Keywords

18.1 Introduction

The radioactive contamination of forests caused by the accident at the TEPCO’s Fukushima Daiichi Nuclear Power Plant has had various effects on people’s lives and livelihood in the satoyama areas of Fukushima Prefecture. The forest products are diverse both in variety and for use. Even if the amount of radioactive materials that fell because of the nuclear power plant accident and the intensity of their radiation are the same, the type of forest products used and how they were used has brought a significantly different impact.

The radioactive contamination of the forests resulted in high air dose rates, which hindered activities such as logging, afforestation, and other forestry work and recreation. This is a serious matter; there are still areas designated as difficult-to-return zones (evacuation zones). The long-term effects on the recovery of daily life and livelihood are enormous. In contrast, the use of forest trees and other forest products contaminated by radioactive materials has been restricted according to the radioactivity levels of the forest products, which also have a significant impact on people’s lives and livelihood. The severity of the impact determined by the radioactivity concentration of the forest products is more complicated than in the case of air dose rates. The overall picture of such radioactive contamination of forests, forestry industry, and the livelihood of people living in satoyama is described in detail in a book based on published data (Hashimoto et al. 2022). To summarize the effects of radioactive contamination on the forestry industry sector, low regulation values of radiocesium activity concentration were set for edible forest products and raw materials for foodstuff production, relatively high index values were set for other cases, or no regulation values (index values) were set for building materials. Under these circumstances, more than 10 years after the nuclear accident, one of the most important issues for the forestry and forest industry in Fukushima is the resumption of full-scale hardwood forest operations (Miura 2016; Hashimoto et al. 2022). In particular, there is a strong desire among forestry business entities in the contaminated areas to seek a pathway to resume the use of hardwood forests for mushroom log production.

18.2 What Is Needed to Resume Hardwood Mushroom Log Forestry in the Abukuma Region?

The Fukushima Daiichi Nuclear Power Plant is located on the eastern side of Fukushima Prefecture facing the Pacific Ocean. Its western hinterland, called Abukuma Highlands, is a gently undulating mountainous area composed of granite, stretching 50 km from east to west and 150 km from north to south. Before the nuclear power plant accident, this area formed one of Japan’s largest mushroom log production areas and supplied many logs outside of Fukushima Prefecture (Miura 2016). However, because of radioactive contamination caused by the nuclear power plant accident, the bed logs and logs for mushroom production were contaminated soon after the accident, and the production and shipments were completely halted across the area. Even now, no one in the Abukuma region has resumed full-scale production of logs as in the past. Only some pulp chips for paper manufacturing are being shipped, and experimental log production is being conducted.

When hardwoods are used as mushroom logs, the greatest concern is the concentration of future trunks used for bed logs. Immediately after the nuclear power plant accident, the Forestry Agency conducted an emergency survey to determine the actual situation regarding the contamination of the logs and bed logs for mushroom production and the radioactive cesium concentration in the produced mushrooms. Based on this survey, in March 2012, the Forestry Agency tentatively determined and noticed index values of 50 Bq/kg for mushroom logs and 200 Bq/kg for sawdust, which can be used for mushroom bed cultivation, and provided guidance to log and sawdust producers and mushroom farmers (Hashimoto et al. 2022). Each producer has hesitated to decide whether to continue log production, because they do not have a clear idea of how many years they should wait before they can resume log production in the hardwood forests where production is currently halted. In other words, forest owners and forestry business entities are faced with the necessity to decide whether to continue the log production business or withdraw from log production. The predicted radiocesium concentration at the time of future harvesting is needed for this purpose.

