Circulation of 137 Cs in Various Forest Plants in the Chornobyl Exclusion Zone during the Year

: This study investigated the content of 137 Cs (a long-lived radioactive isotope of caesium) in various parts of Pinus sylvestris L. (Scotch pine) and Dicranum polysetum Sw. (rugose fork-moss) at three different sites within the exclusion zone of the Chornobyl nuclear power plant over two years. The Leliv site is located within the 10 km zone, while the Paryshiv and Dytiatky sampling sites are within the 30 km zone. Samples of different P. sylvestris organs were collected, including 1-and 2-year-old branches and needles and wood and outer bark, and the entire D. polysetum . Sampling was conducted every two weeks throughout the year during 2014 and 2015. The speciﬁc activity levels of 137 Cs in the samples were measured using gamma spectrometry with a CANBERRA gamma spectrometer unit and a coaxial high-purity HPGe semiconductor detector. The study found that at the Leliv and Paryshiv sites, the highest content of 137 Cs in living organs of P. sylvestris was found in the wood. At the Dytiatky site, the needles and branches of the ﬁrst and second years had anomalously high concentrations of radiocaesium ( 137 Cs). This could be due to a thin layer of forest litter (1.5 cm) at that site. The study also found signiﬁcant changes in the speciﬁc activity levels of 137 Cs in living pine organs throughout the year. The highest concentration was observed in pine branches and needles in summer, and the maximum values in wood were observed in winter. The study suggests that a constant circulation of 137 Cs in the soil–plant system can cause seasonal changes in the content of 137 Cs in living pine organs. Symbiotic mycorrhizal fungi can play an important role in the circulation of radiocaesium in forest ecosystems. The outer bark of P. sylvestris did not show any seasonal changes in the content of 137 Cs. It may not be involved in radiocaesium redistribution inside the plant but can serve as a long-term source of this radionuclide entering the forest litter. The study found no seasonal changes in the accumulation of 137 Cs by D. polysetum , which might be due to the physiological characteristics of this plant species. Based on the analysis of the conducted studies, the recommendation is to consider the seasonal changes in the content of 137 Cs during monitoring activities and when using Scots pine in areas potentially contaminated with this radionuclide.


Introduction
Since the mid-20th century, long-term studies have examined the effects of radionuclides that contaminated the Earth due to global fallouts from nuclear weapons testing and nuclear accidents on different ecosystems. In the case of the Chornobyl Nuclear Power Plant (ChNPP) disaster, large areas of the northern hemisphere were contaminated with radiocaesium, with the most significant contamination occurring in Europe. The ongoing research into the effects of radiocaesium contamination highlights the importance of monitoring and the possibility of mitigating the impacts of nuclear accidents on the environment [1][2][3][4][5][6][7][8][9][10].
In the case of the Fukushima Dai-ichi nuclear disaster, the release of radiocaesium and other radionuclides resulted in the contamination of the surrounding areas, including air, water, and soil. This accident served as a reminder that the use of atomic energy by humans still carries the risk of releasing various radionuclides, including radiocaesium, The content of 137 Cs in mushrooms of the same species can vary by an order of magnitude throughout the year. The highest level of specific activity of this radionuclide in mushrooms from June to December was observed in October [46].
In the radioecology of the forest, there is still no unambiguous explanation for the fluctuations in the values of the coefficients of radiocaesium transfer from the soil to various plants and mushrooms [25,26,44]. Significant sudden changes in the levels of 137 Cs-specific activity in different forest objects during the study of long-term dynamics [47] have yet to be fully explored. Changes in weather conditions are most often indicated as the causes of these phenomena [48]. However, there may be other explanations. Sharp changes in the levels of specific activity of 137 Cs in the studied objects in different years can be explained by seasonal fluctuations in the content of this radionuclide.
This study aimed to investigate whether there are seasonal changes in the concentration of the radioactive isotope 137 Cs in plants within the forest surrounding the Chornobyl nuclear power plant and to interpret the implications of any observed changes. Seasonal changes in 137 Cs concentration in plants could provide insight into how this radioactive material is transported and distributed within the ecosystem.
To achieve the study's goal, sampling sites were laid in the Chornobyl NPP's exclusion zone, and a sampling technique was developed at the established landfills. Samples of different parts of Scotch pine and Dicranum moss were collected following the proven methodology, and the content of 137 Cs was measured in the samples. The research results were analysed, and a possible explanation was put forward. Finally, conclusions were drawn on the stated goal.
The findings may contribute to a better understanding of the behaviour of 137 Cs in forest ecosystems and help to develop more effective strategies for mitigating the impact of radioactive contamination.

