Keywords

12.1 Introduction

Fukushima Prefecture and the surrounding areas are among the most forested areas in Japan (Fig. 12.1); ~70% of the contaminated areas are covered with forests (Hashimoto et al. 2012). Accordingly, although the amount of total deposition on the land is still in debate, ~70% of it was deposited onto forest ecosystems (Kato et al. 2019a). Therefore, the forest is one of the key ecosystems in the radioactive contamination caused by the Fukushima Daiichi Nuclear Power Plant accident.

Fig. 12.1
A map of the Fukushima area with forests highlighted on the map in a dark shade of color. The F D N P P is indicated with a large triangle on the east coast.

Spatial distribution of forests in Fukushima area (Ministry of Land, Infrastructure, Transport and Tourism 2020). Black triangle indicates the location of the Fukushima Daiichi Nuclear Power Plant

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In Fukushima, there are 0.97 million ha of forests. The forests in Fukushima and adjacent regions are rich in tree species, like the forests in other areas of Japan. The most dominant species are the Japanese cedar (Cryptomeria japonica), the Japanese cypress (Chamaecyparis obtusa), the red pine (Pinus densiflora), and the Konara oak (Quercus serrata). These trees in artificial forests are used for building, paper, and mushroom cultivation. Furthermore, in forests, local inhabitants collect wild edible mushrooms and vegetables, and hunt wild game for meat (e.g., sika deer, Asian black bear, and wild boar). Therefore, the radioactive contamination by the Fukushima Daiichi Nuclear Power Plant accident seriously affected the lives of the local inhabitants.

Therefore, the radiocesium dynamics in the forest are of great concern to people and the authorities. The studies conducted after the Chernobyl (Chornobyl) Nuclear Power Plant accident have reported that the radiocesium deposited onto forests migrates within forest ecosystems (IAEA 2006; Shaw 2007). The radiocesium is first trapped by the tree canopy and migrates to the forest floor afterward. The mineral soil under the organic layer of the soil surface becomes the largest reservoir of radiocesium eventually. With time, spanning from days to years, migrations occur and are driven by various physical and biological processes in forest ecosystems. However, the overall picture described above is not necessarily inferred from the abundance of observation data (particularly limited data in the first few years).

In Fukushima, immediately after the fallout, many researchers and governmental authorities collected huge amounts of data that described the spatiotemporal radiocesium dynamics. This chapter describes the overall radiocesium dynamics in forest ecosystems that were revealed by various research studies conducted in the first decade after the accident. Also, it discusses the future status of forest radiocesium with predictions from modeling studies.

12.2 Decrease in Radiation Dose in Forests

The important characteristic of radioactive contamination is its radioactive decay. Figure 12.2 shows the temporal changes in the air dose rates measured in forests in Fukushima Prefecture (Fukushima Prefecture Forestation Division 2021). The long-term monitoring clearly illustrates the decreasing trend of the radiation dose in forests, which is consistent with the theoretical changes caused by the physical decay of 134Cs and 137Cs. As in other ecosystems, the decrement in the air dose rate is more rapid in the first several years due to the decay of 134Cs. In the later phase of the first decade after the accident, the decrement slows down, because most of the remaining radiocesium is 137Cs, which has a half-life of 30 years. The decrease in the air dose rate may occur faster than predicted based on the theoretical curve due to the further migration of radiocesium in the deeper layers of mineral soils. However, the decrease would basically follow the theoretical curve, because the migration of radiocesium within the soil is very slow, and most radiocesium will stay in the shallowest layer of forest soils. Besides, the forest retains most radiocesium without substantial outflow via stream flows. These characteristics of temporal changes in the air dose rate in forests are the fundamental fact for local people and authorities that take safe and effective countermeasures against contaminated forests.

Fig. 12.2
A line graph of air dose rate versus year. The curve begins at approximately (2011, 0.9) and decreases to (2021, 0.4).

