Concentrations of radiocesium in branches and leaves show seasonal changes

Seasonal changes in radiocesium and potassium concentrations in current-year shoots of saplings of three tree species in Fukushima, Japan

Kenzo Tanaka (a), Satoshi Saito (b), Satoru Miura (c), Takuya Kajimoto (d), Natsuko I. Kobayashi (e), Keitaro Tanoi (e)

(a) Department of Plant Ecology, FFPRI, Tsukuba, Ibaraki, Japan.

(b) Kansai Research Center, FFPRI, Kyoto, Japan.

(c) Center for Forest Restoration and Radioecology, FFPRI, Tsukuba, Ibaraki, Japan.

(d) Tohoku Research Center, FFPRI, Morioka, Japan.

(e) The University of Tokyo, Bunkyo-ku, Tokyo, Japan.

Journal of Environmental Radioactivity, 223–224: 106409 DOI：10.1016/j.jenvrad.2020.106409( External link )

As a result of the accident at the Fukushima Dai-ichi nuclear power plant, forests over an extensive area were contaminated with radiocesium. To undertake forestry and forest management, it is important to reveal the movement of radiocesium in forest, not only in trees that were directly contaminated at the time of the accident, but also in saplings that regenerated after the accident. Although these saplings absorb radiocesium from the soil, almost nothing is known of the seasonal changes in radiocesium concentrations in the trees. Therefore, over a two-year period, we examined seasonal changes in radiocesium concentrations in three tree species: Pinus densiflora (Japanese red pine), Cryptomeria japonica (Japanese cedar), and Quercus serrata (konara oak). In all three species, concentrations were high in the spring, decreased greatly over the summer, and showed almost no changes in the fall and winter. Such changes corresponded to the faster growth of young leaves and shoots that started in early spring, which was a factor that diluted concentrations in the trees.

In addition, potassium, which is an essential nutrient for plants, also showed the same types of seasonal changes, and we could see that radiocesium moved in trees with potassium, which has similar elemental properties. By understanding seasonal changes in radiocesium concentrations and their mechanism, it will become possible to estimate such things as the movement and accumulation of radiocesium in trees and forests with a high degree of accuracy.

Photos of leaves of Cryptomeria japonica (top) and Quercus serrata (bottom) in different seasons.

The leaves are small in the spring when shoots are developing, grow dramatically through the summer, and almost completely stop growing in the fall.

Figure：Seasonal changes in radiocesium concentrations in leaves.
Radiocesium concentrations in leaves are high in the spring, decrease significantly through the summer with the growth of leaves, and are relatively stable in the fall and winter when growth has stopped. Radiocesium concentrations in current-year branch and potassium concentrations show almost the same seasonal changes. In addition, from spring to summer, the rate of radiocesium degrease is much greater in C. japonica than in Q. serrata, This may be because C. japonica grows more than Q. serrata from spring to summer and this has a high dilution effect.