Keywords

4.1 Radiocesium and Material Cycles in Forests

The absolute amount of radiocesium is extremely small and does not affect the material cycle of the ecosystem.

In the forest ecosystem, various substances (carbon, nitrogen, water, nutrients, and various elements) are transferred/circulated through biological, chemical, and physical processes. For example, the so-called carbon cycle begins with the absorption of carbon dioxide from the atmosphere by trees. The trees use the carbon dioxide absorbed and the water and nutrients absorbed from their roots to carry out photosynthesis and grow. During the growth process, some parts of the trees die and fall to the ground. The organic matter is decomposed on the ground surface, and some of it is released as carbon dioxide into the atmosphere, while the rest stays there. In this cycle of carbon, nutrients also circulate. In addition to nitrogen, phosphorus, and other elements that are important for life support (essential elements), plants and animals also contain non-essential elements that circulate through the ecosystem in the same way as carbon. The movement of these substances is driven by the physiological functions of plants themselves, gravity, water movement, and the actions of organisms.

It can be said that radiocesium released into the atmosphere as a result of the Fukushima nuclear accident suddenly entered the forest (although some radiocesium was already there, as described in Sect. 4.4). Did the influx of this new anthropogenic element directly and significantly change the movement of materials in the forest? As it turns out, the amount of radiocesium itself is not enough to directly change the material cycle in the forest. Radiocesium is an alkaline element and behaves very similarly to potassium. Cesium-133 , the stable isotope of cesium, is also found in nature, including forests. Cesium-133 has been circulating in the forest regardless of whether radiocesium enters or leaves the forest. Compared to cesium-133, the amount of radiocesium (cesium-134 and cesium-137) that entered the forest ecosystem as a result of the accident was very small, ranging from 1/1000 to 1/1,000,000 (Fig. 4.1). Therefore, the presence of newly entered radiocesium is not influential enough to change the movement of the elements in the first place.

Fig. 4.1
figure 1

A schematic image of circulating amounts of cesium-133 (133Cs) and cesium-137 (137Cs) in relation to the circulating amount of potassium (K). Cesium is an element that behaves in a similar way to potassium. The respective circulating amounts are shown instinctively in the form of circle sizes, but in reality the circulating amount of cesium-133 is about 1/100,000 of the amount of potassium, and that of cesium-137 is about one part in 10 billion of the circulating amount of potassium

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On the other hand, the transfer of radiocesium newly introduced into the forest is driven by the movement of various materials in the forest, as mentioned earlier, and is related to a very wide range of research fields such as forest hydrology, material cycle, plant nutrition, dendrology, and tree physiology. For example, radiocesium on leaves and branches transfers to the forest floor via rainfall (forest hydrology), and via litterfall (the fall of dead leaves and branches to the ground) (material cycle). In the soil, it also migrates from the soil surface to the deeper layers through water infiltration and decomposition of dead organic matter such as fallen leaves and branches (soil science and material cycle). In addition, the root system of trees absorbs radiocesium with nutrients that are mixed in the soil and it enters the tree (soil science, plant nutrition, tree physiology). The movement of radiocesium accompanying such dynamic material flows is a characteristic of forests, which are natural ecosystems with perennial and gigantic characteristics not seen in agricultural ecosystems.

The existence of a wide variety of organisms is also a characteristic of forest ecosystems. From small animals such as earthworms and insects to large wildlife, radiocesium is taken up by them. The uptake of radiocesium by organisms is influenced by their diet, living place, life cycle, etc. Disturbance of soil by earthworms and activities of large wildlife can be said to contribute to the migration of radiocesium to some extent.

The idea of using fungal uptake to recover radiocesium from the soil is another way of utilizing the ecological functions of forest ecosystems. Soil, leaf litter, and dead wood contain a wide variety of fungi in the form of mycelium, in addition to the occasional “mushrooms” (fruiting bodies) that are visible to the eye. The mycelium decomposes organic matter and takes in nutrients, or enters the roots of trees to live in symbiosis and exchange nutrients with each other. Radiocesium transfers along with this flow.