Temporal changes in air dose rate observed at 362 forest sites in Fukushima Prefecture (solid circle) (Fukushima Prefecture Forestation Division 2021). The line shows the theoretical curve based on the decay of 134Cs and 137Cs

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12.3 Intensively Investigated Parts of Forests

The obtained forest radioactivity data have been published in scientific journals, government reports, and the Web. Published data on activity concentration and inventory of trees, soils, and mushrooms observed in forests were comprehensively compiled (Hashimoto et al. 2020b). The compilation showed that trees were investigated more intensively, but soils and mushrooms were also moderately investigated (Table 12.1). The inventory data for the soil were actively obtained, whereas those for the tree part were less investigated.

Table 12.1 Data count for trees, soils, and mushrooms (activity concentration and inventory) (Hashimoto et al. 2020b)
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As expected, the compilation found much data for the Japanese cedar, because it is the most widely distributed tree species in managed forests (Table 12.2). However, the Konara oak, an important tree species for mushroom cultivation in Japan, was less studied.

Table 12.2 Counts for activity concentration data for trees by species and parts (Hashimoto et al. 2020b)
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Concerning the parts of the Japanese cedar, the leaf (needle) was mostly investigated intensively (Table 12.2). However, the woods (bulk wood, heartwood, and sapwood) were well studied, probably because wood is commercially the most important forest product. Interestingly, the pollen of the Japanese cedar was also well investigated, because it is the main cause of the hay fever in Japan, and the potential pollen contamination could be of great concern to the public.

12.4 Radiocesium Dynamics in Forests

12.4.1 Trees to Soils

Most of the radiocesium deposited onto forests was first trapped by the tree canopy (Kato et al. 2012, 2019a; Gonze et al. 2021). The reported interception rates vary between studies due to the different observation periods, methods, and forest types (Hashimoto et al. 2020c). Gonze et al. compiled the early data and back-calculated the interception rate for coniferous trees during deposition; the estimated value was 0.7–0.85 in March 2011 (Gonze et al. 2021). The interception ratio for the deciduous trees, which had no leaves in March, was lower than that for coniferous trees.

As reported in many studies, the radiocesium then rapidly moved from the tree canopy to the forest floor in time scales of days to years through litterfall and rainfall (Itoh et al. 2015; Kato et al. 2019b). Figure 12.3 shows the percentage of the radiocesium inventory in the tree canopy and soil (soil surface organic layer + mineral soil layer) relative to the total inventory observed in the forests in Fukushima Prefecture (Forestry Agency 2022). Note that the sampling was conducted during the summer yearly, and that the first data were taken 5 months after the fallout in 2011. Although the tree inventory ratio varies among sites and species, the tree inventory decreases with time, and accordingly, the soil inventory increases with time. Within a few years, the soil surface organic layer (A0 layer) and mineral soil retain more than 90% of the total inventory in the forests. The activity concentration of 137Cs in needles and branches exponentially decreased with time from 2011 to 2015, with effective ecological half-lives of 0.45–1.55 and 0.83–1.69 years for needles and branches, respectively (Imamura et al. 2017).

Fig. 12.3
A multiple line graph of percentages to the total inventory versus year for soil and tree for different sites. The plot depicts an increasing trend for soil and a decreasing trend for trees for different sites.

Changes in percentages of radiocesium inventory (134Cs + 137Cs) in trees and soil (organic layer + mineral soil) to total inventory in the forest observed at long-term monitoring sites operated by FFPRI (Forestry Agency 2022). The red vertical line in 2011 is the tree interception ratio in March 2011 estimated by Gonze et al. (2021)

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12.4.2 Soil

As shown above, the radiocesium trapped by the tree biomass moved to the forest floor (Fig. 12.3). However, most of the radiocesium deposited onto the forests is now in mineral soils, not in the soil surface organic layer (Fig. 12.4). The soil surface organic layer retains almost the same amount of inventory as in the mineral soil layer in the early stage, but the percentage continuously decreases over time (Fig. 12.4). Imamura et al. compiled the radiocesium distributions in the soil surface organic and mineral soil layers observed between 2011 and 2017, and found that the radiocesium fraction in the soil surface organic layer drastically decreased in the first 6 years from ~60% in 2011 to ~10% in 2017 (Imamura et al. 2017). Probably because the soil surface organic layer is thinner in Japan than in the forests in European countries affected by the Chernobyl accident, radiocesium did not remain long in the surface organic layer in Japan.