Materials and Methods
The author developed a unique methodology for studying the redistribution of 137 Cs in forest ecosystems after the Chornobyl accident. The method involves the frequent and year-round sampling of cosmopolitan plant species, including living and dead plant parts. The selection of easily accessible plant parts for sampling and the careful laying of the test sites are additional features of the methodology.
The study was conducted almost 30 years after the accident, allowing the investigation of these processes' long-term and chronic nature. Through the analysis of their results, the researcher could identify regular patterns of 137 Cs redistribution in the studied plant species, which occurred throughout the year and could be assessed in terms of their frequency and scope. Overall, this study sheds light on the long-term effects of 137 Cs contamination on forest ecosystems and how these ecosystems continue to be impacted over time.
However, the methodology may have limitations when applied to deciduous forests due to the annual fall of leaves, which reduces the volume of information gathered about the redistribution of 137 Cs in living plant parts throughout the year.
Overall, the methodology appeared well-suited for studying the redistribution of 137 Cs in forest ecosystems and was developed per methodological recommendations for monitoring work in the Chornobyl exclusion zone [49].

Sampling Sites
137 Cs circulation in various organs of the Scots pine and mosses in the exclusion zone of the Chornobyl nuclear power plant (ChNPP) was researched. The sampling sites are located at various distances and directions from the Chornobyl NPP ( Figure 1) chosen sampling sites in the Chornobyl Exclusion Zone are identified by the following coordinates: Dytiatky (30.12449 E, 51.13088 N, distance from ChNPP, 29.9 km), Paryshiv (30.32473 E, 51.30069 N, distance from ChNPP, 18.8 km), Leliv (30.16031 E, 51.32326 N, distance from ChNPP, 8.4 km). A sampling of 1st and 2nd branches and needles was conducted in 2014 and 2015, once every two weeks. He outer bark was sampled from February 2014 to December 2015, and moss from May 2014 to December 2015. Wood was sampled only from November 2014. The studied sampling sites had an area of approximately 30 m 2 and a sod-podzolic soil. The vast majority of woody vegetation was the Scots pine (Pinus sylvestris L.).

Sampling of Various Objects of Forest Ecosystems
The sampling regime was once every two weeks. Such organs of P. sylvestris as the wood, the outer bark, branches, one-year and two-year needles and moss Dicranum polysetum Sw. were chosen as the objects of the study. The main reasons for using P. sylvestris as the main object of research were: -in the forest massifs of the Chornobyl exclusion zone, pine forests occupy almost 60% of the total area of 91,565.0 ha [50], and this species is dominant; -samples of photosynthetic organs of this plant are available throughout the year; -the capacity to simultaneously sample branches and needles of different ages.
Samples of branches and needles were taken with a pruner. A sampling of the outer bark was carried out with the help of a knife. The wood was selected with the help of a 30 cm long Preissler drill. Before using the drill, a trunk part was cleaned of the outer and inner bark. After sampling, the places of the trunk with the bark removed were covered with a cut paste to prevent the tree's death. Moss samples were taken using a 10 × 10 cm frame. Moss Dicranum was chosen as an object of research because this plant is evergreen, so the processes of redistribution of 137 Cs can occur throughout the year.
Fifteen model trees were used in the study at each sampling site. Samples were taken each time in the following amount:

Sampling of Various Objects of Forest Ecosystems
The sampling regime was once every two weeks. Such organs of P. sylvestris as the wood, the outer bark, branches, one-year and two-year needles and moss Dicranum polysetum Sw. were chosen as the objects of the study. The main reasons for using P. sylvestris as the main object of research were: -in the forest massifs of the Chornobyl exclusion zone, pine forests occupy almost 60% of the total area of 91,565.0 ha [50], and this species is dominant; -samples of photosynthetic organs of this plant are available throughout the year; -the capacity to simultaneously sample branches and needles of different ages.
Samples of branches and needles were taken with a pruner. A sampling of the outer bark was carried out with the help of a knife. The wood was selected with the help of a 30 cm long Preissler drill. Before using the drill, a trunk part was cleaned of the outer and inner bark. After sampling, the places of the trunk with the bark removed were covered with a cut paste to prevent the tree's death. Moss samples were taken using a 10 × 10 cm frame. Moss Dicranum was chosen as an object of research because this plant is evergreen, so the processes of redistribution of 137 Cs can occur throughout the year.
Fifteen model trees were used in the study at each sampling site. Samples were taken each time in the following amount: -15 one-year and 15 two-year branches with needles; -10-12 pieces of bark; -10-12 wood samples.
A moss sample was taken from the soil surface in triplicate at each sample site. There were 6 samples of Scottish pine organs: needles of the first year, branches of the first year, needles of the second year, branches of the second year, bark, wood, and 1 sample of moss. The branches were detached from the needles in a stationary laboratory. Then, all samples were dried separately for 1-1.5 months at room temperature (18-20 • C), ground using a laboratory mill and placed in disposable calibrated plastic cups with a diameter of 9 cm and a height of 6 cm for gamma spectrometric studies.

Radiometry
The specific activity of 137 Cs was measured using a CANBERRA gamma spectrometer unit with a coaxial high-purity HPGe semiconductor detector (GC6020 model). The detection unit was covered with a 100 mm lead screen, which allowed practical measurements of samples with relatively low radionuclide-specific activity. A schematic block diagram of the gamma spectrometer is shown in Figure 2. Figure 3 shows a photo of the gamma spectrometric setup used to determine the specific activity of 137 Cs in the samples and the spectrum displayed on a computer screen.
There were 6 samples of Scottish pine organs: needles of the first year, branches of the first year, needles of the second year, branches of the second year, bark, wood, and 1 sample of moss. The branches were detached from the needles in a stationary laboratory. Then, all samples were dried separately for 1-1.5 months at room temperature (18-20 °C), ground using a laboratory mill and placed in disposable calibrated plastic cups with a diameter of 9 cm and a height of 6 cm for gamma spectrometric studies.

Radiometry
The specific activity of 137 Cs was measured using a CANBERRA gamma spectrometer unit with a coaxial high-purity HPGe semiconductor detector (GC6020 model). The detection unit was covered with a 100 mm lead screen, which allowed practical measurements of samples with relatively low radionuclide-specific activity. A schematic block diagram of the gamma spectrometer is shown in Figure 2. Figure 3 shows a photo of the gamma spectrometric setup used to determine the specific activity of 137 Cs in the samples and the spectrum displayed on a computer screen. The obtained graphs showed measurement data on the specific activity of 137Cs with measurement errors. The measurements were carried out for 600-14400 s, depending on the specific activity of 137Cs. The measurement errors did not exceed 10% and were within 3-5% of radionuclide activity [51].
The study of the dynamics of the content of radiocaesium (the physical half-life of 137 Cs is 30.05 years [52]) in an object requires the exclusion of the influence of its physical decay. April 26, 1986 was chosen as the "zero" date, on which the results of the studies were recalculated.
The concentrations of 137 Cs in various objects of the forest ecosystems were calculated using the radioactive decay formula; the specific activity of 137 Cs in the samples (dry weight) is expressed in Bq/kg.

Correlation Analysis
A correlation analysis was carried out to check the presence or absence of a relationship between the content of 137 Cs in the various studied objects. Spearman's rank correlation coefficients were calculated. This coefficient is used to identify and determine the magnitude of the relationship between data. The ranks of the compared values are used as data. No preliminary assumptions about the nature of the distribution of features in the general population are required to calculate the Spearman coefficient.
The following formula calculates the Spearman's rank correlation coefficient:  The obtained graphs showed measurement data on the specific activity of 137Cs with measurement errors. The measurements were carried out for 600-14,400 s, depending on the specific activity of 137Cs. The measurement errors did not exceed 10% and were within 3-5% of radionuclide activity [51].
The study of the dynamics of the content of radiocaesium (the physical half-life of 137 Cs is 30.05 years [52]) in an object requires the exclusion of the influence of its physical decay. April 26, 1986 was chosen as the "zero" date, on which the results of the studies were recalculated.
The concentrations of 137 Cs in various objects of the forest ecosystems were calculated using the radioactive decay formula; the specific activity of 137 Cs in the samples (dry weight) is expressed in Bq/kg.