The Miyakoji Office of Fukushima Central Forestry Association, located in Miyakoji Town, Tamura City, in the Abukuma region, was known as a producer of high-quality mushroom logs (Fig. 18.1a, b). In the satoyama of this area, hardwood trees once used as firewood and charcoal forests were no longer being used owing to the energy revolution. In the Miyakoji area, the Forestry Association took the initiative in converting the forests for firewood and charcoal to log forests for mushroom production in the 1970s. A hundred hectares or more of secondary hardwood forests were cut down each year in the 1990s and 2000s to produce and sell mushroom logs. The forests were cyclically utilized to mushroom log forests, whereby the target area was moved sequentially, returning to the same forest 20 years later to harvest again the coppices that had grown to a size where they could be used as logs (Fig. 18.2). The Forestry Association acted as a coordinator, and the entire community worked together to form sustainable coppicing forests. Under such circumstances, the nuclear power plant accident occurred in March 2011, and all production activities came to a halt. Harvesting forests take 20 years from the time they are cut down to the next harvest. To resume such cyclical sustainable forestry, which was cut off by the nuclear accident, reliable predictions must be made as to whether or not the trunk of mushroom log forests harvested and regenerated now will have dropped below the index value of 50 Bq/kg in 20 years. Although the dynamics of radiocesium in forest ecosystems is complex, (1) the structure of the prediction equation for future forecasting (mechanism of radiocesium dynamics) must be understood, and (2) it must be clarified which of the explanatory variables in the prediction equation have a large contribution to the magnitude of fluctuation (i.e., clarifying the range of variation of the explanatory variables).

Fig. 18.1
A photograph of water-logged paddy fields with a thick forest in the background.figure 1

(a) A mushroom log forest and paddy fields in early summer around the satoyama area in Abukuma Highland at Miyakoji Town, Tamura City, Fukushima. (b) A mushroom log forest in winter at Miyakoji Town

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Fig. 18.2
An illustration of the cycle of management for coppicing hardwood forest. It depicts mature coppice forests, harvesting, regeneration by coppicing, and regeneration by new planting.

Cycle of management for coppicing hardwood forest producing mushroom logs, repeated every 20 years

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This book contains seven such reports on Quercus serrata (Konara oak) or mushroom log forests. After the Fukushima nuclear accident, the resumption of operations in mushroom log forests became a major forestry issue, but it remains unresolved. Studies on mushroom log forests initiated at the University of Tokyo (e.g., Kobayashi et al. 2019a, 2019b) have been vigorously pursued at the Forestry and Forest Products Research Institute (FFPRI; Kanasashi et al. 2020; Kenzo et al. 2020; Sakashita et al. 2021). This chapter introduces the latest major study results of those research groups. They attempted to promote the resumption of mushroom log forests and, in particular, explained the significance of focusing on current-year shoots in the study of coppice forests of Konara oak.

18.3 Overview of Radioactive Contamination in Forests: What We Know So Far

Surveys began immediately after the Fukushima nuclear power plant accident to determine the actual status of radioactive contamination of forests (Kato et al. 2012). According to the results of this prompt survey, the amount deposited on trees above ground exceeded 50% of the total amount of fallout. In addition, the Forestry Agency and national and prefectural agencies, such as the FFPRI and the Fukushima Prefecture, set up new test sites and began fixed-point observation after the nuclear accident (Hashimoto et al. 2022). The survey results revealed that radiocesium was translocated throughout the trees promptly after the onset of radioactive contamination. In August 2011, 5 months after the occurrence of radioactive contamination, the FFPRI surveyed major forest tree species, such as Japanese cedar, in three areas in Fukushima Prefecture, where the initial deposition differed by a hundredfold, to clarify the radioactive cesium concentration and deposition in the major components of the forest ecosystem (Kuroda et al. 2013; Komatsu et al. 2016; Imamura et al. 2017). The higher the radiocesium activity concentration in the wood inside the tree, the more contaminated the study site was; it was proportional to the amount of radiocesium deposited in the forest (Kuroda et al. 2013). Radiocesium had been taken up inside the trees and transported and diffused to a certain extent into the interior of the trunks as early as 5 months after the accident. There are two uptake routes of radiocesium by trees: absorption from the surface of the tree, such as leaves and bark, and absorption from the soil via the roots. Attempts have been made to clarify the contribution rate of these two pathways (Imamura et al. 2021), but the contribution rate is considered to change over time and remains unclear.

