「森林総合研究所研究報告」（Bulletin of FFPRI）Vol.21-No.1（No.461）27-38 March 2022

論27

文（Original article）
137

Cs concentration observational errors in bark and wood causedby partial sampling from tree stems contaminated by the Fukushima
nuclear accident
Shinta OHASHI 1), *, Katsushi KURODA 1), Takeshi FUJIWARA 2) and Tsutomu TAKANO 3)
AbstractLong-term monitoring of radiocesium (137Cs) contaminations in the bark and wood caused by the Fukushima
Dai-ichi Nuclear Power Plant accident in 2011 requires partial sampling from a standing tree without felling it.
Because 137Cs distributions within the bark and wood are assumed not to be uniform, it is necessary to understandthe observational errors in 137Cs concentration caused by partial sampling and to check the validity of this method.
The objectives of this study are to examine 1) the circumferential distributions of 137Cs concentration in the bark
and wood and 2) the observational errors in 137Cs concentration determined via partial sampling compared withbulk (felling) sampling. The circumferential distributions were investigated by dividing the tree stems collected in2015 into eight directional segments (four segments for small trees). The relative standard deviations of the 137Csconcentration among the directions (the mean of all trees) were 34% and 13% for the bark and wood, respectively.
The patterns of the circumferential distributions were not biased toward a specific direction and were not speciesdependent. Partial sampling was achieved by collecting bark pieces (3 cm × 3 cm) from four directions and woodcores (12 mm in diameter) from 1–2 directions during the period of 2016–2020. The observational errors caused by
the partial sampling were estimated to be approximately 38% and 8%–18% for the bark and wood, respectively, andwere considered random (unsystematic). These results indicate that, for example, to estimate the mean value of the137
Cs concentration in the bark in a forest stand with the same accuracy as the bulk sampling with n = 3, n for thepartial sampling should be increased to 6–8.
Key words : F ukushima Dai-ichi Nuclear Power Plant accident, radiocesium, circumferential distribution,
increment core

1. Introduction

and an increment borer, respectively (Photos 1c and 1d), is

137

Radiocesium ( Cs) derived from the Fukushima Dai-ichi

required for sustainable monitoring. However, it is necessary

Nuclear Power Plant (FDNPP) accident has been transferred

to determine any observational errors caused by such partial

into the stem wood of trees from foliage and bark surfaces

and small-amount sampling. Accordingly, it is also essential(Masumori et al. 2015, Nishikiori et al. 2015, Wang et al.

to understand the variation in the vertical and circumferential

2016, 2018) or from soil via roots (Komatsu et al. 2017).

distributions of the 137Cs concentration in tree stems. Because

137

Cs concentrations

the vertical distribution has been reported in several studies

in the bark and wood of tree stems is critically important to(Ohashi et al. 2014, 2020, Masumori et al. 2015, Ogawa

make appropriate plans for forest management and woodet al. 2016, Yoschenko et al. 2017), we focus here on theuse in radioactively contaminated areas. To obtain data

circumferential distribution.

Accurate and prolonged monitoring ofconcerning the 137Cs concentration that is representative of

A few studies have investigated the circumferential

an individual tree, bark samples should be collected from the

distribution in stem wood (Mahara et al. 2014, Hirano et al.

entire circumference of the stem, and wood samples should

2016). The data from these studies indicate that the relative

be collected as disks by felling the tree (Photos 1a and 1b).

standard deviation (RSD) of the 137Cs concentration among

However, because such bulk sampling decreases the number

four or eight evenly divided directions of a tree stem wasof trees and disturbs the environment in a forest stand, it isapproximately 10%–50% in Japanese cedar (Cryptomeria

not suitable for decades-long monitoring programs. Partial

japonica) and konara oak (Quercus serrata) trees collected

sampling of bark and wood from a standing tree using a chisel

in 2012 (Mahara et al. 2014) and konara oak trees collected

Received 28 June 2021, Accepted 7 October 2021
1) Department of Wood Properties and Processing, Forestry and Forest Products Research Institute (FFPRI)
2) Forest Bio-Research Center, FFPRI
3) Center for Forest Restoration and Radioecology, FFPRI*	Department of Wood Properties and Processing, FFPRI, 1 Matsunosato, Tsukuba, Ibaraki, 305-8687 JAPAN; e-mail: shinta.res@gmail.com

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Shinta OHASHI et al.