Correlation Analysis
A correlation analysis was carried out to check the presence or absence of a relationship between the content of 137 Cs in the various studied objects. Spearman's rank correlation coefficients were calculated. This coefficient is used to identify and determine the magnitude of the relationship between data. The ranks of the compared values are used as data. No preliminary assumptions about the nature of the distribution of features in the general population are required to calculate the Spearman coefficient.
The following formula calculates the Spearman's rank correlation coefficient: where n-number of paired ranked signs (sample size); D-difference between the ranks of the conjugate values of the features; ∑(D 2 )-sum of the squared differences in ranks [53].

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To assess the existence of a relationship between variables, the Chaddock scale is used, according to which, with a correlation coefficient from 0 to 0.3, the strength of the relationship is considered very weak; from 0.3 to 0.5, it is weak; from 0.5 to 0.7, it is average; from 0.7 to 0.9 it is high; from 0.9 to 1 it is very high.
With a negative correlation, the values of the strength of the connection between the variables are reversed.

Statistical Analysis
Statistical analysis and graphical visualisation were performed using GraphPad Prism 9.5.1 (GraphPad Software, San Diego, CA, USA). Statistics were performed using two-way ANOVA and Tukey's multiple comparisons tests. The data are presented as means ± standard deviation (SD). A p-value ≤ 0.05 was considered statistically significant. In the graphs, the variables of significance are labelled with asterisks (* p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001).

Results
The research identified trends in the changes of 137 Cs concentration in one-and twoyear-old branches, needles, and wood of P. sylvestris. The value of the specific activity of radiocaesium in these pine organs on the territory of all sampling sites was found to depend on the year's season (Figures 4-6).
n-number of paired ranked signs (sample size); D-difference between the ranks of the conjugate values of the features; ∑( )-sum of the squared differences in ranks [53].
To assess the existence of a relationship between variables, the Chaddock sca used, according to which, with a correlation coefficient from 0 to 0.3, the strength of relationship is considered very weak; from 0.3 to 0.5, it is weak; from 0.5 to 0.7, it is a age; from 0.7 to 0.9 it is high; from 0.9 to 1 it is very high.
With a negative correlation, the values of the strength of the connection between variables are reversed.

Statistical Analysis
Statistical analysis and graphical visualisation were performed using Graph Prism 9.5.1 (GraphPad Software, San Diego, CA, USA). Statistics were performed u two-way ANOVA and Tukey's multiple comparisons tests. The data are presented means ± standard deviation (SD). A p-value ≤ 0.05 was considered statistically signific In the graphs, the variables of significance are labelled with asterisks (* p ≤ 0.05, ** p ≤ 0 *** p ≤ 0.001).