The author has outlined the uptake of cesium by trees and its movement within the tree in Fig. 18.3. Tree trunks are covered with bark. Inside the bark is a thin cambium layer where cell division occurs. The bark is divided into two parts: the inner bark, which is living tissue and forms the phloem, and the outer bark, which is composed of dead cork-like tissue. Immediately after the nuclear power plant accident, it is believed that radioactive cesium adhering to the outer bark surface was absorbed into the tree through the bark surface. The phloem, which forms the inner bark, functions to translocate nutrients produced by the leaves to other parts of the tree, and sap flows through it. In contrast, a tissue called the xylem is formed on the inner side of the cambium. The xylem supports the tree and also functions as a water-conducting structure, transporting water absorbed from the roots to the leaves. Ionized radiocesium in the soil and litter is absorbed within the water.

Fig. 18.3
A schematic diagram of the uptake and transport of deposited radiocesium. Water transport occurs through the xylem and transportation and diffusion from the bark of sapwood to heartwood, absorption also takes place via roots along with the dying and shedding of roots.

Schematic overview of the uptake and transport of deposited radiocesium into and within trees

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Recent experimental studies using Japanese cedar have shown that radiocesium injected outside the Japanese cedar sapwood moves toward the center in the radial direction of the sapwood with a speed on the order of days by a combination of active transport and diffusion (Kuroda et al. 2020). Although this experiment was conducted on Japanese cedar, it is likely that radiocesium translocating through the phloem also moves faster than expected in the radial direction inside the trunk in other tree species, such as Konara oak.

The author and coresearchers conducted a whole-tree digging surveyes in 2014 in a coppiced Konara oal forest. The radiocesium in the above- and below-ground biomass of the Konara oak forest distributed as a result of reflecting such transport of cesium within the tree. Three years after the nuclear accident, radiocesium was detected in all parts of Konara oak, from fine roots to the central core of the root stump. An analysis of root data collected in radiocesium surveys of various forests after the Fukushima nuclear accident reported that the radiocesium activity concentrations in roots were relatively higher than those of the soil going down deeper in the soil (Sakashita et al. 2020). Sakashita et al. (2020) gathered that root mortality and shedding may be a driving force for the vertical downward movement of radiocesium in the soil.

These studies on the distribution and movement of radiocesium within trees suggested that radiocesium that fell in forests was incorporated into the movement of material cycles in forest ecosystems more quickly than initially expected and circulated within the forests. Such radiocesium distribution in forests and future radiocesium activity concentrations in trees have also been predicted by models since the earlier phase after the Fukushima nuclear accident (Hashimoto et al. 2013; Thiry et al. 2018). The models have been updated with the addition and enrichment of new observational data (Hashimoto et al. 2020; Thiry et al. 2020). Comparative studies among multiple models have been conducted (Hashimoto et al. 2021; Hashimoto 2023). Given the different responses of multiple models to the same data set, Hashimoto (2023) noted the importance of a better understanding of the root uptake ratio and the biogeochemical processes. The key to the appropriate use of models is to identify factors that significantly impact the radiocesium dynamics within the forest ecosystem and within trees.

18.4 Key Factors of Radiocesium Uptake in Mushroom Log Forests

As mentioned in the previous section, radiocesium in forests moved significantly between soil and trees weekly to monthly. Considering that 10 years have already passed since the fallout of radiocesium into the forest, it is expected that the radiocesium distribution in the trees has neared a state of equilibrium in the soil environment. The average harvesting cycle of hardwood forests, such as Konara oak, used for mushroom cultivation is approximately 20 years, more than half of which has already passed since the nuclear power plant accident occurred. The coppicing ability of hardwood trees decreases with age, so forest owners have to determine whether or not to continue the coppice forest operations by harvesting and coppicing regeneration of the trees, and the time limit for this decision is approaching.