Photo 1. Sampling of (a) bark from the entire circumference of a tree stem, (b) a wood disk, (c) a bark piece,
and (d) a wood core.
in 2014 (Hirano et al. 2016). On the other hand, theBecause the radial distribution of the 137Cs concentration in thecircumferential distribution in bark has not been reported forstem wood is often heterogeneous (e.g., Mahara et al. 2014,

trees affected by the FDNPP accident. Even though studies

Ohashi et al. 2014, 2020), stem wood should be sampled as aon the circumferential distribution in the bark are scarce forcircular sector in cross-section. However, because the shape

the trees affected by the Chernobyl Nuclear Power Plant

of a core is columnar, sampling with an increment borer(ChNPP) accident, one study showed that the RSD of the 137Cs

extracts too much wood from near the pith. This may result

concentration in bark among four directions of a tree stemin an underestimation of 137Cs concentration in the stem wood

was 33%–47% at some sites even nearly 30 years after thein some cases because the concentration tends to be lower on

accident (Cosma et al. 2016). Therefore, the circumferential

the pith side than on the bark side (Mahara et al. 2014, Ohashi

distributions in bark and wood are assumed to be highly

et al. 2014, 2020). Therefore, it is necessary to check whether

heterogeneous in some individual trees. To reduce the

137

Cs

concentration observational errors associated with partial

there is a systematic error in the 137Cs concentration in woodcores compared with that in wood disks.

sampling from a tree stem, it is crucial to determine whether

I n t h i s s t u d y, w e i n v e s t i g a t e d t h e c i r c u m f e r e n t i a lthere is a tendency for the 137Cs concentration to be high in

distributions of 137Cs concentration in stem bark, sapwood, anda specific direction and whether the presence or absence ofheartwood collected in 2015 and examined 1) the variability

such a tendency differs among tree species. Additionally, itof concentration among different directions, 2) the presence

is also important to determine whether there is a correlationof a directionality that is common among individual trees,

between the circumferential distributions in bark and wood (i.e.,and 3) the correlation between the stem parts. Additionally,

whether the 137Cs concentration in wood is high in the same

we investigated the 137Cs concentrations in partial samples

direction that the concentration in bark is high) to understandcollected without felling trees and those in bulk samples

the characteristics of 137Cs contamination in tree stems.

collected by felling trees in the period of 2016–2020. Finally,

In addition to heterogeneities in the circumferential
distribution of 137Cs, the shape of a core sample likely affects
137

we discuss the validity of partial sampling for a long-termmonitoring survey of 137Cs concentrations in a forest.

Cs concentration observational errors in the stem wood.
森林総合研究所研究報告 第 21 巻 1 号, 2022

137

Cs observational errors via partial sampling29

the cases of two small-diameter trees. The disks of the small-

38° N

diameter trees, the M- and S-sized oaks from site 1, weredivided into four directions. The wood disks were split using
a bandsaw after collecting bark samples from each direction
Site 3using a chisel. Each split disk sample was further divided into
Site 2Site 1

sapwood and heartwood using a chisel and a hammer. Then, allSite A1

samples were chipped using a cutting mill with a 6-mm sieve

20 km

and oven-dried at 75 °C for 48 h.