Results
The research identified trends in the changes of 137 Cs concentration in one-and t year-old branches, needles, and wood of P. sylvestris. The value of the specific activit radiocaesium in these pine organs on the territory of all sampling sites was found to pend on the year's season (Figures 4-6).    The specific activity level of 137 Cs varied among branches and needles of differ ages. On average, the specific activity was higher in one-year-old needles compared two-year-old needles and one-and two-year-old branches. The lowest content of 137 Cs found in two-year-old needles at the Paryshiv and Leliv sampling sites, whereas at Dytiatky site, the concentration of 137 Cs in two-year-old needles was higher than in t year-old branches (Table 1, Figure 7a  The specific activity level of 137 Cs varied among branches and needles of different ages. On average, the specific activity was higher in one-year-old needles compared to two-year-old needles and one-and two-year-old branches. The lowest content of 137 Cs was found in two-year-old needles at the Paryshiv and Leliv sampling sites, whereas at the Dytiatky site, the concentration of 137 Cs in two-year-old needles was higher than in two-year-old branches (Table 1, Figure 7a,b).      Figure 7 display the average 137 Cs content in the wood and outer bark of P. sylvestris at the sampling sites. At the Leliv and Paryshiv sites, the highest 137 Cs content among living tissues was observed in the wood, and the highest concentration among all studied pine organs was found in the dead outer bark. At the Dytiatky sampling site, the most contaminated part of P. sylvestris was one-year-old needles, with almost three times higher concentrations of radiocaesium compared to that at the Leliv site and 11 times higher than that at the Paryshiv site.
The performed statistical analysis (Figure 7) indicated the reliability of the differences in 137 Cs accumulation at the three sampling sites in annual needles, annual branches and outer bark. For the rest of the studied pine organs, the differences in the concentration of this radionuclide at different sites were not statistically significant.
The maximum specific activity of 137 Cs in one-year-old branches and needles was reached between May and August 2014 and 2015 in all sampling sites. The subsequent increase in specific activity levels of 137 Cs in branches and needles was observed in the second year of their life during the next growing season (see Figures 4-6).
The content of 137 Cs in different parts of P. sylvestris was analysed, and the parameters' correlation was determined using the Spearman method. The results, which can be found in Table 2, showed a strong direct relationship for the radiocaesium content between annual needles and annual branches, as indicated by Spearman's correlation coefficients exceeding 0.7 in all sampling sites. Consequently, changes in 137 Cs content in these pine organs occur almost simultaneously. Similarly, high correlation coefficients were observed for other pairs of objects, such as 1st branches-2nd branches, 1st needles-2nd needles, and 2nd needles-2nd branches, but this was not the case for all sampling sites. These results indicate that changes in the level of 137 Cs in different organs of P. sylvestris during the study period often do not occur simultaneously.
Significant seasonal changes in the content of 137 Cs in wood in all areas of the study could not be recorded. One can speak of a seasonal trend in the accumulation of this radionuclide in Leliv and Paryshiv (Figure 8). In the territory of these sampling sites, the highest values of 137 Cs specific activity levels were observed from November 2014 to March 2015 and at the end of 2015. The lowest values of the specific activity of 137 Cs in wood in Leliv and Paryshiv were observed from mid-summer to mid-autumn. In Dytiatky, the minimum values of the particular radiocaesium activity were noted in different seasons-winter, summer and autumn. onuclide in Leliv and Paryshiv (Figure 8). In the territory of these sampling sites, the highest values of 137 Cs specific activity levels were observed from November 2014 to March 2015 and at the end of 2015. The lowest values of the specific activity of 137 Cs in wood in Leliv and Paryshiv were observed from mid-summer to mid-autumn. In Dytiatky, the minimum values of the particular radiocaesium activity were noted in different seasonswinter, summer and autumn.  During the research period, low levels of specific activity in wood, which did not exceed 200 Bq/kg, were observed at two sampling sites-Paryshev and Dityatki. However, at the Leliv sampling site, the content of 137 Cs in wood ranged from 400 to 703 Bq/kg.
The lowest values of 137 Cs specific activity in wood at the Leliv and Paryshiv sampling sites were observed from mid-summer to mid-autumn. In contrast, at the Dytiatky site, the minimum specific activity of radiocaesium was noted during various seasons, including winter, summer and autumn. It was impossible to establish any seasonal trends in the accumulation of 137 Cs in wood at this site. Figure 9 shows the results of the measurements of the specific activity of 137 Cs in the outer bark of P. sylvestris. Seasonal trends in the accumulation of radiocaesium in this pine organ were not observed at any of the sampling sites.
Ecologies 2023, 4, FOR PEER REVIEW 10 During the research period, low levels of specific activity in wood, which did not exceed 200 Bq/kg, were observed at two sampling sites-Paryshev and Dityatki. However, at the Leliv sampling site, the content of 137 Cs in wood ranged from 400 to 703 Bq/kg.
The lowest values of 137 Cs specific activity in wood at the Leliv and Paryshiv sampling sites were observed from mid-summer to mid-autumn. In contrast, at the Dytiatky site, the minimum specific activity of radiocaesium was noted during various seasons, including winter, summer and autumn. It was impossible to establish any seasonal trends in the accumulation of 137 Cs in wood at this site. Figure 9 shows the results of the measurements of the specific activity of 137 Cs in the outer bark of P. sylvestris. Seasonal trends in the accumulation of radiocaesium in this pine organ were not observed at any of the sampling sites. As a result of the research, seasonal trends in the specific activity levels of 137 Cs in moss were not detected ( Figure 10). During the year, there were increases and decreases in specific activity levels in moss at all sampling sites, which practically did not correlate with the seasonal changes in radiocaesium content in different pine organs. As a result of the research, seasonal trends in the specific activity levels of 137 Cs in moss were not detected ( Figure 10). During the year, there were increases and decreases in specific activity levels in moss at all sampling sites, which practically did not correlate with the seasonal changes in radiocaesium content in different pine organs.
As a result of the research, seasonal trends in the specific activity levels of 137 Cs in moss were not detected ( Figure 10). During the year, there were increases and decreases in specific activity levels in moss at all sampling sites, which practically did not correlate with the seasonal changes in radiocaesium content in different pine organs.

