Impacts of Radioactive Contamination of Forest on Life

Hashimoto, Shoji; Komatsu, Masabumi; Miura, Satoru

doi:10.1007/978-981-16-9404-2_6

Shoji Hashimoto⁴,
Masabumi Komatsu⁵ &
Satoru Miura⁶

2403 Accesses
3 Citations

Abstract

In this chapter, we will look at how radioactive contamination itself and regulations to prevent human exposure to radiation have affected the lives of the people living there and their social and economic activities. In forests, various regulations based on the criteria described in Chap. 5 have been established to reduce internal and external exposure. The impact of these regulations on access to forests, restrictions on the use of timber, wildlife, mushrooms and wild plants, and the effects of these restrictions will be explained, as well as the measures taken.

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6.1 Effects of Increased Air Dose Rates

Air dose rates in forests behave differently from those in urban areas and agricultural lands.

Air dose rates are used as a criterion of external exposure due to activities such as living and working, and restrictions on entry and other uses are imposed based on the standard limits described in Chap. 5. In this section, the characteristics of air dose rates in forests will first be explained. This is followed by an explanation of restrictions on entry and activities set based on air dose rates, and finally, the effectiveness and limitations of forest decontamination, which is a measure to reduce air dose rates.

6.1.1 Characteristics of Air Dose Rates in Forests

First, let us look at the characteristics of air dose rates measured in forests, and how they differ from those in urban areas and flat open lands.

6.1.1.1 Air Dose Rates in a Forest Change Generally According to the Radioactive Decay of Radiocesium, But Changes in the Distribution of Radiocesium in the Forest also Have an Effect

When considering exposure due to activities in forests, external exposure due to radiation from radiocesium in the environment is the main exposure route. Therefore, it is important to know the distribution of air dose rates in the forest and their changes with time. In general, the air dose rate increases in proportion to the amount of radiocesium in the surrounding area and decreases with time due to radioactive decay of radiocesium. In the results of surveys conducted by Fukushima Prefecture since 2011 in fixed-point observations conducted at 362 forest locations, it has been observed that air dose rates decrease in accordance with radioactive decay of radiocesium (Fig. 6.1a). The same trend was also confirmed in a survey conducted by the Forestry Agency of Japan and the Forestry and Forest Products Research Institute (Fig. 6.1b).

However, Fig. 6.1b shows that the air dose rate at a height of 10 cm from the ground surface is higher than that at a height of 1 m, even at the same point. This can be attributed to the fact that a large amount of radiocesium accumulates in the soil in forests (Fig. 6.2). In addition, the values for 2012 and 2013 are slightly higher than the estimate based on the decay curve from 2011. This is probably considered to be due to the fact that in 2011, there was a relatively large amount of radiocesium attached to the tree canopies at a distance from the measurement height at 1 m above the ground, whereas in 2012 and 2013, radiocesium attached to trees had migrated to the ground.

6.1.1.2 Decrease in Air Dose Rate Due to Radioactive Decay (About Half in 3 Years)

Another thing that should be noted is the speed at which the air dose rate decreases. The amount of radiocesium decreases due to radioactive decay, but the air dose rate decreases at a faster rate than the amount of radiocesium. For example, if it is assumed that radiocesium falls on a certain place and does not move, and the air dose rate changes only due to radioactive decay, the amount of radiocesium (cesium-134 and cesium-137 ) will decrease to 65% of the initial amount in 3 years after the accident, while the air dose rate will be 52% (Fig. 6.3). This is because cesium-134 emits radiation approximately 2.7 times more intense than cesium-137 per decay. Of the radiocesium emitted in the ratio of 1:1 in the Fukushima nuclear accident, cesium-134, which has a stronger impact on air dose rates, decreases faster, resulting in a faster decrease in air dose rates than the amount of radiocesium. This tendency to decrease air dose rates due to radioactive decay is a common phenomenon not only for forests but also for radiocesium in the environment. However, it is important information when considering external exposure to radiation from forests.

6.1.1.3 Spatial Distribution of Air Dose Rates in Forests Is Uneven

The spatial distribution of radiocesium is non-uniform and varies with time, which in turn affects the air dose rate. For example, air dose rates may be higher at forest edges (forest ends bordering farmland and residential areas) or in areas where sediment tends to accumulate due to the movement of topsoil. In fact, forest edges are known to have higher air dose rates due to their location where airborne radiocesium is easily trapped. Figure 6.4 shows the results of a spatial survey in a forest conducted by the Forestry and Forest Products Research Institute, showing that air dose rates are distributed unevenly in a small forest [113]_.

6.1.1.4 Air Dose Rates in Forests Are Higher than in Nearby Residential Areas

It is known that a very small percentage of the radiocesium that falls on forests flows out of them (Sect. 3.6). Also, most of the radiocesium in the soil stays in the surface layer. Since the distribution of radiocesium does not change and the amount of radiocesium flowing outside is small, the air dose rate in forests can be considered to generally decrease in accordance with radioactive decay of radiocesium over time. The air dose rates of forests with such characteristics were compared with those of other land uses (Fig. 6.5). Comparing the results of measurements on roads conducted by car-borne surveys and fixed-point measurements conducted on open flat land, it was found that the air dose rates in forests did not decrease easily over time [114]. This is due to the fact that air dose rates for land use other than forests are characterized by a faster decrease in air dose rates than the decrease due to radioactive decay of radiocesium. Because radiocesium tends to flow along roads, and even on flat land, radiocesium migrates in the direction of deeper soil and decontamination works reduce the amount of radiocesium.

6.1.2 Access Control Based on Air Dose Rates

6.1.2.1 Designation of Areas Under Evacuation Orders and Their Changes

The designation of evacuation order zones based on air dose rates after the Fukushima nuclear accident had a strong impact on the lives and activities of residents. To protect residents from radiation exposure immediately after the accident, the area 20 km around the plant was first designated as an evacuation order zone (warning zone), and the area to the northwest, which was highly contaminated, was designated as a planned evacuation zone, where entry was restricted (prohibited) (Fig. 6.6). Subsequently, on April 1, 2012, the evacuation zones were reorganized into three areas with reference to the annual exposure dose of 20 mSv (3.8 μSv/h, Sect. 5.4) as recommended by the ICRP to facilitate the return of residents and the rehabilitation and reconstruction of the region. First, the areas where the annual dose is certain to fall below 20 mSv were designated as preparation areas for lifting the evacuation order, with the aim of lifting the evacuation order as soon as possible. The areas with annual exposure doses of over 20 mSv were designated as restricted residential areas, where people were prohibited from living (or staying), while decontamination and other restoration work was carried out to rebuild the infrastructure of daily life. Areas where the annual exposure dose exceeded 50 mSv and where the annual exposure dose was expected to exceed 20 mSv even after 5 years were designated as difficult-to-return areas where entry by the public was prohibited in principle. Thereafter, as the air dose rate decreased due to decontamination and the radioactive decay, the evacuation order was lifted mainly in the preparation areas for lifting the evacuation orders and the restricted area, and the area of the evacuation order became smaller [116].

However, only some of the difficult-to-return areas with high radiation doses have been lifted, and as of March 2020, evacuation order zones have been established across seven municipalities. Forests in the difficult-to-return areas are inaccessible as well as residential areas. Although the government is promoting a plan for the further lifting of the evacuation order, according to the results of airborne monitoring, there are still places where the air dose rate exceeds 10 μSv/h. To meet the criteria for the lifting of the evacuation order, it is necessary to wait for a decrease in the radiation dose rate due to radioactive decay or to take measures such as decontamination to further decrease the radiation dose rate.

6.1.2.2 The Limit of Air Dose Rate for Forestry Activities is 2.5 μSv Per Hour or Less

Regulations are also in place for activities in the forest. Work in forests is also managed in accordance with the Ionizing Radiation Ordinance for Decontamination , as explained in Sect. 5.4 (See footnote in 5.4.1). In other words, when working in forests where the air dose rate exceeds 2.5 μSv/h, it is necessary to control exposure doses in compliance with the “Guidelines on Prevention of Radiation Hazards for Workers Engaged in Works under a Designated Dose Rate”.^{Footnote 1} Furthermore, in the case of work involving soil , if the radiocesium concentration in the soil at the work site exceeds 10,000 Bq/kg, the work is classified as “Works for Handling Designated Contaminated Soil and Waste” and the “Guidelines on Prevention of Radiation Hazards for Workers Engaged in Decontamination Works” are applied. Soil handling work includes not only decontamination work but also tree planting and nursery (e.g. thinning) works (Table 6.1).

Table 6.1 Classification of work in contaminated forests

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However, in practice, the principle is to reduce exposure doses as much as possible and have workers work under an air dose rate (2.5 μSv/h or less) that does not require dose control. Therefore, except for highly urgent work such as restoration in areas affected by natural disaster such as earthquake, works in forests in areas where the air dose rate exceeds 2.5 μSv/h are refrained. In addition, forestry activities had not been allowed in the evacuation order zones due to the access control. As a result, as of August 2012, forestry activities were restricted in 13% of the forest area in Fukushima Prefecture (130,000 hectares) [118].