37° N

2.2 Samples for estimating the 137 Cs concentration

60 km

observational errors via partial sampling

We compared the 137Cs concentration in the stem wooddetermined from partial sampling (core sampling) with that

Deposition densityof ¹³ Cs (kBq m ²)

36° N

3000<
1000–3000
600–1000
300–600
100–300
60–100
30–60
10–30
10

determined from bulk sampling (disk sampling) to estimate theobservational error. First, to check whether the observational

Site 5error is affected by the number of cores collected from a

150 kmtree, we investigated the 137 Cs concentration determinedfrom a single core and that determined by averaging the140° E

1 41 ° E

Fig. 1. Locations of the sampling sites and deposition densityof 137Cs (as of December 28, 2012) in Fukushima and
Ibaraki Prefectures, Japan.
Open circles indicate sampling sites, and a cross markindicates the Fukushima Dai-ichi Nuclear Power Plant.
Data on the deposition density were provided by MEXT
(2013).
2.1 Samples for investigating the circumferential
distribution ofold cedar (Cryptomeria japonica; DBH = 20.7–33.1 cm),31-year-old cypress (Chamaecyparis obtusa; DBH = 14.9–20.6cm), and 31-year-old oak (Q. serrata; DBH = 9.4–12.3 cm)
grown at site 1 were used for this investigation. Wood cores
were collected from two directions (180° opposite) on eachtree stem at a height of 1 m using a 12-mm-diameter increment
borer in August 2016. After the core sampling, the trees

2. Materials and Methods

137

concentrations of two cores. Three individuals each of 48-year-

Cs

were felled with a chain saw, and wood disks were collectedfrom near the locations where the cores were collected. After

Samples were collected from sites 1 and 2 (Kawauchi

separating the wood samples into sapwood and heartwood, theVillage, Fukushima Prefecture, Japan; Fig. 1) in August, 2015.

core and disk samples were chipped using a cutting mill withThree individuals each of 47-year-old cedar (Cryptomeria

2-mm and 6-mm sieves, respectively.

japonica [L.f.] D.Don), 30-year-old cypress (Chamaecyparis

Second, to check the observational error caused by the partial

obtusa [Siebold et Zucc.] Endl.), and 30-year-old oak (Quercus

sampling that we expected to use in practice, we investigatedserrata Murray) at site 1 and 61-year-old cedar at site 2 werethe 137Cs concentration in the stem wood determined fromselected from three different diameter classes, i.e., large

two cores roughly chipped using pruning shears instead of a(L), medium (M), and small (S) diameters. The diameter at

cutting mill (i.e., the cores were less homogenized than coresbreast height (DBH) of each tree is shown in Fig. 2. Before

chipped normally using a cutting mill). Using the same method

felling the trees, the north (magnetic north) sides of the tree

previously described, core and disk samples were collected

stems were marked with a permanent marker and the stems

from six pines (Pinus densiflora Siebold et Zucc.) and threeCschestnuts (Castanea crenata Siebold et Zucc.) at site A1

contamination by litter and soil. Then, the stem disks were

(Kawauchi Village, Fukushima Prefecture, Japan) in Augustcollected from the stems near breast height after felling the2017 and 2019. The trees were approximately 50 years old andtrees using a chain saw, and the magnetic north direction was

had DBH values of 17.9–38.1 cm. The samples were separated

again marked on the upper side of each disk. Later, during

into sapwood and heartwood. Here, two cores collected fromdata processing, the ordinal directions of the stem disks weredifferent directions on the same tree were treated as a single

determined by assuming a magnetic declination of −7.5° at the

sample and chipped into semicircles with a thickness ofsampling sites.

1–2 mm using pruning shears, while the disk samples werenear breast height were wrapped in tarpaulin to prevent

137

To divide the disk into eight directions, we drew lines onprocessed in the same way as described previously.

the disk surface, making the center angle of each circular

The observational error of the 137Cs concentration in the

sector 45° (here we treated the pith as the center), except inbark determined from partial sampling (piece sampling)

Bulletin of FFPRI, Vol.21, No.1, 2022

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Shinta OHASHI et al.

was also investigated by comparing it with that determinedfrom bulk sampling (entire circumference sampling). Twelve

2.4 Estimation of 137Cs concentration observational errors
via partial samplingpines (P. densiflora) and nine chestnuts (Castanea crenata)

