesium distribution in the typical Japanese cedar (Cryptomeria japonica D. Don) forest ecosystem was determined. In four years after the Fukushima accident, about 74% of the total radiocesium inventory was localized in soil, 20% was in the litter, and only 6% was associated with the aboveground biomass. Most of the radiocesium that was initially intercepted by the tree canopies has been already transported to the ground surface. The importance of the processes for removal of radiocesium from the tree canopies decreased in the order litterfall > throughfall >> stemflow. Within the tree compartments, the largest radiocesium activity fraction, about 46%, was observed in old foliage, which indicates that the process of removal of the initial deposit from the tree crowns has not yet completed. The aggregate soil-to-wood transfer factor was 1.1⋅10-3 m2 kg-1 d.w., which is close to the geometric means of transfer factors recommended by IAEA for other coniferous tree species. Further studies in Fukushima forest are necessary to assess the variation of this parameter under various soil-landscape conditions. Presence of the residues of the initial deposits does not allow to obtain the accurate values of the annual radiocesium fluxes in the ecosystem. Based on the conservative assumptions, the ranges of the fluxes were estimated. Analysis of the flux structures shows that up to percents of the total radiocesium activity in the ecosystem may be involved into biogenic cycling.

Morphological abnormalities in Japanese red pine (Pinus densiflora) at the territories contaminated as a result of the accident at Fukushima Dai-Ichi Nuclear Power Plant.

Our research, carried out in 2014-2016 at eight sites in the radioactive contaminated territories of Fukushima Prefecture, showed that the young trees of Japanese red pine (Pinus densiflora) are sensitive to radiation. Irradiation induced cancellation of the apical dominance in this species. The effect is similar to that observed in young trees of Scots pine growing in the Chernobyl zone. At the same time, we did not observed any morphological abnormalities in mature trees of Japanese red pine. The probability of cancelling the apical dominance in Japanese red pine increased to 0.11 and 0.14 in the two less irradiated populations, and to 0.5 and 0.9 at sites were the absorbed dose rates were approximately 14 and 25 µGy h-1, respectively. Most of the observed abnormalities appeared in the second whorl after the beginning of exposure. No new abnormalities were observed in the fifth whorl. This temporal pattern is similar to those reported for Scots pine in Chernobyl and for Japanese fir in Fukushima. Additional detailed studies are necessary for interpretation of the observed temporal pattern and, in general, for explanation of the mechanism of formation of the morphological abnormalities.