4.2 Radiocesium in the Food Chain

Bioaccumulation in the food web has not been confirmed, but high radiocesium concentrations have been detected in some organisms.

4.2.1 Radiocesium Concentration in Earthworms Is Lower than that in the Soil Surface Organic Layer

Earthworms, which are soil animals, play an important role in the material cycle of forest soil and the formation of soil structure by feeding on soil along with organic matter such as fallen leaves and excreting them as feces [69]. Since the Fukushima nuclear accident, several studies have been conducted on the dynamics of radiocesium in earthworms. It has been found that most of the radiocesium deposited in forests stays in the shallow layers of mineral soil. Therefore, it was feared that earthworms, which feed directly on the soil, would concentrate the radiocesium. As a practical example, it is known that when eating fallen leaves containing toxic substances such as dioxin, the concentration of toxic substances in the earthworm’s body increases more than the concentration in the fallen leaves [70, 71]. Furthermore, it was noticed that bioaccumulation of radiocesium may take place through the subsequent food chain in the forest. Therefore, Hasegawa et al. collected earthworms at fixed plots every year for 3 years from 2011 and examined the radiocesium concentration. As a result, the radiocesium concentration in earthworms showed a decreasing trend every year, which was faster than the decrease due to radioactive decay (Fig. 4.2).

Fig. 4.2
figure 2

Changes in radiocesium concentration (total of cesium-134 and -137) in earthworms collected from cedar forests in Kawauchi Village. Solid squares (■) indicate actual measured values, and open circles (○) indicate estimated values based on the assumption that the concentration was reduced from the actual measured values 1 year ago by radioactive decay alone. It can be seen that the radiocesium concentration in earthworms decreased faster than the decrease due to radioactive decay (Source: Redrawn from the Forestry and Forest Products Research Institute 2014 [72], the original paper is Hasegawa et al. [73])

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In addition, when the radiocesium concentration in earthworms was compared with those in the soil surface organic layer and mineral soil layer in their habitats, it was found that the radiocesium concentration in earthworms was intermediate between that in the organic layer and mineral soil layer at all three survey sites (Fig. 4.3). Therefore, bioaccumulation of radiocesium by earthworms is not considered to have occurred.

Fig. 4.3
figure 3

Comparison of radiocesium concentrations (sum of cesium-134 and -137) in the organic layer, surface mineral soil layer (0–5 cm), and earthworms in cedar forests at three locations in Fukushima Prefecture collected 6 months after the Fukushima nuclear accident. The radiocesium concentration in earthworms varied depending on the contamination level of the study site, and was intermediate between the organic layer and the soil layer (0–5 cm) at all sites (Source: Data from the Forestry and Forest Products Research Institute 2014 [72]; the original paper is by Hasegawa, et al. [74])

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To investigate why earthworms do not concentrate radiocesium, Fujiwara et al. studied the change in radiocesium concentration when earthworms were cultured in soil with a high radiocesium concentration and then transferred to soil with almost no radiocesium. As a result, the radiocesium concentration in the earthworms increased when they were cultured in soil with a high radiocesium concentration, but when they were transferred to soil containing no radiocesium, the radiocesium concentration in the worms decreased to an undetectable level in just 1 day [75]. Therefore, it was considered that the radiocesium in the soil eaten by the earthworms was immediately excreted as feces without being absorbed by the body. Later, when the earthworms were dissected and examined, it became clear that most of the radiocesium in the earthworms was derived from their intestinal contents [76]. The radiocesium in soil is strongly adsorbed by clay minerals, and earthworms may not be able to absorb it.