Discussion
The concentration of 137 Cs in plants in territories contaminated by nuclear accidents depends on the level of contamination [54,55]. The results obtained from the Leliv and Paryshiv sampling sites (see Figures 4,5 and 8-10, Table 1) confirmed this regularity. The average radiocaesium content in various parts of the examined pine in Leliv, which is situated within the 10 km exclusion zone, was higher than that in Paryshiv. The decreasing order of the average 137 Cs content in different pine organs at these sites is outer bark > wood > firstyear needles > first-year branches > second-year branches > second-year needles.
At the Dytiatky sampling site (see Figure 6 and Table 1), a deviation from this regularity was observed, with needles of the first year showing higher levels of 137 Cs compared to the other organs, followed by branches of the first year, needles of the second year, branches of the second year, outer bark and wood.
The peculiarities of 137 Cs accumulation by various pine organs in Dytiatky cannot be explained by the magnitude of 137 Cs contamination of the soil at this site because the content of this radionuclide in the soil at the Dytiatky sampling site was the lowest. The A 0 l layer in Leliv contained 137 Cs 598 ± 321 Bq/kg, that in Paryshiv, 164 ± 80 Bq/kg, and that in Dytiatky, only 149 ± 107 Bq/kg. The concentration of 137 Cs in A 0 f + A 0 h in Leliv was 49,171 ± 16,054, that in Paryshiv −6059 ± 2949, and that in Dytiatky −2490 ± 867 Bq/kg. An exception was the soil layer of 0-5 cm, in which the content of 137 Cs was higher in Dytiatky than in Paryshiv. The radiocaesium content in the 0-5 cm soil layer in Leliv was 12,844 ± 5855, that in Paryshiv, −862 ± 428, and that in Dytiatky, −1034 ± 566 Bq/kg [56].
According to [57], the level of 137 Cs accumulation by plants may depend on various factors, including the thickness of the litter layer. The thickness of the A 0 l + A 0 f + A 0 h soil layer at the Dytiatky sampling site is significantly different from those at other sites. At Dytiatky, the litter layer is only 1.5 cm thick, while at other sites, it is 7 cm thick. The A 0 l + A 0 f + A 0 h layer with less fungal mycelium and another soil biota can allow radiocaesium to penetrate plants more easily and its outflow into the soil. Symbiotic organisms can retain a significant portion of 137 Cs from the soil, as evidenced by this radionuclide's higher content in symbiotic fungi than symbiont plants [44,58]. It is likely that 137 Cs first accumulates in shoots and needles before being redistributed into the wood and retained in the living bark once it enters a plant. At Dytiatky, the lower number of symbiotic organisms increases the availability of 137 Cs for plants but also allows it to "leak" easily from pine trees into the soil, decreasing the possibility of its accumulation in wood and living bark, with the greater outflow of caesium from the pine.
The concentrations of radiocaesium in the living organs of P. sylvestris exhibited a seasonal trend, varying throughout the year. At all sampling sites, a significant increase in the levels of specific activity of 137 Cs in the needles and branches of the first year was recorded since the beginning of their growth in late April to early May. This trend was also observed in two-year-old needles and branches (see . This suggests that as the summer progresses, the growth of young needles and branches slows down, leading to a decline in the concentration of 137 Cs in these organs in the middle of summer.
The seasonal changes in the levels of specific activity of 137 Cs in pine branches and needles can be explained by the existence of two differently directed processes occurring in the plant: accumulation and excretion of radiocaesium, i.e., the circulation of this radionuclide in the soil-plant system. These processes occur simultaneously, and the predominance of one of them leads to an increase or decrease in the content of 137 Cs in the needles and branches of P. sylvestris during the year. A seasonal trend in the redistribution of 137 Cs may indicate ascending and descending fluxes of this radionuclide in wood.
Based on the study of some tree species in Japan after the accident at the Fukushima Dai-ichi nuclear power plant, the authors of the work suggested that in these plants, there is both an inflow and an outflow of radiocaesium, and a decrease in its content in plants is a consequence of an excess of outflow over the entry of this radionuclide into them [59]. In [19], a downward flow of 137 Cs in wood is indicated as a possible interpretation of the obtained research results. The constant circulation of the chemical analogue of caesium, potassium, in the soil-plant system was described [60,61]. Probably 137 Cs also circulates in plants. With the beginning of the growing season, its additional entry into plants occurs. By the time the intensive growth stops, the outflow of radiocaesium from the needles and branches begins to prevail over its accumulation, and the levels of the specific activity of this radionuclide decrease.
The main radiocaesium fluxes in forest ecosystems may occur in the fungal symbiontpine system rather than in the soil-pine system. In the soil-pine system, the circulation of 137 Cs can be influenced by symbiotic soil organisms, particularly mycorrhiza-forming fungi, which are essential for P. sylvestris. Early studies after the Chornobyl accident suggested that the fungal mycelium can contain up to 63% of the total 137 Cs reserve in the forest soil, making it a long-term depot for this radionuclide [62,63]. The symbiotic relationship between the pine tree and its fungal symbiont may allow the tree to receive additional radiocaesium from the mycelium during the growing season as the tree's need for water increases, resulting in a higher volume of water passing through the fungal barrier and potentially transferring more radionuclides to the plant. In mid-autumn, the highest levels of 137 Cs in mycorrhizal fungi suggest an outflow of this radionuclide from the plant to the fungi after the end of the growing season [46]. This means fungi could retain 137 Cs until the following spring, making them a depot of this radionuclide in forest ecosystems.
No seasonal fluctuations in the specific activity of 137 Cs were found in the outer dead bark. This suggests no radiocaesium transfer between the tree's living parts and the outer bark. The movement of 137 Cs within living parts of the tree may contribute to a gradual reduction in its concentration, unlike in the dead outer bark, where the decrease in concentration is solely due to the physical decay of this radioactive element (see Figure 9).
The studies that were carried out could not identify any seasonal fluctuations in the levels of 137 Cs activity in moss. This may be due to the characteristics of this moss physiology, which lacks symbiotic mycorrhizal fungi. As a result, the content of 137 Cs in moss experienced fluctuations throughout the year, as depicted in Figure 10.