The primary factor determining the degree of radioactive contamination of forest trees is first the amount of initial deposition of radioactive cesium. The second most important factor is how much radiocesium is absorbed by the trees from the soil. Kanasashi et al. (2020) clearly showed this by the 2016–2017 survey results of current-year shoots and soils at 34 sites of coppiced Konara oak forests that were harvested and regenerated after the nuclear accident. The mushroom log production by the Forestry Association in the Miyakoji area had completely stopped because of radioactive contamination caused by the nuclear accident. The study results (Kanasashi et al. 2020) in mushroom log forests in various locations in such an area showed a clear negative correlation between the amount of exchangeable potassium in forest soils at a depth of 0–5 cm and the radiocesium activity concentrations in current-year shoots of Konara oak on both logarithmic graph (Fig. 18.4 [left]). In contrast, the radiocesium activity concentration of current-year shoots did not correlate with the amount of 137Cs in the soil (Fig. 18.4 [right]). Under the homogeneous initial deposition of radiocesium in the study area, the amount of exchangeable potassium was apparently the main factor determining radiocesium uptake by Konara oak rather than the radiocesium inventory. However, this does not mean that the initial deposition of radiocesium, which varied by a factor of 1000 in Fukushima Prefecture, does not affect radiocesium absorption. Undoubtedly, the degree of contamination determined by the amount of initial deposition is the primary factor in the amount of radiocesium absorbed by trees in the wider area. As mentioned earlier, in a survey conducted in summer 2011 in cedar forests with different degrees of contamination, the radiocesium concentration in the wood of trees was proportional to the degree of contamination (Kuroda et al. 2013). However, under the condition of being in an area with the same degree of radiocesium contamination as Miyakoji Town, the amount of exchangeable potassium in the soil has a stronger influence on radiocesium absorption than the slight difference in radiocesium amount in the soil.

Fig. 18.4
2 error bar plots of current shoot versus exchangeable K amount and total C 137 s amount. Both plots depict the majority of error bars lying between 10 and 1000 of the y-axis and 1 and 10 and 10 and 100 of the x-axes for the first and second plots.

Relationship between exchangeable potassium amount (left) or total 137Cs amount (right) in the surface soil of 0–5 cm in depth and 137Cs activity concentration in current-year shoots of Konara oak (Quercus serrate). (Source: Reproduced with data from Kanasashi et al. 2020)

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18.5 Uptake Competition Between Cesium and Potassium

Plant roots absorb alkaline elements, such as potassium and cesium, through transporters and channels. Although cesium is not an essential element for plants, it is a homologous element of potassium. It causes strong absorption competition due to its close ionic radius (White and Broadley 2000; Zhu et al. 2000). Potassium is a major essential element for plants, and the plants actively try to absorb the necessary amount for growth. However, when there is not enough of it in the growth medium, the absorption of cesium, which has a similar chemical form to potassium, increases. Conversely, when there are sufficient potassium ions, cesium absorption is suppressed. This competition with potassium in cesium absorption by plants was known even before the Chernobyl accident (Zhu et al. 2000). Absorption suppression of cesium by potassium was also actively used as a measure against radiocesium contamination in crop production after the Fukushima nuclear accident. For example, testing exchangeable potassium concentrations in soil and applying potassium fertilizers were required for paddy rice cultivation in Fukushima Prefecture (Kato et al. 2015). This competition between cesium and potassium uptake has likely been caused by the amount of exchangeable potassium in forest soils where Konara oak grows (Kanasashi et al. 2020).

In this book, the plant physiological characteristics of cesium absorption in trees are presented in Chap. 5, presenting the results of hydroponic trials conducted with Konara oak and rice as experimental material (Kobayashi et al. 2023, 2019a; Kobayashi and Kobayashi 2023). The competition between potassium and cesium ions also works in root uptake in Konara oak, as confirmed by hydroponic tests (Kobayashi et al. 2019b). However, the selective absorption of potassium observed in rice was not observed in Konara oak (Kobayashi et al. 2019a). Only competition for ion absorption was observed in Konara oak. In addition, in this hydroponic experiment, the difference in the uptake rate of cesium absorbed by leaves and branches was only twofold compared to a 60-fold difference in the potassium ion concentration in the culture medium. In contrast, the multipoint field surveys showed a 100-fold difference in radiocesium activity concentrations in Konara oak shoots against a tenfold difference in exchangeable potassium in the soil at 0–5 cm depth (Kanasashi et al. 2020). In the first place, it is not possible to directly compare the results of hydroponic experiments that examined the characteristics of plant physiological uptake of potassium and cesium ions by roots to the results of studies on the amount of exchangeable potassium in forest soils in the field. However, even taking this into account, it is highly probable that the differences in the amount of exchangeable potassium in forest soils significantly affect radiocesium absorption by Konara oak. This point should be further investigated in other areas to verify the universality of its nature. Furthermore, the possibility that factors other than ion competition, which has been clarified in hydroponic tests, may be involved must also be confirmed from various perspectives.

