Decadal trends in 137Cs concentrations in the bark and wood of trees contaminated by the Fukushima nuclear accident

Abstract

Understanding the actual situation of radiocesium (137Cs) contamination of trees caused by the Fukushima nuclear accident is essential for predicting the future contamination of wood. Particularly important is determining whether the 137Cs dynamics within forests and trees have reached apparent steady state. We conducted a monitoring survey of four major tree species (Japanese cedar, Japanese cypress, konara oak, and Japanese red pine) at multiple sites. Using a dynamic linear model, we analyzed the temporal trends in 137Cs activity concentrations in the bark (whole), outer bark, inner bark, wood (whole), sapwood, and heartwood during the 2011–2020 period. The activity concentrations were decay-corrected to September 1, 2020, to exclude the decrease due to the radioactive decay. The 137Cs concentrations in the whole and outer bark samples showed an exponential decrease in most plots but a flat trend in one plot, where 137Cs root uptake is considered to be high. The 137Cs concentration ratio (CR) of inner bark/sapwood showed a flat trend but the CR of heartwood/sapwood increased in many plots, indicating that the 137Cs dynamics reached apparent steady state within one year in the biologically active parts (inner bark and sapwood) and after several to more than 10 years in the inactive part (heartwood). The 137Cs concentration in the whole wood showed an increasing trend in six plots. In four of these plots, the increasing trend shifted to a flat or decreasing trend. Overall, the results show that the 137Cs dynamics within forests and trees have reached apparent steady state in many plots, although the amount of 137Cs root uptake in some plots is possibly still increasing 10 years after the accident. Clarifying the mechanisms and key factors determining the amount of 137Cs root uptake will be crucial for predicting wood contamination.

Radiocesium mobility in different parts of the two major tree species in Fukushima

“…However, much longer monitoring and modeling analyses in forests with the same initial deposition levels of 137 Cs have revealed that, the activity concentration of 137 Cs is higher even in the inner tree parts, such as the sapwood, of konara oak than in Japanese cedar [e.g., Ohashi et al (2022) 18 ; they analyzed samples from plots KU1-S and KU1-Q]. Moreover, the increasing trends in this activity concentration in konara oak have been estimated to continue much longer 54 .…”

“…The uptake of 137 Cs from soil to trees is often evaluated by using an aggregated transfer factor, namely, the ratio of the 137 Cs activity concentration in each tree part to the 137 Cs inventory in the soil (unit: m 2 kg −1 ) 18 , 54 , 55 . In 2015, when the impact of the initial 137 Cs deposition would have been diminished, this ratio for oak trees was similar to, or greater than, that for cedar trees 55 .…”

“…In addition to the foliar uptake of 137 Cs that occurred in the canopy in the initial phase after the accident, before the washing out of 137 Cs 13 , part of the 137 Cs in the forest soils was taken up by tree roots in competition with nutrients—mainly monovalent cations of K 14 – 16 . The internal translocation of 137 Cs in trees is complicated: not only does the 137 Cs activity concentration vary in different parts such as leaves/needles and sapwood 17 , 18 , the translocation processes to, and affinities for, each plant tissue also vary 19 – 22 . In addition, 137 Cs translocated to the external parts of trees—leaves/needles, branches, and bark, which were the main targets of this study—is partly leached by rainwater (via throughfall or stemflow) and then transported again to the forest floor in a bioavailable form 12 , 23 – 26 .…”

“…However, in previous studies conducted in the initial phase after the accident, 137 Cs cycling was considered to be less dynamic in stands of konara oak than in Japanese cedar; because the accident occurred before the flush of shoots on deciduous trees, and the larger part of 137 Cs falling on konara oak stands was deposited directly on the forest floor and more quickly fixed by clay minerals 5 , 24 . However, 137 Cs contamination of the inner parts (such as the sapwood) of konara oak is still a severe problem 18 . It is therefore important to compare and clarify 137 Cs movement in these two species.…”