6.1.2.3 There Are No Restrictions on Temporary Entry into the Forest

On the other hand, there are no restrictions on entry into forests for recreational purposes, except for entry into evacuation order zones. This is related to the fact that the external exposure dose is calculated by multiplying the air dose rate by the time spent in the forest (Fig. 5.2). Because entry into forests for leisure is for a short time compared to work, and external exposure is considered to be small.

The Ministry of the Environment of Japan has estimated the external exposure dose in the case of leisure activities in Fukushima Prefecture (Ministry of the Environment, Environmental Remediation Website, “June 15, 2015, Committee on Environmental Remediation (15th Meeting) Document 4” [108]). The results of the calculations for each age and region show that the annual exposure dose will be limited to 0.06 mSv at most. Even when forests are used for recreation, the annual exposure dose is not expected to exceed 1 mSv, which is the upper target value of annual exposure dose for the public.

6.1.3 Forest Decontamination

6.1.3.1 Removing the Organic Layer Reduces the Air Dose Rate

In the farmland after the accident, a wide range of countermeasures against contamination has been taken, including removal of surface soil and inversion tillage (replacing topsoil with subsoil) to reduce radiocesium in the surface layer that is related to absorption by crops, and potassium fertilization to reduce the transfer of radiocesium to crops [119]. Agricultural land is generally flat and spread out over a wide area, which makes it possible to perform efficient work using agricultural machinery. On the other hand, in forests in Japan where the slopes are steeper and more undulating than in farmland, and trees and their roots are irregularly distributed, countermeasures are more limited. In forests with many technical limitations, the removal of the soil surface organic layer (so-called ‘forest decontamination’) became the main technical measure (Fig. 6.7).

Decontamination of forests is carried out by gathering and transporting contaminated organic layer out of the forests. As shown in Chap. 3, monitoring surveys conducted after the accident showed that much of the radiocesium in the forest transferred to the organic layer and surface mineral soil layer in the years following the accident. Therefore, the decontamination of forests involves the removal of the organic layer (fallen leaves and branches some of which are coarsely decomposed) of the forest. However, the forest decontamination did not cover the entire vast forest area, but was limited to the area within 20 m from the edge of the forest bordering residential areas, roads, and other living areas to reduce radiation exposure to the living environment (Ministry of the Environment, Environmental Remediation, “About Forest Decontamination, etc.”) [120].

6.1.3.2 The Effective Range of Forest Decontamination is 20 m

This section describes a test in which the range of forest decontamination was determined to be 20 m [121]. The test was conducted in a coniferous forest and a broadleaf forest in Koriyama City, Fukushima in September 2011. A 20 m × 20 m test site was set up in the middle of a forest slope in each type of forest, and the changes in air dose rates were measured while gradually expanding the decontamination area from the center. Figure 6.8 shows the spatial distribution of the change ratio in air dose rates in the test site after decontamination of 12 m × 12 m and 20 m × 20 m. As the decontamination area was expanded, the area where air dose rates decreased also expanded. After the decontamination of 20 m × 20 m, air dose rates at the central point decreased to about 70% and 60% of those before decontamination in the coniferous and broadleaf forests, respectively. However, even when the decontamination area was extended from 12 m, the air dose rate in the central area did not decrease further. Thus, while forest decontamination is effective in lowering air dose rates, the effect reaches a ceiling as the decontamination area expands. In addition, since decontamination generates a large amount of waste , the decontamination area at a forest edge was set at 20 m to achieve a balance between effectiveness and cost .

After that, decontamination of forests was carried out in satoyama forests and other areas where people enter on a daily basis, and in some areas, a model project was carried out by the Ministries of the Government and Fukushima Prefecture to test surface decontamination (Satoyama Restoration Model Project) [122]. The target areas were the sites for bed-log cultivation of shiitake mushrooms, campsites, and walking trails. In the test sites that were flat and had no risk of soil runoff, scraping of the topsoil was also conducted, resulting in a reduction in air dose rates of up to 50% or more.

In addition to decontamination, tests have been conducted to reduce the air dose rate by spreading wood chips and to reduce the transfer of radiocesium from soil to trees by applying potassium fertilizer. In addition, to utilize the high cesium-absorbing capacity of fungi for decontamination, tests have been conducted to absorb radiocesium by mycelium infected in the laid chips [30]. However, this has only been done on a trial basis and not on a large scale.

As described above, removal of the organic layer has been an effective method for reducing air dose rates in forests. However, caution must be exercised when decontamination should be conducted. As described in Chap. 3, a large amount of radiocesium, which was contained in the organic layer immediately after the accident, has shifted to the surface layer of mineral soil over time. Since the purpose of decontamination is to remove as much radiocesium as possible, it will only be possible to do so effectively for a few years up to 10 years after the accident.

6.1.3.3 Does Cutting Down Forest Trees Reduce Air Dose Rates?

It was explained that the removal of the organic layer is effective in reducing the air dose rate in forests. Subsequently, changes in air dose rates in the forest were also investigated by combining forest management practices such as clear-cutting (cutting all trees) and thinning (partial cutting), in addition to removal of the organic layer. As a result, although air dose rates decreased in the thinned area, they also decreased in the control area without thinning, and the effect of thinning on air dose rates was not clear (Fig. 6.9). In the case where litter removal was combined with thinning or clear-cutting, there was no significant difference from the values in the test area where only litter removal was conducted. The fact that the effect of the treatment was not clearly observed can be considered to be due to the fact that most of the radiocesium had already migrated to the ground surface in the winter of 2012. On the other hand, the effect of litter removal was found to be long-lasting.

6.1.3.4 Is It Realistic to Decontaminate All Forests?

Decontamination was carried out in residential areas and farmland, prioritizing areas where people spend a lot of time as living areas. Forest decontamination was carried out within a 20-meter width area adjacent to the living area from the forest edge, but the purpose was only to reduce the air dose rate in the living area. Now that most of the plans for decontamination of residential areas and farmland have been completed, forests might be candidates for new decontamination. However, it is not realistic to decontaminate all the forests. The area of forests is huge and the topography is complex, so much of the work needs to be done by human power. Decontamination also generates a huge amount of waste , which results in huge costs for storage. Yasutaka et al. [124] estimated the cost of decontamination in Fukushima Prefecture, including the storage of the decontaminated soil. As a result, the cost of decontaminating 20 m from the forest edge was estimated to be 23–46 billion US dollars, while the cost of decontaminating the entire forest would be over 145 billion US dollars. In addition, as a result of estimating the impact of decontamination on the external exposure of residents, it was calculated that even if the decontamination area was expanded to include all forests, the effect on reducing external exposure would be very small. As well as the enormous costs involved, from the standpoint of cost-effectiveness in that the return from effort is not deemed rewarding, it is considered difficult to actively implement forest decontamination. Furthermore, decontamination of the entire forest is not feasible due to the increased risk of topsoil runoff caused by the loss of understory vegetation and soil surface organic layer and the additional exposure of workers due to the required extensive decontamination. We believe that decontamination undertaken to date has given a certain sense of security to local residents. However, it is also important to explain the limitations of the effect of decontamination on a huge area for reducing radiation exposure to residents (Fig. 5.1).

The concept of forest decontamination is also discussed in Chap. 7.

6.2 Wood-Related Regulations and Their Impact

The index values of regulations differ among wood products depending on the type and use.

In wood-related regulations, the term “index value” is used in this book to distinguish the regulations for wood products from those based on the criteria described in Chap. 5. The index values are recommended by a notice from the department in charge of the Forestry Agency and are not regulated by law unlike the criteria (“standard limit”) for food, which means there is slightly more flexible, but still substantial regulation by the Government of Japan. The “provisional permissible values” set by the Ministry of Agriculture, Forestry and Fisheries is also the same type of regulations which provides a notice from the department in charge of the ministry. See also Sect. 5.4 for “standard limit”, which is a similar term.

6.2.1 Regulations Related to Wood

6.2.1.1 External Exposure from Living in Wooden Houses Is Negligible

Wood from trees can be used for a variety of purposes. The first use that comes to mind would be for housing. Since external exposure is determined by multiplying the air dose rate by the time spent on that spot, the effects of wood building materials used in living spaces have attracted attention. However, no index values have been set for the wood building materials from the time of the accident to the present. This is because even if the radiocesium concentration in the wood materials is high, the amount is much smaller than that of soil in the surrounding area, and the effect of the wood building materials on air dose rates is small. When living in a house made of wood obtained from where access is currently permitted, the annual exposure dose from wood has been estimated as 0.04 mSv at most [125] (Fig. 6.10, the Forestry Agency, “Approximate Calculation of Exposure in an Occupied Room Surrounded by Timber, IAEA-TECDOC-1376”). In addition to these results, the Fukushima Prefectural Timber Cooperative Associations has voluntarily conducted surface dosimetry of wood (inspection of radiation levels detected on the surface of wood). In 2018, the maximum value was reported to be 44 cpm (equivalent to 0.001 μSv/h in terms of air dose rate) [126].