Fig. 12.4
A multiple-line graph of the percentage of r C s in the litter layer to the total in soil versus year for different sites. The line for the site cedar 2 has the highest initial value decreasing from 74% to 13% approximately.

Time dependency of percentage of radiocesium inventory (rCs: 134Cs + 137Cs) in the soil surface organic layer (litter layer, A0) to total inventory in soil (soil surface organic layer + mineral soil layer) observed in long-term monitoring sites operated by the FFPRI (Forestry Agency 2022)

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In the mineral soil layer, most radiocesium materials remain in the shallow layer (Fig. 12.5). Figure 12.5a shows the temporal changes in the activity concentration in each soil layer in a forest in Kawauchi village, Fukushima (Forestry Agency 2022). First, the activity concentration at depths of 0–5 cm increases from 2011 to 2012. Afterward, the highest concentration between layers is maintained. The vertical distribution of the radiocesium inventory observed in 2011 and 2015 is shown in Fig. 12.5b (Imamura et al. 2017). The radiocesium inventory in the surface layer has remained the largest since 2011.

Fig. 12.5
2 multiple line graphs of activity concentration of r C s versus year and depth versus inventory of r C s. For a depth between 0 to 5 centimeters, the activity concentration of r C s increases from 20 k B q per k g with an initial peak up to 45 k B q per k g and decreases up to 43 k B q per k g. The depth versus inventory of r C s depicts an increasing trend for both years.

Vertical distributions of radiocesium (rCs: 134Cs + 137Cs) in Japanese cedar forest in Kawauchi village (long-term monitoring sites of FFPRI). (a) Temporal changes in activity concentrations (Forestry Agency 2022), and (b) vertical distributions of radiocesium inventory observed in 2011 and 2015 (Imamura et al. 2017)

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12.4.3 Wood

The activity concentrations of wood, which is the most important part of a tree in terms of commercial use, differ among tree species and forests. Figure 12.6 shows the temporal trends in the activity concentrations of the Japanese cedar, Japanese cypress, red pine, and Konara oak observed in forests in Fukushima. These trends differ even for the same tree species. However, the Konara oak shows increasing trends, whereas the cedar shows stable or small increasing and decreasing trends. The normalized activity concentration, which is almost comparable to Tag, mainly ranges from 10−4 to 10−3 (m2/kg, dry weight).

Fig. 12.6
4 scatterplots of normalized concentration versus year for cedar, cypress, pine, and oak for four different sites. The points are between 1 multiplied by 10 power negative 4 and 1 multiplied by 10 power negative 3.

Activity concentration of 137Cs for Japanese cedar, Japanese cypress, red pine, and Konara oak normalized with total inventory of 137Cs in 2012. [The data was originally from Ohashi et al. (2017), and were normalized by the author]

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The Tag values for the wood observed in Fukushima were collated and compared with those in Chernobyl studies (Hashimoto et al. 2020c). The range and geometric mean of the Tag values for the conifer (pine excluded), pine, and oak are similar to those observed after the Chernobyl accident. Although the record number is small, those for the pine are probably lower than those reported for the Chernobyl accident, including the data measured for hydromorphic soils (Hashimoto et al. 2020c). The similarity indicates that the overall magnitude of transfer to the wood was similar and comparable, although the number of samples and different species and timing since the fallout must be considered. The comparison suggests the mutual global applicability of radioecological parameters and the need for further collation of parameters. However, Fig. 12.6 shows that the transfer to wood and its time dependence vary among species and sites, thereby emphasizing careful monitoring of trees and soils at affected areas where accidents occurred is always essential. The cause of the variability remains unclear; thus, future studies should be conducted.