4.2.2 Bioaccumulation Through the Food Chain Is Not Occurring

It was found that bioaccumulation of radiocesium does not occur in earthworms. However, biological communities in ecosystems have a series of predator-prey (eat and be eaten) relationships, creating a food chain. In addition, the actual predator-prey relationship is not linear, but rather a complex web of multiple organisms, which is why food chains are also called food webs. To verify the bioaccumulation of radiocesium through the food web of the ecosystem, it is necessary not only to examine radiocesium concentration in some organisms, but also to examine radiocesium concentration in several organisms located at the top and bottom of the food web. There was concern about contamination of small aquatic insects (lower in the food chain), large aquatic insects, and stream fish (higher in the food chain) in the stream. It is thought that radiocesium in river water does not dissolve in the water as ions (dissolved formFootnote 1), but moves around in a suspended form attached to fine organic matter and soil particles. It has also been found that the radiocesium concentration in algae and fallen leaves collected in rivers is higher than that in sand on the river bottom [77]. Therefore, the radiocesium concentration was investigated for aquatic insects in the family Stenopsychidae that feed on algae and large aquatic insects that feed on smaller aquatic insects. As a result of this investigation was that the radiocesium concentration in aquatic insects was not higher than that in algae, and the possibility of radiocesium being concentrated in the food web higher up in the food chain of aquatic organisms was considered to be low. The results of no bioconcentration of radiocesium through the food chain have been reported for terrestrial insects as well as for aquatic insects [78].

In general, radiocesium taken into the body by animals through ingestion of food is excreted from the body in a period of several tens of days, which is much shorter than the physical half-life of radiocesium. Such a half-life that takes into account the elimination of radioactive materials by living organisms is called the biological half-life (Sect. 2.1). The reason for the short biological half-life of animals is that radiocesium does not have the property of binding to or being adsorbed by specific organs of animals. Therefore, broadly speaking, the radiocesium concentration in forest animals, insects, and other living creatures depends on the degree of contamination of the environment in which they get their food. Also, if the creatures can be moved to an uncontaminated environment, the contamination will decrease quickly.

4.2.3 Radiocesium Taken up by Large Wildlife

As we saw in Chap. 3, radiocesium is contained in forest plants and soil, and also in small animals such as soil animals and insects, as we saw in the previous section. As a result, radiocesium has also been detected in large animals at the top of the food chain. As we will see in detail in Chap. 6, different species have different levels and seasonality of radiocesium concentration in their muscles. These can be considered to be due to differences in the diet of different species (e.g., whether they sometimes eat surface soil with relatively high contamination concentration or not), their lifespan, and their ability to discharge radiocesium in the first place.

We consume the meat of wildlife, which raises the issue of regulation as food in addition to the perspective of cycling and diffusion of radiocesium in the ecosystem. The issue of radiocesium contamination of large wildlife and its regulation will be dealt with more in Sect. 6.3.

4.2.4 Fungi and Radiocesium

Another factor that cannot be ignored when considering dynamics of radiocesium in forest ecosystems is fungi. As we saw in Chap. 3, the major movement of radiocesium, especially in the early phases after the accident, was driven in large part by rain and fallen leaves and branches, but once that movement slowed down, movement by other mechanisms also became noticeable. As we have seen in Sect. 3.4, the movement of radiocesium from the soil to the soil surface organic matter, for example, can also occur. The movement of radiocesium by mycelial functions is also expected to have some influence on the future behavior of radiocesium in the forest.

The most common form of fungi that we see is the fruiting body (mushrooms). Especially in mountain villages, there is a culture of collecting and eating wild mushrooms like in northern Europe and Eurasian region. For more details, please refer to Sect. 6.4, but the radiocesium concentration in mushrooms varies depending on the species, even if they were collected from the same place. Although it has been known for a long time that the ecology and function of mushrooms differ from one mushroom to another, the difference of radiocesium uptake among different mushroom species gives us another glimpse into the diversity of mushrooms [79].

4.3 Effects of the Fukushima Nuclear Accident on Forest Ecosystems

What direct and indirect effects did the radiation have on the creatures in the forest?