6.2.1.2 Disposal of High Concentrations of Bark Is a Problem

Although there was no problem in using lumber from the affected areas as building materials, the disposal of the bark generated during the lumbering process became a problem. Before the accident, the bark was generally used as compost or bedding material for livestock. However, the radiocesium concentration on the bark is higher than that of the wood because it was directly contaminated with radiocesium during fallout (Fig. 3.3 and Fig. 3.8). Depending on the conditions, there was a possibility of exceeding the criterion (8000 Bq/kg, Sect. 5.4) that must be properly disposed of as designated waste. Therefore, based on the relationship between the concentration of bark and the air dose rate, Fukushima Prefecture set 0.5 μSv/h as the index for forests that can be logged without inspections. Then, for logs felled from forests with air dose rates exceeding 0.5 μSv/h, the inspection of the radiocesium concentration of bark was required [127]. As of November 2014, 90% of the privately owned forests in the Fukushima Prefecture were below the index value of 0.5 μSv/h and could be logged without inspections [128].

6.2.1.3 Criteria (Index Values) for Firewood, Chips, and Charcoal

On the other hand, various regulations have been established for the use of trees for purposes other than building materials (Table 6.2). In the case of logs used for mushroom cultivation (bed-logs), to ensure that mushrooms do not exceed the standard limit for food of 100 Bq/kg, index values of 50 Bq/kg and 200 Bq/kg have been set for bed-logs and sawdust medium (a mixture of wood sawdust and nutrients such as rice bran), respectively [129] (to be explained in detail in Sect. 6.5). In the case of firewood, charcoal, pellets, and other combustion materials, the index values were set as ash generated after combustion not to exceed 8000 Bq/kg, which can be disposed of as general waste. When comparing firewood and charcoal, the index values are 40 Bq/kg and 280 Bq/kg, respectively, with firewood having a stricter limit. This is because the change in weight of firewood is greater when it is burned (more radiocesium is concentrated per weight). The Ministry of the Environment of Japan has estimated that the exposure dose from the use of firewood is low. According to the calculations, the annual exposure doses for children using wood stoves and baths boiled with firewood with combustion ashes of 8000 Bq/kg were 5.8 μSv and 5.0 μSv, respectively [130]. With regard to fertilizers and livestock bedding containing bark compost made by fermenting tree bark, an index value (a provisional permissible value) of 400 Bq/kg has been set as a criterion that will not exceed 100 Bq/kg, which is within the range of variation of past radiocesium concentrations in farmland soil, even after 40 years of continuous application [131]. Although there is no index value for wood chips, business or public entities that handle chips as fuel often set the acceptance criterion at 40 Bq/kg in accordance with the index for firewood. On the other hand, in the case of chips used for paper manufacturing, many companies set the index value at 400 Bq/kg, the same as for compost and bedding. In this way, the index values for the use of wood and its by-products are based on the respective exposure dose estimates, and different values are set depending on the subject. If the regulations for use are adhered to, it will be possible to reduce the additional exposure of users.

Table 6.2 Current index values for mushroom logs, firewood, charcoal, pellets, etc.

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6.2.2 The Impact on Forestry from Statistics

In this way, as a countermeasure to the radioactive contamination of the forests caused by the Fukushima nuclear accident, access to the forests and the use of timber were restricted. Let’s take a look at the impact that the forestry has suffered as a result through statistics. When looking at the impact of the accident on the industry, it is necessary not only to look at the changes before and after the accident, but also to compare them with changes in the rest of the country, and to examine whether the changes are a common phenomenon across the country or a unique phenomenon that occurred in the prefectures affected by the disaster (or for other reasons). The following is a list of changes that were observed before and after the accident.

First of all, looking at material production of logs (harvest volume of logs), the supply of domestic timber had originally been on a downward trend as foreign imports had been increased, but the national average bottomed out in 2002 and has been on an upward trend even after the accident. On the other hand, production of logs in Fukushima Prefecture declined after the accident and has been recovering, but the recovery has been slow, and the ratio of 2015 to 2010 is 104%, lower than the national average (117%) and results of neighboring prefectures (114–121%) [118]. Looking at the demand and production of logs in the prefecture by region in Fukushima Prefecture, a significant change was observed. Both supply and demand declined significantly in the Soso region (a coastal area consisting of 12 cities and towns located around the Fukushima Daiichi Nuclear Power Plant, including Soma City and Futaba Town), while demand increased in central region extending from north to south and Iwaki (south coastal) region, and supply increased in Ken-Nan (southern-central) and Aizu (north-west) region (Fig. 6.11a). As a result, demand exceeded supply in the prefecture as a whole [118]. The Soso region includes many municipalities that have been designated as difficult-to-return areas, such as Futaba Town and Okuma Town, where the Fukushima Daiichi Nuclear Power Plant is located. It is expected that the industries in these areas will be depressed in a wide range from before the accident. It is thought that the restriction due to the high air dose rates have reduced forestry activities, leading to a decline in the production of logs.

In addition, hardwood (broadleaf) production declined while softwood (conifer) production increased in Fukushima Prefecture, and the balance of supply between hardwood and softwood has changed significantly (Fig. 6.11b). The decline in hardwood can be attributed to lower production of fuel chips and logs for mushroom cultivation. The radiocesium contamination of hardwood for bed-log cultivation is a major problem not only in Fukushima Prefecture but also in neighboring prefectures. The details will be described in Sect. 6.5.

Looking at the area of forest maintenance in Fukushima Prefecture, it has been reduced by half compared to the area before the accident [125] (Fig. 6.11c). The purpose of forest maintenance is to bring out the various functions of forests through activities such as planting, clearing, thinning, and maintenance of forest roads. In the short term, it is difficult to see the effects of stagnation in forest maintenance, but in the long term, there is concern that forest functions will be degraded, resulting in a decline in the quality of wood and carbon sequestration capacity, as well as an increase in the risk of disasters during heavy rainfall (e.g. landslides), and other effects on the multiple functions of forests.

6.2.3 Utilization of Contaminated Forests

Some experimental ideas have been tested on how to resume the use of forests that had stopped due to high levels of radiocesium. However, except for the common method of treating combustible waste , which is to burn it, reduce its volume, and store it in a storage facility, there is no practical method that has been implemented on a large scale at this point.

6.2.3.1 Volume Reduction of Contaminated Wastes

The highly concentrated radioactive byproducts (unused parts) of logging in contaminated areas and the organic matter yield from forest decontamination have turned into waste. The waste shall be transported and stored to a storage facility, which is costly, so the volume needs to be reduced for efficiency. In forests, the use of wood crushers to reduce the volume of waste was considered. The volume reduction rate using wood crushers has been reported to be 45–63%. To further reduce the volume of combustible wastes including fallen leaves and branches from decontamination, a temporary incineration facility was built in Fukushima Prefecture. Tests with the high-temperature incineration facility showed that the volume reduction rate was very high (96–99%) and that the transfer of radiocesium to the exhaust air was very low (up to 0.3 Bq/m³, test in Okuma Town) [133]. However, since radiocesium concentrates in combustion ash at high concentrations (up to two million Bq/kg in Okuma Town), special attention is required for subsequent management.

6.2.3.2 Conversion to Energy Use and to Other Uses

Instead of using wood from contaminated areas for construction or for mushroom cultivation, it can be used as biofuel (e.g. pellets) for biomass power generation. The index value for fuel wood and pellets is set at 40 Bq/kg, which is stricter than 50 Bq/kg for mushroom logs (Table 6.2). However, the radiocesium concentration in tree trunks is highest in the bark (Sect. 3.3), and if the bark is removed to make chips, the radiocesium concentration in the chips can be lowered, so there are cases where materials that cannot be used as logs can be used as chips. However, it is necessary to take measures to ensure that the combustion ash does not exceed the criterion of 8000 Bq/kg for designated waste, and to be careful about the release of radiocesium into the environment during combustion, since cesium has the property of vaporizing at high temperatures. On the other hand, volume reduction while utilizing energy from bark and wood containing radiocesium can be considered as an option by using biomass power generation facilities on the premise that the combustion ash will become designated waste. However, as mentioned in the section on volume reduction, it is necessary to take appropriate measures for exhausting radiocesium, and to fully explain to local residents and obtain their understanding when implementing such measures.

For other uses, Otsuka et al. [134] developed a technology for methane fermentation of woody biomass, which had been technically difficult. It was found that methane gas does not contain radiocesium and most of it remains in the fermentation residue. Since this technology has two advantages: energy production from wood and volume reduction of contaminated biomass, it is expected to be utilized in contaminated areas (Fig. 6.12).

6.3 Radioactive Contamination of Wildlife

Radiocesium levels in large wildlife are high in a wide area, complicating the problem of recent population growth.