Conclusions
As a result of this study, it was found that in all investigated parts of pines and in the moss, the content of 137 Cs was not constant. Seasonal fluctuations in the levels of specific activity of this radionuclide were manifested only in the living parts of P. sylvestris-to a greater extent in the branches and needles and to a lesser extent in the wood. Seasonal changes in the concentration of 137 Cs were not found in the dead outer bark of pine and moss. The absence of a symbiotic relationship between D. polysetum and mycorrhizal fungi may be the reason for the lack of seasonal fluctuations in 137 Cs content in this plant.
The changes in the content of 137 Cs in the living organs of the pine did not occur simultaneously. In summer, the maximum concentrations of this radionuclide in the branches and needles were observed, while in winter, the maximum values were observed in the wood.
In forest ecosystems, there is likely a constant circulation of 137 Cs in the soil-plant chain. Radiocaesium enters the plant from the soil, and the fungal mycelium can be a source of an additional amount of this radionuclide during the intensive growth of branches and needles. Fungal mycelium is a depot of 137 Cs in forest soil. An increase in the concentration of 137 Cs in mycelium by mid-autumn may be due to the outflow of this radionuclide from the living organs of pine.
This study showed that the redistribution of 137 Cs in plants largely depends on the peculiarities of the sampling site. The Dytiatky sampling site differs from the others in the low thickness of the forest litter layer, which may be the reason for the imbalance of the 137 Cs ratios in different pine organs between this and other sampling sites. In addition, the low thickness of the forest litter in this area might be the reason for the anomalously high levels of 137 Cs specific activity in the fast-growing organs of the pine, i.e., branches and needles.
An analysis of the results indicated that the seasonal changes in 137 Cs specific activity levels in Scots pine are essential to consider for monitoring activities and in relation to the use of this plant by the population in radiocaesium-contaminated areas. Further research is needed to determine if these seasonal changes in 137 Cs content are typical for other tree species. This highlights the need for continued research in this area, as it could have implications for monitoring and managing the impacts of radiocaesium contamination on the environment and human health. Institutional Review Board Statement: The statement of the Institutional Review Board is not applicable.

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Data Availability Statement:
The author confirms that all relevant data are included in the manuscript.