4.3.1 Radiation Effects on Living Things

To begin with, there are many naturally occurring radioactive materials on earth. Radiation also comes from space. Therefore, living things on earth are constantly exposed to natural radiation. However, regardless of its origin, it is known that exposure to high doses of radiation can damage cells beyond their natural repair function, resulting in a variety of effects, including morphological abnormalities and even death. The degree of sensitivity to radiation varies from organism to organism. The radiation effects on such organisms can be divided into acute exposure, where the organism is exposed to a relatively high level of radiation for a short time immediately after the accident, and chronic exposure, where the organism is exposed to a relatively low level of radiation for a long time afterwards [80]. In addition, radiation effects need to be considered at various scales: genes, cells, individuals, populations, and ecosystems.

Did the Fukushima nuclear accident in 2011 cause any changes in the plants and animals living in the forest as a result of the radiation? The papers reporting the effects range from insects and birds to mammals and trees [80, 81]. The studies have collected plants and animals in areas of Fukushima with various radiation doses and examined them. In the case of trees, there are reports that there was a correlation between the frequency of morphological abnormalities in fir and pine trees and the intensity of air dose rate [87, 88]. These morphological abnormalities of trees were also seen in the Chernobyl area. While there are many scientific papers showing that radiation had an effect in the contaminated areas, there are also those who say that some reported effect is not clear, and hence the debate is still going on [80, 82,83,84,85]. In fact, there is still continued debate on what the radiation effects were in the Chernobyl nuclear accident, which occurred in 1986, 25 years before the Fukushima nuclear accident [86]. Other factors (e.g., soil, topography, climate, genetic characteristics, etc.) also changed at the same time due to the different collection sites, making it difficult to determine whether the radiation dose actually affected the plants and animals. The effects of earthquakes, tsunamis, loss of human activity, and the lack of pre-accident data have also been pointed out as problems. One distinct effect on forest in the Chernobyl area is Red Forest. In the case of the Chernobyl nuclear accident, the reddish-brown, dead and dying vegetation, was caused by the exposure to very strong radiation (Fig. 4.4), but there was no such high radiation at Fukushima, and no forest die-off occurred. Although we need to continue monitoring, radiation levels will decrease over time, and it may be difficult to imagine any new major effects in the future [89].

Fig. 4.4
figure 4

View of Red Forest (whitish forest in lower right) and Chernobyl Reactor No. 4 (back of photo). In addition to dead trees, you can see the areas that were burned in a forest fire that occurred in 2016 (Source: Reprinted from Beresford et al. [86])

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4.3.2 Forest Ecosystems Without Human Activity

The forest ecosystem has been affected by the large decrease in human involvement in the forest, including in the difficult-to-return areas (areas where people have not lived for a long time, see Sect. 6.1). This is thought to be due to changes in vegetation caused by the lack of human intervention and also due to the absence of humans as a direct threat. Using satellite data, Ishihara et al. found that farmland had changed to grassland in the difficult-to-return areas [90]. With the disappearance of people from mountain villages, animals that used to live in forests began to appear in the villages that had become grassland and in forests near the villages (Fig. 4.5). It has also been observed that the decrease in human use of forests and the absence of humans has led to the flourishing of trees in cultivated areas and also to an increase in wildlife populations [91,92,93]. It has also been reported that the diversity of bird species was proportional to the radiation dose [94]. In particular, some species have been reported to increase the most in the difficult-to-return areas. These significant changes in the area can be attributed to indirect effects of radiation.

Fig. 4.5
figure 5

Wildlife photographed in the evacuation zone. (a) Japanese macaque, (b) Japanese red fox, (c) wild boar (Courtesy of James C. Beasley, University of Georgia, using an automated camera (a, b), and Hirofumi Tsukada, Fukushima University (c))

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The Chernobyl nuclear accident occurred in the spring of 1986, exactly 25 years before the Fukushima nuclear accident. Recent studies have confirmed that the area around Chernobyl is now home to a large amount of wildlife due to the designation as a protected area and the artificial introduction of several species [95].