Large wildlife such as wild boar, bear, and deer are deeply involved in the lives of people in mountain villages (Fig. 6.13). They are harmful animals that appear in the living area and cause damage to fields and residents. On the other hand, such wild animals and birds are hunted as game animals and their meat is consumed as “gibier (game meat)”. The radiocesium concentration in the muscles of large wildlife has been high since the accident. Based on the results of inspections, restrictions on shipping and intake have been imposed in a wide area. In 2020, a total of 10 prefectures have set restrictions on shipping of the three large wildlife species mentioned above. In addition, 20 municipalities in Fukushima Prefecture have imposed intake restrictions of wild boars (Table 6.3). However, some prefectures and municipalities have taken measures to “partially lift” the restrictions, allowing shipments after establishing safety confirmation schemes such as testing all animals slaughtered (all wild meat taken in the region is inspected before shipping).

Table 6.3 Prefectures where restrictions on shipping of meat from wild animals have been imposed (as of November 16, 2020)

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6.3.1 Large Wildlife Populations Are Increasing Across the Country

These shipping restrictions have brought new problems in wildlife management. The distribution of large mammals such as wild boar and deer in Japan has been declining since the Edo period (1603–1868) due to habitat changes caused by hunting and land development [136]. However, since the latter half of the twentieth century, the distribution range of large wildlife has expanded rapidly and their populations have increased (Fig. 6.14). The increase in the population is thought to be influenced by various factors such as conservation policies of the wildlife, the extinction of wolves as predators, the decrease in the number of hunters due to aging, the increase in abandoned land due to the depopulation of mountain villages, and warmer winters due to global warming [137]. In particular, in the Abukuma Highlands (between Hamadori and Nakadori) near the Fukushima nuclear power plant, there are no deer and the distribution of bears is limited, but wild boars have established themselves over a wide area, and crop damage has become a problem. In eastern Japan, the Fukushima nuclear accident led to a decline in willingness to hunt contaminated wildlife and a decrease in the population leading to an increase in abandoned land, resulting in a marked increase in the number of wildlife [138]. In a questionnaire survey of hunters, the number of people who stopped hunting is significantly higher in Hamadori and Nakadori (Eastern coastal and central areas of Fukushima Prefecture, respectively) than in other areas, indicating that radioactive contamination has a strong effect on the lowering of the motivation of hunters [140].

6.3.2 Trend of Radiocesium Concentration

The trend of the radiocesium concentration in wildlife since the accident is important to know for assessing the human internal exposure due to ingestion and to consider the future prospects of shipment restrictions. Therefore, the radiocesium concentration in the muscles of hunted wildlife is continuously being monitored through hunting and capturing for inspections. Wild animals are sampled in different locations at different times of the year, and the higher the amount of radiocesium in their habitat, the higher the radiocesium concentration in the muscle. Therefore, instead of simply comparing the radiocesium concentration in muscles, it is effective to compare the radiocesium concentration with the aggregated transfer factor (unit: m²/kg) (Fig. 3.9), which is calculated by dividing the radiocesium concentration by the amount of radiocesium accumulated in the soil at the capture point (amount of radiocesium per unit area, unit: Bq/m²). As a result of the comparison, it was shown that the radiocesium concentration in wild boars and black bears in Fukushima Prefecture tended to decrease with the passage of years [141]. On the other hand, clear temporal trend in radiocesium concentration in Japanese deer has not been observed in the data up to 2015.

Seasonal variation of radiocesium concentrations in wildlife has also been studied. In Europe, after the Chernobyl nuclear accident, there have been many publications on seasonal variations in the radiocesium concentration in wildlife. A study of wild boars and black bears in Fukushima Prefecture also revealed seasonal variations in the radiocesium concentration in muscle meat. As shown in Fig. 6.15, the radiocesium concentration in both wild boars and black bears was lower in spring and summer, and higher in autumn and winter [142]. According to European studies, the radiocesium concentration in wild boars is revealed to be higher in summer and lower in winter, and it is said that the fact that they eat a lot of high-concentration mushrooms called “deer truffles” (Elaphomyces granulatus) in summer has an effect on the seasonal increase in concentration [143]. On the other hand, Japanese wild boars are believed to be omnivores, and there are no reports of them foraging for mushrooms. In addition, wild boars feed mainly on plant underground stems during the winter [144], and it is thought that they take in high concentrations of surface soil with them, but the detailed mechanism is not clear.

6.3.3 Countermeasures: Testing All Animals Slaughtered and Population Control

The increase in the population of wildlife caused by the complex factors as mentioned above has been accelerated by radioactive contamination in eastern Japan. Now the complexity of wildlife issues becomes very difficult to take countermeasures. To reduce wildlife damage to crops and forests, we must not only control the population by hunting, but also develop different measures according to the degree of contamination in the area, as the number of hunters is decreasing.

After the Fukushima nuclear accident, restrictions on the shipping meat of wildlife have been extensively imposed in a wide area of eastern Japan, Fukushima and surrounding prefectures (Table 6.3). However, the level of contamination decreases as the distance from the nuclear power plant increases. Furthermore, depending on the species, it is expected that the radiocesium concentration will decrease over time, and the percentage of wild meat with radiocesium concentration lower than the standard limit for food will increase. Therefore, in areas where meat of wildlife were originally used for food , there was a movement to ensure safety by conducting inspections of all slaughtered individuals and preventing the shipping of meat that exceeded the standard limit. In fact, wild boar meat processing facilities in Nakagawa Town, Tochigi Prefecture, and Ishioka City, Ibaraki Prefecture, have adopted a system to test the radiocesium concentration in specific parts (e.g. thigh meat) of all slaughtered animals and ship wild boar meat only if it is below the standard limit [145].

On the other hand, in areas where forests are highly contaminated and the consumption of wildlife is restricted, the meats of most large wildlife may exceed the standard limit of 100 Bq/kg, making it difficult to use them for food. Therefore, to control damage by wildlife, it is necessary to manage the population through active extermination with the premise of disposal. For example, in Fukushima Prefecture, a management plan has been implemented to adjust the wild boar population from an estimated 49,000 in 2014 to 5200, taking into account the balance between maintaining the population and reducing agricultural damage caused by the wildlife [138]. If active disposal is to be undertaken, the method of disposal also needs to be considered. At present, in many cases, the bodies are buried where they were captured, and some are disposed of in existing incinerators. Therefore, the burden on hunters and local residents should be considered. Under these circumstances, there is a movement to develop special incinerators or biological treatment facilities using microorganisms with the help of subsidies [146].

6.4 Radiocesium Contamination of Wild Mushrooms and Wild Plants

The impact on leisure activities in the forest was probably greater than the impact on the forestry.

6.4.1 The Value of the Forests’ Bounty to Local Communities

One of the blessings of the forest is edible wild mushrooms. Most of the mushrooms that we see in grocery stores are grown on medium in factories or cultivated in villages using logs cut from the forests (these are called cultivated mushrooms and will be discussed in Sect. 6.5). Wild mushrooms differ from such artificially cultivated mushrooms in that they are found growing wild in the forest. In Japan, there is a culture of collecting and eating a variety of wild mushrooms. One of the most common mushroom species in Japan is matsutake (Tricholoma matsutake), which cannot be cultivated artificially. Wild mushrooms have a different flavor from factory-grown mushrooms, and the fun of searching for mushrooms in forests has led to widespread mushroom hunting in the mountainous areas.

Another great blessing of the forest is edible wild plants, which are called “Sansai” in Japanese. It is difficult to clearly define what wild plants are and how they differ from vegetables. The Japan Special Forest Products Promotion Association [147] defines wild plants as edible plants that grow naturally in the forests and fields of Japan and have been eaten for a long time. In general, the vegetables differ from wild plants in that they have been bred for cultivation over a long period of time. However, in recent years, some of the wild plants, such as water dropwort (Oenanthe javanica), Japanese honeywort (Cryptotaenia canadensis subsp. japonica), Japanese butterbur (Petasites japonicus), Japanese spikenard (Aralia cordata), and taranome (Aralia elata), have been developed for forcing cultivation. Although the boundary between wild plants and cultivated vegetables is not clear, most wild plants are not suitable for mass production. In addition, Saito [148, 149] mention that the characteristics of wild plants are that different species and parts are used (or recognized) in each region, that many of them have a unique taste and bitterness, that they require a lot of work to be eaten, such as removing harshness, and that many of them have few calories and are not used for “subsistence”.