12.4.4 Wild Food in Forests

The radiocesium deposited onto forests was also transferred to wild edible foods, such as wild mushrooms, wild edible plants, and game animals. The consumption of these contaminated foods in forests is an even more sensitive issue in affected areas. After the accident, the government and scientists have monitored the radioactivity levels in wild foods in the forests. The data revealed huge variations of radiocesium transfer among species (Tagami et al. 2016; Komatsu et al. 2019, 2021; Hashimoto et al. 2020c). For example, Komatsu analyzed open government monitoring data for wild mushrooms and found that the normalized concentration ranged from 1.1 × 10−4 to 2.3 × 10−2 (m2/kg, fresh weight) and that the mycorrhizal species tend to have higher radioactive concentrations than saprophytic species (Komatsu et al. 2019) (Fig. 12.7). It was also confirmed that wild plants generally had lower normalized concentrations than mushrooms, although some species of wild plants showed much higher transfer (e.g., koshiabura, Eleutherococcus (Chengiopanax) sciadophylloides) (Komatsu et al. 2019, 2021). A similar analysis was also conducted using the monitoring data for game meat (Tagami et al. 2016). The analysis of the data collected between 2011 and 2015 revealed that the geometric mean values of the Tag values of 137Cs in 2015 for the Asian black bear, wild boar, sika deer, and copper pheasant were of similar values, ranging from 1.9 × 10−3 to 5.1 × 10−3 (m2/kg, fresh weight), whereas those for the green pheasant and wild duck were about one order of magnitude lower than these species, ranging from 1.0 × 10−4 to 2.2 × 10−4 (m2/kg, fresh weight). Also, different time dependencies for different game animals were observed.

Fig. 12.7
A violin plot of median posterior N C of species versus ecological types. The median of mycorrhizal lies between 10 power negative 3 and 10 power negative 2. The median for wood decomposition lies and litter decomposition lies at 10 power negative 3.

Violin plot of normalized concentration (NC: almost comparable to Tag) for mushroom species according to ecological types. Concentration is normalized with total deposition (Komatsu et al. 2019)

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The Japanese government sets and operates the criteria concerning radiation and radioactive concentrations to protect people from radiation exposure. However, the regulations have affected people’s lives in contaminated areas through limited access to forests, restrictions on the use of timber, wild mushrooms and plants, and wildlife. More details are described in the study by Hashimoto et al. (2022).

12.5 New Features Captured in Fukushima

The Fukushima Daiichi Nuclear Power Plant accident is the second-largest nuclear accident since the Chernobyl Nuclear Power Plant accident. There are several new features captured in the research on the Fukushima accident.

In the Chernobyl accident, few data captured the very early radiocesium dynamics in the forests (days to months after the accident). However, in the Fukushima accident, the preinstalled observation system, which had been operated for biogeochemically studying the forest before the accident, monitored the very early dynamics of the radiocesium, including deposition, trapping by the tree canopy, and migration from the canopy to the forest floor. Furthermore, the monitoring activity started early enough in many forests; thus, days to months and a few years of radiocesium in forests were well captured. These data are new and essential in areas of radioecology research, aiding the understanding of radiocesium dynamics in forests.

Another new feature is the transparency of various datasets. Various datasets about forest and forest products were released and opened by researchers and the government, and these datasets are available for researchers and the public. For example, the FFPRI and the Forestry Agency have monitored the forest radiocesium dynamics (i.e., contamination of tree organs and soils and radiation) since August 2011 and published the data annually. Fukushima Prefecture has also published similar monitoring data. The government has also released monitoring data for edible forest products, such as wild mushrooms, wild edible plants, and game meat. The airborne survey of radiation and radiocesium inventory was also demonstrated repeatedly and is available in a very easy-to-see Web interface and GIS format for specialists (e.g., https://emdb.jaea.go.jp/emdb/). These datasets have demonstrated forest contamination, as researchers have used them to study forest contamination. Furthermore, they provided the general situation of the forest contamination to the public with transparency.