In addition, many of the forests in Japan have been maintained by human intervention (forest management) in some way. Now that humans are no longer taking care of the forests, maintaining forest health has become a problem. For example, forests that are poorly managed are more prone to disease and insect damage. Even if pests and diseases do occur, forests managed by humans can quickly eliminate the damaged trees and prevent further spread of the damage. In recent years, Japanese oak wilt caused by an insect called the oak ambrosia beetle (Platypus quercivorus) and its associated fungus (Raffaelea quercivora) has become widespread throughout Japan (Fig. 4.6). Japanese oak wilt is known to cause mass mortality of konara oak and mizunara oak (Quercus mongolica var. crispula) that have grown to large diameters without use [96]. If konara oak, which has been the major species for mushroom log forests , is left unattended due to being isolated because of radioactive contamination, there is a risk of serious damage from Japanese oak wilt. In addition, the pine wilt disease, which causes mass die-off of pine trees (pine dieback), is still causing damage in Japan. Since wilt trees become a new source of infection for both oak and pine wilt, it is necessary to manage dead trees to reduce the spread of damage (Fig. 4.7).

Fig. 4.6
figure 6

Broadleaf forest with mass dieback due to Japanese oak wilt (Courtesy of Shoichi Saito, Yamagata University)

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Fig. 4.7
figure 7

Forest landscape affected by pine wilt disease and treatment work for dead pine trees. (a) Forests attacked by pine wilt disease, (b) fumigation of killed pine trees (Courtesy of Katsunori Nakamura, the Forestry and Forest Products Research Institute, taken outside Fukushima Prefecture)

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4.4 Global Fallout: Cesium-137 Has Been in Forest Ecosystems for Half a Century

Radiocesium has been deposited in Japan since more than 50 years prior to the Fukushima nuclear accident due to atmospheric nuclear testing. The deposited radiocesium has also been used as a tool to investigate soil erosion .

4.4.1 What Is Global Fallout?

The accident at the Fukushima nuclear power plant released a large amount of radioactive materials into the atmosphere and contaminated the environment on a large scale, including forests. This led to an increased interest in the Chernobyl nuclear accident that occurred in 1986. Surprisingly, a large amount of anthropogenic cesium-137 was released into the environment 20–30 years earlier than the Chernobyl nuclear accident, in the late 1950s and early 1960s. By the 1980s, more than 500 atmospheric nuclear weapons testing had been conducted, and cesium-137, which reached the stratosphere in explosions, was spread by the jet stream and fell widely and thinly all over the world, mainly in the northern hemisphere [97]. The total amount of cesium-137 emitted from the Chernobyl nuclear accident was several times greater than that from the Fukushima nuclear power plant accident, and the total amount released from the atmospheric nuclear weapons testing was ten times greater than that released from the Chernobyl nuclear accident. The fallout to the earth’s surface of radioactive materials such as cesium-137 from the atmospheric nuclear tests is called global fallout.

In Japan, the amount of cesium-137 fallout has been observed by the Japan Meteorological Agency and other organizations since the 1950s. Figure 4.8 shows the monthly fallout of cesium-137 from the 1950s to the present. The amount of fallout peaked in 1963 and rapidly decreased after the partial nuclear test ban treaty was signed in the same year; it was momentarily high after the Chernobyl nuclear accident in 1986, but returned to a low level in a few months because Japan was far away from Chernobyl. After that, the Fukushima nuclear accident in 2011 caused a sharp increase to levels much higher than the 1963 peak, and then a gradual decrease again.