Wild mushrooms and wild plants with these characteristics are treated as valuable foodstuffs that enrich the lives of mountain village communities (Fig. 6.16). Not only are they used as seasonal ingredients, but they are also processed as preserved foods and used as offerings for special occasions (called “hare ” in Japanese) such as Bon Festival (a Japanese Buddhist custom in August) and New Year’s Day. Mushrooms and wild plants also serve as a communication tool for the community. Collecting wild mushrooms and plants is also a popular leisure activity for local residents. It is difficult to evaluate the economic value of the collected wild mushrooms and wild plants since they are usually consumed at home. However, Matsuura et al. [150] estimated the economic value of wild mushrooms and plants collected annually in a town of Fukushima Prefecture before the accident to be in the tens of millions of yen (hundreds of thousands of US dollars). It has been shown that the activity of collecting these wild forest foods is more frequent in an area with more days of snowfall [151]. Therefore, wild mushrooms and wild plants have been familiar in the deep snow areas of Fukushima Prefecture. The radioactive contamination of forests caused by the Fukushima nuclear accident may reduce people’s motivation to collect them regardless of whether the bounty of the forest is restricted or not and consequently degrade the vitality of the inhabitants in the mountainous areas.

6.4.2 Radioactive Contamination of Wild Mushrooms

Since the Fukushima nuclear accident, wild mushrooms and wild plants have been subject to shipping restrictions in a wide area. For example, looking at the test results for agricultural products, most of the foods that exceed the standard limit of 100 Bq/kg are wild plants in spring and wild mushrooms in autumn (92% of foods exceeded, data since 2014) (Fig. 6.17). The reason for the high radiocesium concentration in wild mushrooms and plants can be attributed to the fact that radiocesium accumulate in the forest due to the low flowing rate of radiocesium flowing out of the forest ecosystem and the lack of decontamination of the forest, and the fact that wild mushrooms and plants are high in minerals and have the ability to absorb radiocesium efficiently. The circulated quantity of wild food obtained from forests is much smaller than that of cultivated food, so the economic impact of shipping restrictions is considered to be relatively small. However, the role played by the blessings of the forest in mountain villages is not small. Clarifying the levels of radiocesium in wild mushrooms and wild plants is important for people to live in the areas affected by the accident.

6.4.2.1 Species-Independent Batch Restrictions of Shipping

As a result of inspection after the Fukushima nuclear accident, wild mushrooms were found to often exceed the standard limit in a wide area of the eastern Japan, and as of November 2020, 117 municipalities in 11 prefectures have imposed shipping restrictions (including 15 municipalities in 3 prefectures where restrictions were lifted for some species) (Fig. 6.18). It is also said that there are 4000–5000 species of wild mushrooms, and hence it is difficult to identify the species and the concentration characteristics of each species, unlike other agricultural crops. Therefore, shipping restrictions are implemented for all wild mushroom species in a lump, not for individual species.

6.4.2.2 Analysis of Wild Mushrooms Using the Results of Food Monitoring

As a result of compiling research on wild mushrooms conducted mainly in Europe after the Chernobyl nuclear accident, it was found that the trends of radiocesium concentration differed by species and genus. If the characteristics of radiocesium concentration in wild mushrooms in Japan can be classified according to species, it will be helpful in reviewing shipping restrictions and helping local people make decisions of collecting wild mushrooms. However, it is not easy to continuously sample specific species, because the amount, timing, and location of their occurrence are not stable from year to year. In addition, to study the trend of radiocesium in wild mushrooms in Japan as a whole, it is necessary to collect data from multiple locations, which limits the ability of researchers to conduct surveys alone.

Therefore, Komatsu et al. [154] focused on the data of radiation monitoring of foods by the government. The Ministry of Health, Labour and Welfare (MHLW ) of Japan compiles and posts the results on its website [152]. From the data reported from August 2011 to November 2017, measurement data of 3189 edible wild mushroom specimens of 107 species were obtained from 246 municipalities. In addition, the radiocesium concentration of mushrooms is considered to be affected by the contamination level at the point of occurrence. Therefore, Komatsu et al. analyzed the total amount (including both in soils and in plants) of radiocesium deposition per unit area (Bq/m²) by airborne monitoring as an indicator of the contamination level, and the concentration characteristics of each species and region were analyzed.

Komatsu et al.’s analysis assumes that the radiocesium concentration varies depending on the species, municipality, and date of collection. Furthermore, there are variations (errors) in concentration that cannot be explained by these conditions. The concentration characteristics of each species were expressed in terms of a numerical value called “normalized concentration”. The results are summarized in Fig. 6.19. In this figure, two types of fungi are shown: mycorrhizal fungi, which live in symbiosis with trees, and saprotrophic fungi, which obtain nutrients by decomposing dead wood and leaves.

6.4.2.3 The Radiocesium Concentration in Wild Mushrooms Varies Greatly Among Species

Comparing the concentration characteristics, there was a general trend for mycorrhizal fungi to have higher radiocesium concentrations than saprotrophic fungi, but this varied greatly depending on the species. Some saprotrophic fungi such as a scalycap mushroom (Pholiota lubrica) also had higher concentrations. In fact, the scalycap mushroom is a species of which radiocesium concentrations exceeded the standard limit in areas even far from the Fukushima Daiichi Nuclear Power Plant such as Nagano Prefecture (250 km from the power plant). It is thought that the physiological and ecological characteristics of mushroom species would decide the radiocesium concentration, but the mechanism is not clear at this stage.

Results indicating that the radiocesium concentrations in wild mushrooms would be different according to species, are helpful in considering the framework of shipping restrictions of wild mushroom, which are currently implemented in a species-independent manner. However, some caution should be exercised in interpreting the results. There are three points to be aware of: (1) there are still large uncertainties (variations) in the prediction of radiocesium concentration in wild mushrooms, (2) the trend of annual changes is not fully understood, and (3) there are also variations in the radiocesium concentration in mushrooms collected in the same municipality. It is necessary to conduct more detailed surveys in the future to clarify the effects of species and regions on radiocesium concentration and its temporal trends.

6.4.3 Radioactive Contamination of Wild Plants

6.4.3.1 Differences in Restricted Areas of Shipping by Species and Growing Conditions

Table 6.4 shows the number of municipalities with shipping restrictions for each type of wild plants (as of November 16, 2020). Although wild plants used to be obtained from natural conditions such as forests and fields, in recent years some species have been cultivated. Therefore, the shipping restrictions of wild plants are differently imposed according to their growth conditions as well as species. In other words, it would be imposed only for naturally grown products or for both natural and cultivated products. The number of restricted municipalities that do not distinguish between natural and cultivated products indicates the spread of restrictions on shipping of cultivated products, while the sum number of municipalities (“Sum” columns in Table 6.4) indicates the spread of restrictions on naturally grown products. For Japanese spikenard (Aralia cordata), taranome (Aralia elata), Japanese butterbur (Petasites japonicus), etc., which are being promoted for cultivation (forcing cultivation), there are almost no shipping restrictions when their growth conditions are not distinguished. On the other hand, shipping restrictions are more frequently imposed on naturally grown products, indicating that the wild plants growing in the natural conditions are likely to have higher concentration than the cultivated products. Also, the number of municipalities with shipping restrictions varies greatly by species. When results for naturally grown products are included, the shipping of koshiabura (Eleutherococcus sciadophylloides) has been most extensively restricted (113 municipalities in total), about the same as for wild mushrooms (117 municipalities).

Table 6.4 Number of municipalities implementing shipping restrictions for each species of wild plants

Full size table

The number of municipalities with shipping restrictions is 44 on wild taranome, second only to koshiabura . Bamboo shoots were originally introduced from China and are not strictly considered as naturally grown wild plants, but are widely restricted in shipping.

6.4.3.2 Why Is the Concentration in Koshiabura so High?

Wild plants are eaten in various parts, such as leaves, shoots, roots, etc., and surveys have revealed that the radiocesium concentration in wild plants varies depending on not only the species, but also the part of the plant even within the same species [155]. Among wild plants, it has been found that the radiocesium concentration is high in koshiabura and yamadori-zenmai (Osmundastrum cinnamomeum var. fokeiense) , and low in katakuri (Erythronium japonicum) and Japanese red elder (Sambucus racemosa subsp. sieboldiana), while taranome, royal fern (Osmunda japonica), and ostrich fern (Matteuccia struthiopteris) are in the middle. These results also correspond to the high/low number of municipalities that have implemented shipping restrictions for each species (Table 6.4).

Among the wild plants, the concentration of koshiabra is noticeably high. It is a tall tree of the Araliaceae family with five leaves spreading out like a palm from a single bud. The young shoots just about to open like those of the taranome are plucked, and it is commonly eaten as a tempura or boiled dish. A study of the relationship between radiocesium concentrations in the leaves, soil, and organic layer of koshiabra, showed that radiocesium concentrations in the leaves of koshiabra are more strongly correlated with radiocesium concentrations in the organic layer than in the soil [156]. It may be that koshiabura is capable of efficiently absorbing radiocesium from the organic layer and the surface layer of mineral soil, where radiocesium concentration is high. The role of microorganisms living symbiotically in the roots of koshiabra has also been investigated as a reason for the high radiocesium concentration [157]. Measurements of samples collected in 2017 also indicate that radiocesium concentrations in koshiabura leaves may increase further [156]. There is a concern that the radiocesium concentration in koshiabra may increase and exceed the criterion even in areas with low radiocesium deposition, and caution should be exercised.