To evaluate the difference between the 137Cs concentrations

at site A1; six cedars (Cryptomeria japonica), three pines

determined from partial sampling (X) and those determined(P. densiflora), and three oaks (Q. serrata) at site 3 (Otama

from bulk sampling (Y), the mean absolute percentage error

Village, Fukushima Prefecture, Japan); and three cypresses(MAPE) was calculated according to the following equation:(Chamaecyparis obtusa) at site 5 (Ishioka City, Ibaraki
Prefecture, Japan) were used for this investigation. The trees
were approximately 50 years old and had DBH values of 16.6–
38.1 cm. In July–September of 2017–2020, we collected barkpieces (approx. 3 cm in the axial and tangential directions)from four directions (approx. 90° apart from each other) of

where n is the number of samples.

each tree stem at a height of 1 m using a 3-cm-wide chisel

Because not only X but also Y contains the observational

and a hammer. We then felled the trees and collected barkerror (at least the error caused by the radioactivity

from the entire circumference of the tree stems using the samemeasurement), we estimated the observational error in X bymethod as in the monitoring survey by the Forestry and Forest

simulating MAPE with the assumption that the observational

Products Research Institute (FFPRI; Kuroda et al. 2013). The

error in Y was 5% (counting error of the radioactivity

bark pieces collected from different directions on the samemeasurement). The simulation was done by generating 100,000

tree were treated as a single sample and cut into approximately

random numbers from a normal distribution (mean = 100,5-mm pieces in the axial and tangential directions using

standard deviation = 5) for Y and 100,000 random numbers

pruning shears, while the bark samples collected from thefrom the normal distribution (mean = 100, standard deviation

entire circumference were chipped using a cutting mill with a= σ) for X using R version 4.0.4 (R Core Team 2021). Various

6-mm sieve.

values of σ were tested sequentially to search for the optimum

The samples were oven-dried at 75 °C for 48 h, and thevalue that derives the nearest MAPE to the observed MAPE.

sample weights were converted to that dried at 105 °C by

We determined the optimum value of σ to be the observational

multiplying by 0.98 for the bark samples and by 0.99 for theerror in X.

wood samples (Ohashi et al. 2017).
2.3 Radioactivity measurements

3. Results and Discussion

3.1 Circumferential distribution of 137Cs

The samples were packed into 2-L or 0.7-L Marinelli

The circumferential distributions of the 137Cs concentration

containers (MAX-Y20 or MAX-Y07, Sugiyama-gen, Tokyo,

in the bark, sapwood, and heartwood collected in 2015

Japan), U-8 containers (No. 3-20, Umano Kagaku Youki,

are shown in Fig. 2. The mean RSD values of the 137 Cs

Osaka, Japan), or #737 containers (Kartell, Milan, Italy)

concentration among the directions were 34%, 14%, anddepending on the sample amount. The radioactivity of 137Cs

12% in the bark, sapwood, and heartwood, respectively.

in the sample was determined via gamma-ray spectrometry

The relatively large RSD in the bark is likely due to thewith a high-purity Ge detector (GEM20, GEM40, or GWL-

heterogeneous deposition of accident-derived 137Cs (Tanaka et120, ORTEC, Oak Ridge, TN). Peak efficiency calibrations

al. 2013), the non-uniform removal of 137Cs by stemflow withfor the Marinelli, U-8, and #737 containers were done

preferential flow paths (Imamura et al. 2017a), and the random

using standard sources: MX033MR, MX033U8PP (Japan

exfoliation of bark via aging and weathering. The RSD values

Radioisotope Association, Tokyo, Japan), and EG-ML (Eckert

in the sapwood and heartwood observed in this study are& Ziegler Isotope Products, Valencia, CA), respectively. Each

relatively small compared with previous studies that showed an

measurement was continued until the counting error dropped

approximately 10%–50% RSD among the directions (Mahara

to 5% or less; however, for some wood core samples withet al. 2014, Hirano et al. 2016).

