Intake of Radionuclides in the Trees of Fukushima Forests 1. Field Study

Abstract

The earthquake and tsunami on 11 March 2011 led to a meltdown followed by a hydrogen explosion at the Fukushima–Daiichi nuclear power plant in Japan, causing the dispersal of abundant radionuclides into the atmosphere and ocean. The radionuclides were deposited onto trees and local residences in aerosol or gaseous forms that were partly absorbed by rain or melting snow. Here, we show that the radionuclides attached to the surfaces of trees, in which some radiocesium was incorporated into the xylem through ray cells and through symplastic pathways. The level of incorporated radiocesium varied based on tree species and age because of the ability of radiocesium to attach to the surface of the outer bark. After four years, the radiocesium level in the forest has been decreasing as it is washed out with rainwater into the sea and as it decays over time due to its half-life, but it can also be continuously recycled through leaf tissue, litter, mulch, and soil. As a result, the level of radiocesium was relatively increased in the heartwood and roots of trees at four years after the event. In private forest fields, most trees were left as afforested trees without being used for timber, although some trees were cut down. We discuss an interdisciplinary field study on the immediate effects of high radiation levels upon afforested trees in private forest fields.

Intake of Radionuclides in the Trees of Fukushima Forests 2. Study of Radiocesium Flow to Poplar Seedlings as a Model Tree

“…The radiocesium was also localized in the dead cells, which represented the artificially changed sapwood of cypress in Fukushima forest ( Figure 2B). This is in agreement with the observation [5] that the movement of radiocesium could accumulate in the heartwood of the tree stem. The radionuclide could also be further deposited in trees containing high levels of potassium.…”

“…The radiocesium was markedly deposited onto trees and local residences in an aerosol form that was partly absorbed by rain or melting snow [2][3][4]. The radionuclide moved from the surface bark to the inner xylem in tree samples obtained from September 2011 to November 2015, 6 to 51 months after the accident [5]. Through the forest tree analyses, we found that radiocesium could be incorporated into the surfaces of trees and also could migrate throughout the tissues of the entire tree.…”

“…Growing cells within the tree could be a target for the accumulation of the radionuclide, similar to that of potassium. However, its turnover in a tree body occurs via transition to the dead cells, and it also occurs in a forest field [5,6]. Nothing is known about radiocesium infiltration into trees, and very little is known about its movement in trees.…”

“…After the Hiroshima atomic bombing, radionuclides such as 14 C translocated to the tree ring of the fallout year and 90 Sr levels were insignificant from the sapwood to the heartwood [3]. However, in Fukushima forests, the xylem of two Japanese cedar trees showed radiocesium migration to the sapwood in one tree and to the heartwood in the other tree in the same field at the same time [5]. The radionuclide appears to move to the heartwood if the color of the heartwood is black, which indicates that it contains high potassium levels.…”

Intake of Radionuclides in the Trees of Fukushima Forests 5. Earthquake Could Have Caused an Increase in Xyloglucan in Trees

“…The earthquake and tsunami on 11 March 2011 led to a meltdown followed by a hydrogen explosion at the Fukushima-Daiichi nuclear power plant in Japan, causing the dispersal of abundant radionuclides into forests and local residences by gaseous or aerosol forms [1][2][3][4]. Radioiodine and radiocesium were likely incorporated and fixed in forest trees.…”