6.4.4 Impact on Leisure Activities of Local Inhabitants

We explained that forests are not only a place for timber production, but also provide forest blessings such as wild mushrooms and wild plants. In addition, forests are also used for other activities such as fishing in mountain streams and mountain climbing. In this way, the forest has been used as a place for various leisure activities (recreation) by local people. However, comparing the data obtained from surveys and the number of users before and after the Fukushima nuclear accident, it became clear that recreational activities using natural reserves such as forests were definitely affected by the accident. For example, a survey revealed that the number of people participating in nature-based recreational activities (number of tourist arrivals) and the number of mountain climbers in Fukushima Prefecture dropped significantly in 2011, the year of the accident, and have not returned to normal since 2012 until at least 2016 (Fig. 6.20a). In the case of mountain climbers in particular, the decline was stronger in the Abukuma Highlands (between Hamadori and Nakadori), which is closer to the nuclear power plant and has a higher level of contamination than the Ou mountain range (between Nakadori and Aizu). The number of urban tourist arrivals also showed a similar downward trend, indicating that the Fukushima nuclear accident has affected various recreational activities, both nature-based and non-nature-based. Furthermore, a survey of the number of newspaper articles in Fukushima Prefecture shows that the number of articles related to outdoor activities such as nature and forests, including mountain climbing, forestry, and environmental education, has decreased since the accident (Fig. 6.20b). On the other hand, there was no change in the number of articles on crop damage caused by wildlife, suggesting that the trend of the damage by wildlife has not changed due to the accident.

The accumulation of radiocesium in wild plants, mushrooms, and mountain stream fish was confirmed in a wide area due to radioactive contamination of forests, and the motivation to collect these items in mountain village areas was greatly reduced. Figure 6.21 shows the changes in the percentage of households that collect wild plants and mushrooms, and go mountain stream fishing surveyed by a questionnaire of all households in two municipalities in Fukushima Prefecture in 2015; Hamadori, near the nuclear power plant, and Minami-Aizu, far from the nuclear power plant [159]. As can be seen here, these recreational activities were enjoyed by many people in the forest areas before the accident, but after the accident, there was a large decline especially in Hamadori, which is close to the nuclear power plant, and also even in Minami-Aizu, which is far from the nuclear power plant. This result clearly shows that the people in the mountain (forest) areas of Fukushima have moved away from leisure activities in the forest. In addition to the aging of the population, which is common in mountain villages, the delay of return of residents to the areas around the Fukushima Daiichi Nuclear Power plant is also thought to affect the decline in outdoor leisure activities.

6.4.5 Reduction of Radiocesium Concentration in Wild Plants by Cooking

There are many ways to prepare different kinds of wild plants. For example, taranome and butterbur scapes are cooked as tempura (Japanese deep-fried dish), and a harshness of wild ferns such as royal ferns and bracken ferns is removed using baking soda. Also, since the harvesting period of wild plants is limited, drying and salting methods have been developed for long-term preservation. Therefore, Kiyono et al. [160] compared the effect of removing radiocesium from different cooking methods by the ratio of the amount of radiocesium contained in the wild plants before and after cooking (food processing residual coefficient). As a result, it was found that the amount of radiocesium in wild plants decreased compared to that before processing by the following treatments: soaking in hot water, boiling in salted water, removing the scum, and pickling in salted water (Table 6.5). In particular, the reduction in the amount of radiocesium was significant for royal ferns that had been treated with a combination of removing scum by baking soda and drying which is used for long-term preservation, and for wild plants that had been salted and removed after several months. On the other hand, the concentration per weight of tempura was apparently smaller due to the increased weight by tempura batter coating, but the residual rate of radiocesium was almost the same as before processing.

Table 6.5 Residual ratio of radiocesium in wild plants by species and cooking method

Full size table

Among the salted treatments, it was found that although the residual rate decreased in the case of Aleutian ragwort (Senecio cannabifolius, “Hangonso” in Japanese), the reduction effect was smaller than that of other wild plants. The same treatment may have different effects on different wild plants. In addition, Nabeshi et al. [161] reported that the reduction effect of baking soda was greater than that of flour or salt, even though they were used in the same process of removing scum. Even in areas with relatively high levels of radioactive contamination, it is possible to avoid ingesting radiocesium by understanding the differences in the concentration of different wild plants and devising cooking methods to increase the reduction effect.

6.5 Cultivated Mushroom

Radioactive contamination has halted the bed-log cultivation using local trees.

6.5.1 Mushroom Cultivation Is an Important Industry Within Forestry in Japan

Forest products produced from forests, excluding timber (lumber), are collectively called non-wood forest products. In addition to food products such as mushrooms, nuts, and wild plants, non-wood forest products include medicinal plants, materials for crafts such as lacquer, woody products for craft such as bamboo and paulownia wood, and fuel materials such as firewood and charcoal. In Japan, the non-wood forest products account for more than half (57%, 2.5 billion US dollars) of the total forestry production (4.4 billion US dollars, 2017), and cultivated mushrooms account for 80% of the output of non-wood forest products (2.1 billion US dollars) [162]. There are two methods of mushroom cultivation: bed-log cultivation (Figs. 6.22a and 6.23), in which spawns (the mycelium that grows into mushrooms, or in the case of logs, wooden plug spawns) are driven into logs cut from the forest, and sawdust medium cultivation (Fig. 6.22b), in which spawns are grown in an artificial medium made of sawdust mixed with rice bran and other nutrients. In Japan, bed-log cultivation of shiitake mushrooms (Lentinula edodes) spread in the 1930s. After that, the method of sawdust medium cultivation of various mushroom species developed in the 1980s and was replaced as the main mushroom cultivation method as companies have set up production systems in factories. Currently, bed-log cultivation is mostly for shiitake, and 76% of bed-log production for shiitake is dried shiitake (production on a fresh weight basis). While total mushroom production in Japan has remained flat over the years, the share of bed-log cultivation in production and the number of farmers have been on a downward trend, indicating a consolidation of mushroom production operations while effecting the elimination of small-scale mushroom farmers (Fig. 6.24). Given the heavy labor involved in bed-log cultivation, such as cutting and periodically rearranging the logs, the decline in the number of farmers can be attributed to the aging of farmers who are leaving the industry, as well as to the growing preference among consumers for mushrooms other than shiitake. Under such circumstances, the radioactive contamination caused by the Fukushima Nuclear Power Plant accident has had a significant impact on the cultivation of shiitake.

6.5.2 Contamination of Bed-Logs and Shiitake Mushrooms

After the accident, it was confirmed that fallen radiocesium had directly adhered to shiitake mushrooms cultivated outdoors, or that radiocesium adhered to incubated bed-logs transferred to shiitake mushrooms. As a result, the production and shipment of shiitake mushrooms was restricted in a wide area. Using the transfer coefficient (Fig. 6.25), which is the ratio of the radiocesium concentration in the bed-logs and shiitake mushrooms, we can set the upper limit of radiocesium concentration in the logs that can be used without exceeding the standard limit for mushrooms by backward calculation. Therefore, a survey on the transfer coefficient was conducted in 2011 after the accident. The study showed that although it varied according to logs the maximum transfer coefficient was about 2 [163] (Fig. 6.26). Since the standard limit for food is 100 Bq/kg (fresh weight), 50 Bq/kg was set as the index value for logs.

6.5.3 Radioactive Contamination of Deciduous Broadleaf Trees for Bed-Log Cultivation and Its Impact on Industry

A survey on radiocesium contamination was conducted in broadleaf forests used for mushroom logs , and it was found that mushroom log forests with wood exceeding the index value of 50 Bq/kg were spread over a wide area in eastern Japan, including Fukushima Prefecture. Fukushima Prefecture used to produce a large amount of mushroom logs and supply them to other prefectures before the accident. But after the accident, the supply was completely stopped due to the contamination. In Eastern Japan, the supply of shiitake mushroom logs could not keep up with the demand, resulting in a mismatch between supply and demand (Fig. 6.27). To solve the mismatch, the Forestry Agency worked to find new sources of supply, for example by distributing flyers [164]. In recent years, the mismatch has been resolved in terms of volume. However, the mismatch between supply and demand by tree species remains, with mushroom producers requesting konara oak (Quercus serrata) and the supply of logs mainly consisting of sawtooth oak (Quercus acutissima) .

As mentioned above, the Abukuma region, which is located in the eastern part of Fukushima Prefecture and was heavily contaminated by the Fukushima nuclear accident, was a major producer of mushroom logs (Fig. 6.28).

To grow deciduous broadleaf trees such as konara oak and sawtooth oak for shiitake mushroom logs, the trees used to be cut down about every 20 years, allowing coppice shoots coming out of the stumps to grow large enough. However, due to the accident, the deciduous broadleaf trees used as logs were directly contaminated and the radiocesium concentration was detected to be more than 10 times higher than the index value, so the log production was completely stopped in an extensive area of eastern Japan.