12.6 Future Prediction

More than a decade of research by scientists and authorities revealed the radiocesium dynamics and radiation changes in forests. What will happen to forests in the future? Because most of the 134Cs, which contributed to the rapid decrease in the radiation dose, had decayed in the last 10 years, the decrease in radiation declines and occurs with the decay of 137Cs (Fig. 12.8). Thus, 137Cs will continue to migrate to forests. Modeling analysis is a good tool for understanding the transfer processes and predicting future radiocesium dynamics in forests (Ota et al. 2016; Nishina et al. 2018; Thiry et al. 2018; Kurikami et al. 2019). After the Fukushima nuclear accident, modeling studies were conducted to predict radiocesium dynamics in forests, revealing that, as already demonstrated by many observations, the mineral soil compartment will be the largest radiocesium reservoir (Fig. 12.9). However, a small portion of radiocesium in the forest ecosystem circulates continuously within forests, between trees and soils, probably approximately <1% of the total initial deposition (Hashimoto et al. 2020a) (Fig. 12.10). However, the six-model intercomparison revealed that the future of radiocesium concentration in wood, which is the most important part of a tree in terms of forest products, is quite uncertain. The intercomparison demonstrates that the state-of-the-art models worked well to predict the radiocesium concentration in trees within a decade, but the uncertainty will increase with time (Fig. 12.11). These patterns were also found in a preliminary simulation for the Konara oak. In the equilibrium stage, the key driver of forest radiocesium dynamics between trees and soils is root uptake; the uncertainty and variation in the model predictions are attributable to the different root uptake assumptions among the models. To reduce the future prediction uncertainty, it is essential to quantify the root uptake ratio and the biogeochemical processes of radiocesium in the soil. The radiocesium dynamics in forests will decrease with time, but will never stop. Thus, it is important to continue monitoring and understanding the transfer process of radiocesium within forests.

Fig. 12.8
An area graph of percentage to the initial dose rate versus the year after the accident for C s 134 and C s 137. The area for C s 137 is initially up to 25% and decreases later. The area of C s 134 is high initially up to 100% as a triangular region.

Predicted changes in air dose rate with time based on physical decay. The ratio of 134Cs and 137Cs emitted at the time of the accident was assumed to be 1:1. 134Cs emits radiation ~2.7 times more intense than 137Cs

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Fig. 12.9
A multiple-line graph of the percentage of soil inventory to the total versus year for 6 different models. Model C M F W in 2011 has a percentage of 25 and then increases up to 100 percent for the years 2030 to 2060.

Prediction of soil inventory percentage in coniferous forests by six models (Hashimoto et al. 2021)

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Fig. 12.10
An illustration of 2 trees at an early phase and late phase. Both the illustrations depict arrows from tree to soil and soil to the tree. However, the early phase has a thick large arrow from tree to soil and a thin arrow from soil to tree. The late phase depicts equal-sized arrows.

Conceptual diagram of radiocesium circulation in forests; early phase after the fallout and later phases in the equilibrium state. In the equilibrium state, the emission from trees and the uptake from the soil are almost balanced

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Fig. 12.11
A multiple line graph of N C in wood versus year for 6 models. All the models depict a decreasing trend with the highest decreasing point for the model R I F E 1 approximately equal to 1 multiplied by 10 power negative 3.

Prediction of normalized activity concentration (NC) in conifer wood using six models. The activity concentration is normalized with the total initial deposition (Hashimoto et al. 2021)

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Another important perspective on the future of contaminated forests is the health of forest ecosystems. Contaminated forests, particularly artificial forests, are now abandoned or undermanaged, which risks the health of forest ecosystem services. Additionally, no-go areas and less used forests have changed the balance between humans and wild animals in the affected areas. Note that these changes in forest ecosystems are not direct effects of radiation. It is crucial to monitor the forest health in affected areas and promote healthy forest functioning.

12.7 Conclusion

Immediately after the accident, the forests in Fukushima have been intensively studied. The radiocesium deposited onto the forests migrated within forests. In Fukushima forests, the dynamics were well captured by many scientific studies and monitoring works by the authorities. In the last 10 years, the total amounts of radiocesium and radiation have substantially decreased due to the decay of 134Cs. In the future, the decrease will decline due to the remaining 137Cs, and 137Cs will circulate continuously in forests, although the amount is minimal in terms of the total reservoir in forests. Also, predictive studies using models have been conducted, which have provided useful perspectives on the dynamics of 137Cs in forests. However, they still have significant uncertainty in predicting the contamination of the wood. These predictions emphasize the importance of long-term monitoring and further research on the radiocesium dynamics, such as root uptake by trees. Forest ecosystems have changed due to the change in the balance between humans and nature due to land abandonment and underuse of forests. Furthermore, it is also important to monitor and manage the health of forest ecosystems in affected areas. An even more detailed understanding of the radiocesium dynamics in forests and a reliable future prediction will help rehabilitate the environment and people’s lives in the affected areas.