Fig. 4.8
figure 8

Monthly fallout of cesium-137 observed in various locations of Japan, which was observed by the Japan Meteorological Agency, etc. (Source: Data from Nuclear Regulation Authority, “Environmental Radioactivity Database” [98])

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Ito et al. analyzed cesium-137 in forest soil samples collected throughout Japan just prior to the occurrence of the Fukushima nuclear accident, and found that the average amount of cesium-137 accumulated up to a depth of 30 cm in the soil was 2.27 ± 1.73 kBq/m2, with a higher amount accumulated on the Sea of Japan side from Hokuriku to Tohoku districts (northwestern coastal areas of the main island of Japan) (Fig. 4.9) [99]. As a result of analyzing whether the accumulated amount of cesium-137 based on the above soil analysis differs from those from the long-term monitoring of fallout by the Japan Meteorological Agency, no significant difference was observed between the two, and it became clear that it is highly likely that most of the cesium-137 originating from atmospheric nuclear tests that fell half a century ago still remains in the forest area [99]. The result that cesium-137 remains in the forest even after several decades have actually passed since the fallout is also in line with the results of the study in Sect. 3.6, which showed that the runoff from the forest via stream water is small.

Fig. 4.9
figure 9

Accumulation of cesium-137 in forest soil in Japan before the Fukushima nuclear accident. Decay corrected as of October 1, 2008. Frequency distribution of accumulation is shown in the upper left (Source: Data from Ito et al. [99]; Courtesy of Eriko Ito, the Forestry and Forest Products Research Institute)

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Among the areas contaminated by the Fukushima nuclear accident, in the relatively highly contaminated areas from the eastern to the central part of Fukushima Prefecture, the amount of radiocesium originating from the Fukushima nuclear accident is several orders of magnitude higher than that from the global fallout, but in other areas with low contamination levels, the percentage of detected radiocesium originating from the global fallout is also higher, so care should be taken in interpreting the detected radiocesium. Cesium-134 is not included in the global fallout, since it is rarely produced in nuclear tests and has a short half-life of 2 years. On the other hand, since cesium-134 and cesium-137 were released at a ratio of approximately 1:1 in terms of radioactivity in the Fukushima nuclear accident, it was possible to estimate the ratio of radiocesium originating from the Fukushima nuclear accident by analyzing the ratio of cesium-134 and cesium-137 within a few years of the accident.

4.4.2 Using Radiocesium to Track the Movement of Materials in Forests

Cesium-137 is a radioactive element artificially created by nuclear power plants and nuclear testing, and is also added to forest ecosystems at certain times in the Earth’s history, such as in global fallout, and remains in forests. In addition to being watched and monitored as contaminants for human radiation exposure protection, such radioactive materials have been used in research as tracers to track the movement of materials in forests. A tracer test is a research method used to add a certain amount of easily detectable substance to a system and observe the behavior of the substance within the system, or to investigate the movement of the substance throughout the system. In particular, cesium-137, which is adsorbed by soil and accumulates in the soil surface layer for a long time, has been frequently used as a tracer of soil erosion since the 1970s. Long-term monitoring of the increase or decrease of cesium-137 from its initial deposition provides information on the amount of erosion of soil on the surface and its outflow or storage in the slope [100]. The ratio of cesium-134 to cesium-137 can also be used to determine the depth from which trees and fungi have absorbed radiocesium from the soil surface organic layer and soil. As mentioned above, discriminating the amount of cesium-137 that was previously present in the forest ecosystem as global fallout and the amount of cesium-137 that was newly introduced due to the Fukushima nuclear accident by referring to the percentage of cesium-134 present is also a kind of tracer use. The applicability as a tracer can be enhanced by using multiple elements and isotopes. In this way, cesium-137 originating from the Fukushima nuclear accident also left clear traces in the forest ecosystem, and it can be used in future research as a tool to track the movement of materials in the forest [31, 101, 102].

4.5 Column: Looking Back on that Time (3)

Memoirs of the Fukushima Accident

George Shaw

Emeritus Professor, University of Nottingham

It is impossible to forget the Great East Japan Earthquake and the tsunami which followed. Sitting in our offices and homes in the UK, over 9000 km to the west, we witnessed the terrible sight of the tidal wave washing over Sendai and its surroundings almost in real time via the internet and news media. In the first hours of the disaster on 11th March 2011 we were unaware that, over the next few days, we would witness a succession of explosions at the Fukushima Daiichi nuclear power station, as a direct result of the tsunami. The first of these (reactor 1) occurred on Saturday 12th March and the second (reactor 3) on Monday 14th March. At that point, having watched the accident unfold over the weekend, I emailed some of my colleagues in Chiba and Rokkasho and I was relieved to receive replies that they were safe. News reports in the UK carried shocking movie images of the exploding reactors. I remember thinking clearly that this was very different from the Chernobyl accident because here were the explosions for everyone around the world to see, as they happened.