Next, let’s take a look at changes in shiitake production. Shiitake mushroom production has been flat nationwide over the years, and prices have temporarily increased (Fig. 6.29). On the other hand, shiitake cultivation in Fukushima Prefecture has been greatly affected by the accident. The production of shiitake mushrooms in both cultivation methods of bed-log and sawdust has decreased significantly since the accident. In 2012, it was about one third of the pre-accident level. The shiitake production by sawdust medium cultivation has turned to recovery, and the amount of production in 2018 was to 92% of the 2010 level. But the amount of production in bed-log cultivation has not recovered and remains <10% of the 2010 level. The price of dried and fresh shiitake has dropped to 64% and 86% of the national average, respectively.

6.5.4 Transfer Mechanism of Radiocesium to Shiitake Mushroom

6.5.4.1 Transfer of Radiocesium from Bed-logs to Shiitake

Since the radiocesium concentration varies in different parts of the tree, it is important to know from which part of the log the shiitake absorbs radiocesium. Iwasawa [167] measured the radiocesium concentrations in each part of bed-logs separately and compared them with the concentration of emerging shiitake mushrooms (Fig. 6.30). The results showed that the radiocesium concentration in shiitake mushrooms was more highly correlated with the concentrations in the inner bark, sapwood, and heartwood of the logs (upper panels of the figure) than with those in the whole log or the outer bark (lower panels of the figure), of which the surface was directly contaminated. In addition, when the surface of bed-logs was washed, the radiocesium concentration in the whole logs decreased because the radiocesium attached to the bark surface fell off. However, the radiocesium concentration in the shiitake mushroom did not change regardless of washing treatment [168]. Therefore, it can be assumed that the shiitake mushroom absorbs radiocesium from the inside of logs (wood in Fig. 3.7) rather than from the outer part. The radiocesium concentration in various parts of trees, such as konara oak, changes over time (Fig. 3.8, Fig. 3.10). Therefore, the transfer factor calculated from the radiocesium concentration in the whole log may also change over time and should be monitored carefully.

Even if the radiocesium concentration is high, the bed-logs can be used if the transfer of radiocesium to the shiitake is controlled. Therefore, several attempts have been made to control the transfer. One of them is a test using a substance called Prussian blue (Fe₄[Fe(CN)₆]₃) to inhibit the absorption of radiocesium. Prussian blue is a substance that binds strongly to radiocesium and is used as an antidote to promote the removal of radiocesium from the body after an accidental ingestion. It was found that shiitake mushrooms produced from bed-logs infiltrated with Prussian blue had less than half the radiocesium concentration compared to shiitake mushrooms from untreated logs [169]. However, Prussian blue is a cyanide compound that may adhere to shiitake during emergence, making it difficult to apply to food products. Other methods were also proposed, such as mixing plug spawns with zeolite that would reduce the transfer of radiocesium, and spraying wet blasting (a mixture of water and abrasives) on the bed-logs to remove radiocesium [170, 171]. None of these methods have been put to practical use, but they have been shown to be effective in reducing the radiocesium concentration in shiitake mushrooms.

6.5.4.2 Additional Contamination from the Growing Environment

There are two ways of bed-log cultivation: outdoor cultivation, in which the inoculated bed-logs are placed and incubated in an outdoor environment such as a forest, and house cultivation, in which the bed-logs are placed in a building such as a greenhouse. There have been cases where the radiocesium concentrations in logs and shiitake mushrooms cultivated outdoors have increased. It is believed that additional contamination from the environment may occur in outdoor cultivation. Therefore, tests were conducted to counteract the additional contamination. Since radiocesium is circulating in the forest, various pathways are expected to cause additional contamination in the outdoor condition. One pathway is to transfer from the ground. In the case of nameko (Pholiota microspora ) mushroom cultivation using logs, it has been reported that the radiocesium concentration in the emerging nameko mushroom was reduced when the surface soil in the forest was removed [172]. These results indicate that radiocesium distributed in the soil transfers to mushrooms for some reason. In addition, in forests, radiocesium is contained in the rainfall that passes through the forest canopy (throughfall) (Sect. 3.3), and hence the transfer of radiocesium from the canopy to mushrooms in outdoor cultivation should also be monitored.

At present, the pathways and mechanisms of additional contamination are not fully understood, and countermeasures have not yet been established. The effects of additional contamination may differ depending on the region and conditions. Compared to the tests conducted in the first 2–3 years after the accident (early phase, Fig. 3.2), the tests conducted 4–5 years after the accident (transition phase) report that additional contamination of shiitake mushrooms from the environment has been reduced. In any case, the resumption of outdoor cultivation of shiitake mushrooms in contaminated forests should be done cautiously, using a variety of recommended measures while verifying their effectiveness.

6.5.5 Countermeasures Against Contamination of Cultivated Mushrooms

6.5.5.1 Guidelines

The mechanism of radiocesium transfer to shiitake is not fully understood. However, it is known that we can produce less contaminated mushrooms when we use logs with radiocesium concentrations below the index value, and the cultivation is carried out under the conditions avoiding additional contamination as with cultivating indoor and managing thoroughly. Therefore, the Forestry Agency is promoting the matching of the supply of uncontaminated (free or less radiocesium) logs and thorough management for producing less contaminated shiitake mushrooms based on the guidelines [173]. The restricted area for shipping is gradually decreasing as the guidelines are being followed and the restrictions are being lifted on a per-farm basis.

On the other hand, mushroom farmers are required to adopt different cultivation methods than before, such as using logs from different areas and cultivating indoor conditions. In addition, purchasing logs from outside the prefecture incurs costs. The local farmers used to establish an efficient system of bed-log cultivation using local deciduous broadleaf trees from their own forests, but the radioactive contamination has made it impossible to continue the system. Therefore, it is necessary to consider ways to gradually return to the original way. The radiocesium concentration in deciduous broadleaf trees varies depending on the environment even within the same area, and it is thought that there are logs that can be used even within the contaminated area. Therefore, an attempt is being made to find usable logs by a non-destructive inspection machine for logs (Fig. 6.31).

6.5.5.2 Contamination of Deciduous Broadleaf Trees and Countermeasures

Factors that determine the variation of radiocesium concentration within trees have been vigorously researched for konara oak trees, which have high commercial value, and research results have been reported suggesting the influence of exchangeable potassium in the forest soil as well as agricultural land. A survey of 40 areas of broadleaf forests for logs production in Tamura City, Fukushima Prefecture, revealed a strong negative correlation between the radiocesium concentration in the current year’s branches of konara oak and exchangeable potassium in the soil surface (0–5 cm depth) layer (Fig. 6.32). In this study, the radiocesium concentration in the current year’s branches differed up to 100 times or more among the study sites which received the similar amount of radiocesium deposition. Of course, the degree of contamination of the soil itself is important, but if the soil has a high concentration of exchangeable potassium, the radiocesium concentration in konara oak will be low, and it may be possible to use it as logs for shiitake cultivation. Research has also been conducted to actively suppress radiocesium absorption by potassium fertilization. For example, Kobayashi et al. [28] showed from experiments with konara oak seedlings that cesium absorption by konara oak was determined by absorption competition based on the ion ratio of potassium and cesium in the culture medium. It is also expected that potassium fertilization will make logs available even when soil radiocesium concentration is high.

6.6 Providing Information to Residents

A variety of methods were used to communicate the vast and complex information to the residents.

As we have seen, the dynamics of radiocesium in forests and changes in air dose rates have been clarified through surveys conducted by researchers and the government during the past 10 years. To rebuild the lives and industries in the affected areas, it is important not only to clarify the actual situation of contamination, but also to provide the obtained information to the local residents in a prompt, accurate, and easy-to-understand manner. To this end, the Forestry Agency, the Ministry of the Environment, Fukushima Prefecture, and other government agencies, national research institutes, and universities took the initiative in providing residents with a variety of information. Various methods were used, including symposiums (Fig. 6.33), dialogue meetings (Fig. 6.34), creation of pamphlets (Fig. 6.35), and construction of websites (Fig. 6.36). Recently, there have been symposiums with a popular YouTuber as a guest. In particular, in symposiums and dialogue meetings, in addition to the presentation of the latest research results by researchers and administrators, questions from the public were answered and direct communication took place.

The Forestry Agency of Japan has also taken the lead in producing a comprehensive pamphlet on radioactive contamination of forests. The pamphlet explains the situation of the forests in Fukushima as well as information on countermeasures and future plans, and is updated every year with new results. In addition, the ministries and agencies have been releasing data sets of information on radiocesium in crops and forest products reported by municipalities. These data sets have been used by researchers as well as providing information to residents. These websites and pamphlets can be viewed and downloaded from the links at the end of the book. Compared to the Chernobyl nuclear accident that occurred in the Soviet Union more than 30 years ago, the Fukushima nuclear accident has provided information with a high degree of transparency and speed, and in a variety of ways. It is important that we continue our efforts to return the research results to society.