In contrast, the results of Kanasashi et al. (2020) showed the potential to be used now as a tool for finding available logging areas with low radiocesium absorption. Figure 18.4 (left) shows the relationship between soil exchangeable potassium contents and radiocesium activity concentrations of current-year shoots. However, if we focus only on the radiocesium activity concentration of current-year shoots, one important fact emerges. For a given soil level of exchangeable potassium, the average radiocesium activity concentration of current-year shoots is about one order of magnitude. For example, for an exchangeable potassium level of 20 g/m2 per 0–5 cm depth, the radiocesium activity concentration of current-year shoots is distributed in the range of 200–2000 Bq/kg. At an exchangeable potassium level of 70 g/m2, the radiocesium activity concentrations of current-year shoots range from 40 to 400 Bq/kg. Once the exchangeable potassium level is determined, the range of radiocesium activity concentration in current-year shoots is also determined correspondingly and does not deviate from it by more than two or three orders of magnitude. To predict and estimate radiocesium uptake by Konara oak, it is often assumed that it is necessary to know the amount of exchangeable potassium in the soil, which is the main factor. However, from a different point of view, if one examines the radiocesium activity concentrations of Konara oak shoots in a given forest area, they should fall within a range of about tenfold, no matter where one examines the radiocesium activity concentrations. Although the variation was large (~10 times), a bidirectional 1:1 relationship was observed between the amount of exchangeable potassium in the soil and the radiocesium concentration in current-year shoots of Konara oak. The logging companies need the radiocesium activity concentration in the logs used for mushroom production, not the amount of exchangeable potassium in the soil. Therefore, the results of Kanasashi et al. (2020) indicated that, considering only the practical purpose of determining whether or not a log forest can be used in the future, it is sufficient to examine only the radiocesium activity concentrations in current-year shoots. It is hoped that the development of a practical method to determine the forest area that will be established can be carried out in accordance with this concept, because it is easier to examine the radiocesium activity concentration in current-year shoots than the exchangeable potassium of the soil.

Studies on hardwood log forests focusing on current-year shoots of Konara oak are currently at this stage of development. Methods to predict radiocesium absorption and future radiocesium activity concentrations are still being developed. Further efforts are being made to improve the accuracy of predicting future radiocesium activity concentrations in hardwood forests for mushroom logs by combining the elucidation of mechanisms with the development of practical tools.

18.6 Distinguishing Current-Year Shoots

As mentioned above, the author’s group has focused on current-year shoots, the growing parts of trees that can be easily investigated. Based on this assumption, Sakashita et al. (2021) conducted a detailed study of the seasonal variation of radiocesium activity concentrations in current-year shoots to extend the survey period in the field beyond the winter dormancy period. As a result, the radiocesium concentration in current-year shoots of Konara oak in the Miyakoji area increased rapidly with the opening of leaves in spring, then gradually decreased, and remained relatively stable from August or later (Sakashita et al. 2021). The radiocesium activity concentration of current-year shoots is the lowest and most stable during the dormant period from November after defoliation to April before the next year’s leaf opening. Further analysis of the data obtained in these studies led to the proposal in Chap. 13 of this book of a method to estimate radiocesium activity concentrations of current-year shoots in dormant season from radiocesium activity concentrations of leaves (Sakashita et al. 2023). In addition, enabling surveys with leaves from summer to autumn would have an advantage in forest management planning, because the radiocesium absorption characteristics of the targeted forests can be evaluated before autumn, when harvesting and regeneration of log forests are at full-scale.