Back in 1986, the year of the Chernobyl accident, the world was politically and technologically different to what we knew in 2011. I was in the final year of my PhD studies in which I was using radioisotopes to measure metals and nutrients in plants and fungi under laboratory conditions. Everything changed on 28th April 1986 when the world was alerted to a possibly major nuclear accident somewhere in the Soviet Union. I use the words ‘possibly’ and ‘somewhere’ because nobody in the west knew for certain what had happened. The Chernobyl power station had actually exploded on 26th April 1986, 2 days before the alert was sounded in Sweden which received a cloud of radiocaesium fallout from northern Ukraine. This cloud would reach across the whole European continent over the following days, but we still had no details of the nature and extent of the accident which made emergency response very difficult at the time.

The causes of the Chernobyl and Fukushima accidents are hardly comparable. Both events were a consequence of human activities because nuclear reactors are designed and built by people. However, the Chernobyl accident was most definitely the result of human miscalculation whereas the tsunami which triggered the Fukushima accident can be seen almost as an ‘act of God’. The consequences of both accidents are partially comparable. Chernobyl released approximately 10 times more radioactivity than Fukushima, but the suite of radionuclides released and their impacts on the environment have been similar. However, because the Fukushima accident occurred in full visibility and because the Japanese authorities took immediate measures to shield the affected population, the health consequences of the Fukushima accident have been (and are likely to be) much less severe than those of Chernobyl. The environmental impacts of both accidents continue to make themselves known, primarily as a result of contamination of soils and sediments with caesium-137. This radionuclide has a radioactive half-life of 30 years. The soils contaminated by Chernobyl fallout in countries such as the UK currently contain slightly less than half the caesium-137 originally deposited 34 years ago in 1986. Time will eventually reduce this radioactive burden to almost nothing, but not until several decades have passed. During this time major ecosystems such as forests will continuously recycle the caesium, passing small but significant quantities on to other ‘downstream’ environmental systems including rivers, marshes, lakes and, eventually, the ocean. Remediation, or cleaning up, even small areas of contaminated forests is a huge task which may not be feasible, either due to financial cost or undesirable ecological side effects of actions such as removing forest floor litter or cutting down trees on a significant scale. Thus, it will probably be necessary to learn to live with the ongoing contamination, which means understanding which aspects of our interaction with forests are likely to lead to more or less exposure to radiation from caesium-137.

Since 2011 I have met and befriended numerous colleagues in Japan who were at a similar stage in their scientific lives at the time of the Fukushima accident that I was back in 1986. Their learning curve over the past 9 years has been very steep, as it was for me and many of my European colleagues in the 1980s and 1990s. Some of us with experience of the post-Chernobyl impacts on forest ecosystems in Europe and the former Soviet Union have tried to share our experiences with our Japanese friends. This has been a most gratifying process, especially as we have witnessed the diligent and expert way in which they have made thousands of measurements, amassing superb data sets which, because of earlier access to field sites the open and accountable way in which the science has been carried out, are of much better quality than we could achieve in the years after Chernobyl. They are now using these data to construct and refine computer models which will be of enormous help in managing the impacts of caesium-137 contamination of Japanese forests over the coming decades. I know from numerous conversations with my colleagues that they not only have an increasingly excellent scientific understanding of this problem, they also have a deep appreciation of the human cost of the Fukushima accident and that their work is ultimately intended to help alleviate this cost.

figure a

Sampling in Kopachi pine forest, 3.5 km southeast of the Chernobyl nuclear power station. George Shaw is on the left. Taken in 2015 (Courtesy of George Shaw).