6.7 Column: Looking Back on that Time (4)

Memoirs of the Fukushima Accident

Yves Thiry

Project Manager, French Radioactive Waste Management Agency

I learned the possible occurrence of mass releases of radioactivity to the atmosphere following the earthquake and tsunami of March 11, 2011 in Japan by radio. We were on March 16, 2011 on the road with a colleague of EDF R&D and two other collaborators from Sweden to visit a field trial in the East of France. At that period, we were working on chlorine biogeochemistry in the forest. The surprise was high. In absence of more detailed information, I was worried about possible consequences for Japanese populations and the environment.

In the following weeks and months, the media reported more on the gravity of the situation and the different measures rapidly taken by the national government, the local government and the operator and then those that followed in an evolutionary manner given the situational awareness. The number and rigorousness of countermeasures to minimize the consequences of radiological exposures, notably the large-scale start-up of decontamination of homes and land, impressed me.

In the end of 2011, the Institute for Radiological Protection and Nuclear Safety (IRSN) in France contacted me to join a vast consortium in charge of a new national research project for several years funded by the French National Research Agency (ANR). The general aim of the AMORAD project (https://www.irsn.fr/en/research/research-organisation/research-programmes/amorad/Pages/AMORAD-program.aspx) was the improvement of radionuclides dispersion and impact assessment modelling in the environment. My experience in forest contamination after Chernobyl was solicited to develop a program focusing on radiocaesium biogeochemistry in forested areas, contaminated on large surfaces in Fukushima region, while other programs looked at erosion via rivers and the transfers to/within marine ecosystems. We had our first field sampling in Japan in a cedar stand in autumn 2013 at a site equipped by the University of Tsukuba. Compared to post-Chernobyl projects, that fast cooperation was an exceptional opportunity to have a quick view on the early fate of contamination. Our study was facilitated thanks to additional exchanges with Japanese researchers, and a lot of supplementary information made available in the scientific literature by several Japanese institutes, including FFPRI. The importance of radioactive deposits interception by forest canopy and the further foliar uptake and recycling of radiocaesium by coniferous trees was rapidly confirmed. Nevertheless, it seems that the local climatic and edaphic conditions in the Fukushima area promoted the self-decontamination of soil and trees, compared to certain Chernobyl-affected forest. In my model TRIPS 2.0, initially developed for Scots pine in Belarus, I had to reduce the root uptake flux by a factor 4 in order the simulation agrees with measurement data in Japan. The recycling into deciduous trees remains more uncertain because of unclear foliar uptake and a root uptake still to be assessed. That is why continuous monitoring will be necessary for a couple of years. Overall, the calibration of our models of radiocaesium cycling in forest were consolidated thanks to the new data; those models are now being used to test different scenarios of forest management or countermeasures in cooperation with other Japanese researchers.

The contaminated areas are in a sort of convalescence now. A demanding task is to identify where and when a “normal” or adapted socio-economic situation may be developed, including the best safety conditions for the public and the workers. In highly contaminated zones where forest products contamination including wood will remain too high for a long time, we have to recognize that a recovery of a normal situation is difficult before several decades. It is depressing for local people to see great areas of forest excluded from their life habits. However, in most areas, less affected, there are still specific countermeasures to be tested to accelerate the self-decontamination of soil or trees and different processes of woody products’ valorisation still need to be investigated. For the future, I also expect a shift from individual/independent studies towards more integrated projects for a better control of the contamination fate at different interfaces e.g. the continuum forest-rivers-ocean. The forested watersheds have an essential role in stabilization of the contamination in Japan. Runoff and erosion of soil are primary vectors of possible long-term remobilization of the contamination, especially in Fukushima region due to local relief and climate; thus all that concerns the connectivity between forest and aquatic ecosystems must be carefully monitored and understood. That will also involve the effect of countermeasures that could degrade the contamination stabilization by forest, notably in steep hills regions. More generally, it is very important to consolidate an integrated vision and to have an interdisciplinary approach on the management of contaminated areas (from the soil to the waste repository) thanks to new insights in various disciplines: radiation protection, economical and social sciences in addition to environmental sciences. That is a very challenging task.

In that perspective, the questionings of local public and forest workers are very important to clarify the expectations and to guide a future programme of forested area management in Fukushima region, not disconnected from real life.

Notes

1.
https://www.japaneselawtranslation.go.jp/notices/view/57.

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Department of Forest Soils, Forestry and Forest Products Research Institute, Tsukuba, Ibaraki, Japan
Shoji Hashimoto
Department of Mushroom Science and Forest Microbiology, Forestry and Forest Products Research Institute, Tsukuba, Ibaraki, Japan
Masabumi Komatsu
Center for Forest Restoration and Radioecology, Forestry and Forest Products Research Institute, Tsukuba, Ibaraki, Japan
Satoru Miura

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Hashimoto, S., Komatsu, M., Miura, S. (2022). Impacts of Radioactive Contamination of Forest on Life. In: Forest Radioecology in Fukushima. Springer, Singapore. https://doi.org/10.1007/978-981-16-9404-2_6

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Impacts of Radioactive Contamination of Forest on Life

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Keywords

6.1 Effects of Increased Air Dose Rates

6.1.1 Characteristics of Air Dose Rates in Forests

6.1.1.1 Air Dose Rates in a Forest Change Generally According to the Radioactive Decay of Radiocesium, But Changes in the Distribution of Radiocesium in the Forest also Have an Effect

6.1.1.2 Decrease in Air Dose Rate Due to Radioactive Decay (About Half in 3 Years)

6.1.1.3 Spatial Distribution of Air Dose Rates in Forests Is Uneven

6.1.1.4 Air Dose Rates in Forests Are Higher than in Nearby Residential Areas

6.1.2 Access Control Based on Air Dose Rates

6.1.2.1 Designation of Areas Under Evacuation Orders and Their Changes

6.1.2.2 The Limit of Air Dose Rate for Forestry Activities is 2.5 μSv Per Hour or Less

6.1.2.3 There Are No Restrictions on Temporary Entry into the Forest

6.1.3 Forest Decontamination

6.1.3.1 Removing the Organic Layer Reduces the Air Dose Rate

6.1.3.2 The Effective Range of Forest Decontamination is 20 m

6.1.3.3 Does Cutting Down Forest Trees Reduce Air Dose Rates?

6.1.3.4 Is It Realistic to Decontaminate All Forests?

6.2 Wood-Related Regulations and Their Impact

6.2.1 Regulations Related to Wood

6.2.1.1 External Exposure from Living in Wooden Houses Is Negligible

6.2.1.2 Disposal of High Concentrations of Bark Is a Problem

6.2.1.3 Criteria (Index Values) for Firewood, Chips, and Charcoal

6.2.2 The Impact on Forestry from Statistics

6.2.3 Utilization of Contaminated Forests

6.2.3.1 Volume Reduction of Contaminated Wastes

6.2.3.2 Conversion to Energy Use and to Other Uses

6.3 Radioactive Contamination of Wildlife

6.3.1 Large Wildlife Populations Are Increasing Across the Country

6.3.2 Trend of Radiocesium Concentration

6.3.3 Countermeasures: Testing All Animals Slaughtered and Population Control

6.4 Radiocesium Contamination of Wild Mushrooms and Wild Plants

6.4.1 The Value of the Forests’ Bounty to Local Communities

6.4.2 Radioactive Contamination of Wild Mushrooms

6.4.2.1 Species-Independent Batch Restrictions of Shipping

6.4.2.2 Analysis of Wild Mushrooms Using the Results of Food Monitoring

6.4.2.3 The Radiocesium Concentration in Wild Mushrooms Varies Greatly Among Species

6.4.3 Radioactive Contamination of Wild Plants

6.4.3.1 Differences in Restricted Areas of Shipping by Species and Growing Conditions

6.4.3.2 Why Is the Concentration in Koshiabura so High?

6.4.4 Impact on Leisure Activities of Local Inhabitants

6.4.5 Reduction of Radiocesium Concentration in Wild Plants by Cooking

6.5 Cultivated Mushroom

6.5.1 Mushroom Cultivation Is an Important Industry Within Forestry in Japan

6.5.2 Contamination of Bed-Logs and Shiitake Mushrooms

6.5.3 Radioactive Contamination of Deciduous Broadleaf Trees for Bed-Log Cultivation and Its Impact on Industry

6.5.4 Transfer Mechanism of Radiocesium to Shiitake Mushroom

6.5.4.1 Transfer of Radiocesium from Bed-logs to Shiitake

6.5.4.2 Additional Contamination from the Growing Environment

6.5.5 Countermeasures Against Contamination of Cultivated Mushrooms

6.5.5.1 Guidelines

6.5.5.2 Contamination of Deciduous Broadleaf Trees and Countermeasures

6.6 Providing Information to Residents

6.7 Column: Looking Back on that Time (4)

Memoirs of the Fukushima Accident

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