The following paragraphs will explain how to identify current-year shoots in the field, which are useful as an indicator of radiocesium absorption by trees.

Current-year shoots can be identified by careful observation of the shoots. Deciduous trees form axillary and apical buds at the base of leaves and the tips of shoots in the summer to autumn of the previous year, which are the source of growth for the following year’s leaves and branches. After leaf fall, the axillary and apical buds remain on the shoot covered by bud scales, stop growing, and attach to the shoot over winter. These are called winter buds. In spring, winter buds swell and differentiate into leaves and branches (shoots), which begin to grow; in summer to autumn, winter buds form again on the shoots that have grown that year. Figure 18.5a illustrates a current-year shoot with leaves collected at the beginning of October and a current-year shoot after leaf fall collected in April. Whether or not a branch is a current-year shoot can be determined by how the leaves grow. A shoot with leaves attached directly to the branch is a current-year shoot (Fig. 18.5a, b). After the leaves fall in the autumn, winter buds in the middle or at the apex of the shoot can be used to determine current-year shoots. Some short current-year shoots may not have leaves or winter buds in the middle of the branch. However, if they are current-year shoots, winter buds (apical buds) will still form at the tips of the elongated shoots. Most winter buds on the current-year shoot will open and grow into leaves or shoots during the next growing season. Even if they do not open, they are no longer living buds, so if a dormant winter bud is dead, it is considered an old shoot from the previous year. Especially, because winter buds are more conspicuous on a well-grown branch, it is not so difficult to determine the current-year shoot, if actually experienced.

Fig. 18.5
3 photographs of current year shoots and previous year branches by leaves and winter buds and identification of primary, secondary, and tertiary growths. It depicts node without bud, bud, growth unit boundary, bud and scar, and others for current year shoots and previous year branches.figure 5

(a) Identification of current-year shoots and previous year branches by leaves and winter buds (collected at the beginning of October). Source: The original figure was created by Masaya Masumori (The University of Tokyo). (b) Identification of primary, secondary, and tertiary growths within the current-year by winter buds and nodes (collected in the middle of April)

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Like leaves, current-year shoots are the parts of the tree that grow during the current-year, and they accumulate nutrients and minerals absorbed from the soil. They are also relatively easy to identify along with leaves. However, there are some points to note when actually sampling in the field. During the growing season from spring to summer, current-year shoots may undergo a secondary or tertiary new growth. In the Miyakoji area, where the survey was conducted, winter buds open and new leaves begin to develop around early May. After that, depending on the weather conditions, new shoots may grow again in July, August, or September, when the lingering summer heat is severe. This is called secondary growth. In the upper canopy of trees, even if the tip of a shoot is branched, there are often cases in which the root side of the branch is also a current-year shoot (Fig. 18.5b). Therefore, the determination of the current-year shoot should be based on the attachment of leaves or winter buds to the shoot, not just branching characteristics at the nodes. Because Kanasashi et al. (2020) did not distinguish among primary, secondary, and tertiary growth of current-year shoots, the variation in the relationship between soil exchangeable potassium and radiocesium activity concentrations in current-year shoots may be due in part to the mix of secondary and nonsecondary growth in current-year shoots. The effect of conducting current-year shoots survey without distinguishing between primary-grown and secondary- or tertiary-grown shoots on the uncertainty of radiocesium activity concentration of current-year shoots remains to be evaluated.

18.7 Measures by Potassium Fertilization to Reduce Radiocesium Absorption

Up to this point, studies have focused on the distribution of exchangeable potassium in forest soils and explained how to find usable forests among contaminated mushroom log forests. The idea is to use forests as they are to the extent possible without incurring costs, which can be considered a passive measure. This is an extension of the long-established relationship with forests cultivated by people living in the satoyama area, and it is a rational approach. In contrast, there is also an attempt to apply potassium fertilization to trees in forests, which was carried out on a large scale in Fukushima Prefecture to suppress the absorption of radiocesium in farmland.

