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Update:January 25, 2016

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Forests and Radioactivity

The accident at the Fukushima Daiichi nuclear power station resulted in the wide range dispersal of radioactive materials in eastern Japan. The majority of the contaminated area is covered by forests, which surround residential areas, agricultural fields, and the sources of stream water. In addition, forests are critical environmental resources that provide timber, game meats, edible mushrooms, and wild plants. For the benefit of its readers, in this website, we review current research findings on the extent of forest contamination.

Dispersion and distribution of radioactive cesium in forests

Forests in eastern Japan experienced extensive radioactive contamination from the dispersal of radioactive cesium following the accident at the Fukushima Daiichi nuclear power station. According to a summary by the Ministry of the Environment, forest areas exposed to annual radiation doses of 1 mSv/year or above (as of September 18, 2013) are estimated to include approximately 430,000 hectares in Fukushima Prefecture (44% of Fukushima Prefecture’s forest area) and 360,000 hectares in the rest of the country (excluding Fukushima Prefecture).
According to aerial radiation monitoring reports, the air radiation dose rate in eastern Japan is gradually declining; however, this statement must be confirmed by field surveys to assess the actual conditions in forested areas.
In the forest areas contaminated with radioactivity at 1,000,000 Bq/m2 (1,000 kBq/m2) or above, the total volume of tree biomass is estimated to be 33,000,000 m3 (33 Mm3), and the estimated total weight of all biomass is 21,000,000 tons (21 Tg).


Behavior of radioactive cesium in forests

According to reports by the Forestry Agency and the Ministry of Education, Culture, Sports, Science and Technology, in 2011, most of the radioactive materials deposited in forests after the Fukushima accident were found on the surfaces of trees, i.e., on branches, leaves, and bark, as well as on the litter layer (just fallen and/or decomposed dead leaves and branches) on the mineral soil surface. Evergreen coniferous trees and deciduous broadleaved trees, which constitute the major tree types in Fukushima, differed in their distributions of radioactive cesium: most cesium contaminants were found in the branches and leaves and the litter layer of coniferous trees such as Japanese cedars, whereas for deciduous broadleaved trees such as Japanese oaks, the contamination was mostly in the litter layer, because the leaves of the deciduous trees had fallen at the time of the accident. According to research conducted by the Forestry Agency in 2012 and 2013, most of the radioactive cesium in the forests has already transferred from the leaves, branches, and litter layers to the surface layers of the soil through leaching, defoliation, and decomposition of the litter layers. However, forests in some areas were found to show distinctive patterns of radioactive cesium distribution, thereby suggesting that cesium distribution patterns in forests are not uniform.
The distribution of radioactive cesium in forests initially changed rapidly, whereas almost no change occurred in the total accumulation. This suggests that the radioactive cesium deposited in the forests remained in the forests and the total outflow was small.
The radioactive cesium is known to circulate within the forest. Therefore, long-term research with respect to the various forest types is required to determine the dynamic behavior of radioactive cesium in forests.


Behavior in mountain stream water

Owing to the accumulation of radioactive cesium in the forests, concerns arise about the runoff of radioactive cesium via mountain streams that originate in the forests, since people in mountainous areas often use such streams for their source of drinking water. River water and groundwater are being regularly monitored by the Ministry of the Environment.
In 2012, at six sites in Fukushima Prefecture, the Forestry and Forest Products Research Institute routinely monitored radioactive cesium concentrations in mountain streams. Overall, radioactive cesium was rarely detected (1 Bq/L detection limit) in stream water, with the exception of some samples on rainy days. In stream water samples, the amount of suspended solids—small solid particles of organic/inorganic materials—was correlated with the radioactive cesium concentration, and radioactive cesium was not detected after filtration. This suggests that suspended solids are the major carriers of radioactive cesium in mountain streams.
Similarly, some university research has shown that the proportion of radioactive cesium runoff from forests through mountain streams is small. However, since mountain streams are the main route for radioactive cesium outflow from forests, further investigation is necessary. For example, detailed analyses must be conducted regarding the content of the suspended solids, streams at lower detection limits, and the amount of annual outflow.


Wild plants and animals

In addition, to prevent the consumption of food products in which radioactive cesium concentrations exceed the regulation value of 100 Bq/kg, wild plants and animals that are typically harvested for food are being monitored by the prefectures concerned before being sold or consumed. If an item above the regulation value is found and this phenomenon is regarded as regionally representative by an examination of monitoring data, its distribution is restricted according to government instructions. As of fiscal year 2013, wild mushrooms, edible wild plants, and game meat, such as deer, are restricted from distribution in several regions (please see External link ), External link ) External link )).

Information about the radioactive cesium contamination of wild plants and animals is currently limited and thus monitoring by relevant institutions is underway.


Forest decontamination

The decontamination of forests, involving the removal of fallen leaves and pruning, is currently in progress, in accordance with laws such as the Act on Special Measures Concerning the Handling of Radioactive Pollution. Since forested areas are broadly contaminated, forests located nearby residences have been given the highest priority for decontamination. Forest areas such as campsites and bed log laying yards for mushroom cultivation, which the public frequently access, are also subject to decontamination efforts. These measures are intended to protect the health of citizens by reducing the environmental dose and radioactive cesium transfer to forest products. For the rest of the forest areas or in the interior forest, more efficient decontamination methods, such as the thinning of trees, are being evaluated on a trial basis.
Based on changes in the distribution patterns of radioactive materials and evaluations of the effectiveness of current decontamination work, more development and testing of technologies is required for the removal of radioactive materials and prevention of their dispersal.



In Japan, Fukushima Prefecture has highly productive forestry and forest product industries, so an investigation of the extent of wood contamination is essential. Based on results from forest surveys regarding the distribution of radioactive materials within forests, as conducted by the Forestry and Forest Products Research Institute, radioactive concentrations in bark and wood were found to be proportional to the air radiation dose rates in forests. Moreover, in 40-year-old cedars, red pines, and konara oak trees, concentrations of radioactive cesium in wood remain lower than those in bark.
The potential increase in the air dose rate in a wooden house built with contaminated timbers was evaluated by the Forestry Agency. If one were to be exposed to radiation through the walls of a room built of wood with a radioactivity concentration of approximately 500 Bq/kg, the additional radiation dose per person would be at least 0.012 mSv/year and the health effects of exposure at this level are considered to be minimal.

The absorption of radioactive cesium by trees via roots and tree surfaces, such as leaves and bark, and the mechanism of its migration within the trees, is not yet well understood. Therefore, comprehensive research is required to estimate future concentrations of radioactive cesium in wood and ensure the safety of wooden products.


Special forest products (mushrooms, edible wild plants, firewood, charcoal, wood pellets, and others)

For mushrooms and edible wild plants harvested for food, the regulation values for radioactive cesium were set by the government (Ministry of Health, Labour and Welfare). Food products exceeding the regulation value are restricted from distribution. Some wild mushrooms and edible wild plants (especially Acanthopanax sciadophylloides, Koshiabura in Japanese) may have higher concentrations of radioactive cesium than other edible wild mushrooms and plants, and thus, they should be harvested with caution, especially in forests with high levels of contamination.
For cultivated mushrooms, the same limit as that for wild mushrooms was set by the government and the same safety measures have been implemented. To reduce the migration of radioactive cesium from substrates to cultivated mushrooms, further research is necessary, especially regarding the development of production technologies.