In 2014, a potassium fertilization experiment was conducted when planting Japanese cypress (Chamaecyparis obtusa) seedlings in a radioactively contaminated forest, and the suppression effect of potassium on the absorption of radioactive cesium was confirmed (Komatsu et al. 2017). The author’s group also conducted potassium fertilization experiments in coppice forests of Konara oak and reported the results in Chap. 17 of this book (Masumori et al. 2023). The year after fertilization, soil exchangeable potassium increased and radiocesium activity concentrations in current-year shoots of Konara oak decreased. However, a year later, the exchangeable potassium concentration decreased again and returned to the original level, and the radiocesium activity concentration of current-year shoots also increased again (Masumori et al. 2023). In contrast, in Sweden, a one-time application of potassium chloride fertilizer at 100 kg/ha by weight of potassium was tested in 1992 to suppress radiocesium absorption by trees, mosses, and fungi in shrubland contaminated by the Chornobyl nuclear accident (Rosén et al. 2011). A follow-up study conducted 17 years later until 2009 showed that radiocesium activity concentrations in trees, mosses, and fungi were significantly lower in the potassium-applied area than in the control area. In Japanese coniferous forests, the cycle extends at least 40–50 years until harvest, when trees are used for lumber forests, and even secondary hardwood forests for mushroom logs require 20 years. When potassium fertilization is applied to suppress radiocesium absorption in forests, it is necessary to carefully examine whether the application is sufficiently effective until the harvesting period and whether the radiocesium concentrations in logs and other products at harvest can be lowered to the target level.

As a study focusing on the adsorption characteristics of potassium and cesium on soil, four different extractants were used to extract potassium from soils in Chap. 15 of this book, and tests were also conducted on soil from areas outside of Fukushima Prefecture (Kobayashi et al. 2023). Different extractants were analyzed for nonexchangeable and exchangeable potassium to determine the relationship with the transfer coefficient of 133Cs in current-year shoots of Konara oak, with the result that the concentration of nonexchangeable potassium extracted with thermal nitric acid had the highest correlation with the transfer coefficient (Kobayashi et al. 2023). In agricultural soils, the potassium extraction characteristics of soils have revealed that differences in clay mineral composition are an important factor affecting the effectiveness of potassium application (Eguchi et al. 2015). Cesium and potassium adsorption characteristics in forest soils are expected to vary greatly depending on the soil parent material and other soil environmental factors. Detailed studies focused on clay mineral composition are also desirable.

The research group of the University of Tokyo and Forestry and Forest Products Research Institute, in cooperation with mushroom log or log-cultivated mushroom producers, started trial planting experiments of Konara oak or sawtooth oak Quercus acutissima (sawtooth oak), which was newly planted in an area where log production had been halted due to radioactive contamination. A treatment area was established with and without potassium fertilizer application. Although it will take another 12–15 years before the planted oak trees will be used as mushroom logs, it is important to keep in mind that the research should be conducted in dialogue with those producers to establish with the local people and effectively deliver the research results.

18.8 Conclusion

This chapter described the results of the studies the author has been conducting for the past 8 years to contribute to the resumption of hardwood forestry operations in radiation-contaminated mushroom log forests. The studies focused on using radiocesium activity concentrations in current-year shoots as an indicator to clarify the characteristics of radiocesium absorption from the soil by coppiced Konara oak to predict the future contamination levels. There has been significant progress in the studies of hardwood log forests. Although not mentioned in this chapter, important themes should be further addressed in the future, such as the effects of topography on radiocesium absorption by trees. It is hoped that conducting studies while keeping dialogue with residents and the forestry industry frontliners can contribute to the recovery of the livelihood of people in the affected areas and the forestry industry. In the field of forestry, which has a long production period, a shortcut to solving problems is to gain a deeper understanding of the scientific mechanisms by which Konara oak absorbs cesium from the soil and transports it within the tree. Further studies should focus on developing practical measures backed by scientific evidence.