Cs concentrations, we allowed an

The RSD of the 137Cs concentration among the directions diderror of up to 10%. For measurements combining the well-type

not vary significantly among the tested species and between

Ge detector (GWL-120) and the #737 container, the gamma-

the sites. Even though the 137Cs concentration in the bark

ray self-absorption was corrected by assuming that the bulk

showed biases in the circumferential distributions in somedensities of the samples were the same as that of the standard

trees (e.g., the M-size cedar, L-size cypress, and L-size oak atsource (Ohashi et al. 2021).

site 1 and L-size cedar at site 2), the directions with high 137Cs

small amounts and low

137

concentrations were not the same for all trees. In contrast to our
森林総合研究所研究報告 第 21 巻 1 号, 2022

137

Site 1Site 1Site 2

Cedar

Cypress

Oak

Cedar

N

N

N

N

1.0
0.0 W

E

RSD
Bark 22%
SW
SW 21%HW
HW 18%S

DBH 34.6 cm

W

E

RSD
Bark 53%
SW
SW 17%HW
HW 8%

DBH 21.0 cm

E

RSD
Bark 52%
SW
SW 5%
HW
HW 9%

S

DBH 15.8 cm

W
RSD
Bark 26%
SW
SW 18%HW
HW 22%ES

DBH 46.0 cm

N

N

N

N

2.0

S

W

Size L

1.0
0.0 W

E

RSD
Bark 42%
SW
SW 9%
HW
HW 10%S

DBH 26.4 cm

W

E

RSD
Bark 27%
SW
SW 8%
HW
HW 10%

DBH 19.4 cm

E

RSD
Bark 19%
SW
SW 14%HW
HW 6%

S

DBH 12.5 cm

W
RSD
Bark 39%
SW
SW 24%HW
HW 15%ES

DBH 38.4 cm

N

N

N

N

2.0

S

W

Size M

1.0
0.0 W

E

RSD
Bark 30%
SW
SW 5%
HW
HW 7%

S

DBH 19.2 cm

W

E

RSD
Bark 25%
SW
SW 15%HW
HW 12%S

DBH 16.6 cm

W

E

RSD
Bark 51%
SW
SW 11%HW
HW 12%S

DBH 11.2 cm

W
RSD
Bark 20%SW
SW 21%HW
HW 19%

E

S

Size S

Ratio of 137Cs concentration in a direction to mean concentration in all directions

Site 1
Bark
SW
HW

2.0

31

Cs observational errors via partial sampling

DBH 30.7 cm

Direction

Fig. 2. Circumferential distributions of the Cs concentrations in the bark and wood disk samples in 2015.
SW sapwood, HW heartwood, RSD relative standard deviation (among the directions), DBH diameter at breast height
137

results, Mahara et al. (2014) reported that 137Cs concentration

directions in all stem parts (bark, sapwood, and heartwood)in the stem wood of an oak (Q. serrata) collected in 2012 was(Fig. 3). This fact indicates that sampling from more than one

high in the direction facing the wind direction during the main

direction of a tree stem should be done by collecting samples

137

Cs deposition event. They inferred that such anomalousfrom parts apart from each other to obtain representative

directionality may appear in deciduous tree species such asdata of an individual tree, especially for the bark, whichoak, which did not absorb 137Cs via their leaves in 2011, andshowed a relatively large RSD of 137Cs concentration in thepredicted that this directionality would soon disappear as a

circumferential direction.

consequence of the transport and diffusion of 137Cs. Therefore,

Correlations in the circumferential distribution patterns

directionality might have also existed at our study sites shortlyof the 137 Cs concentration between the stem parts (bark,after the accident; however, four years after the accident, we

sapwood, and heartwood) were checked using the Pearson

could not find any evidence of directionality. The relatively

product-moment correlation coefficient (Fig. 4). There was no

small RSD in this study also indicates that the circumferential

correlation between the distribution patterns in the bark anddistribution of 137Cs concentration in the stem wood may havethose in the sapwood (R = 0.180, p ≥ 0.05). Even though it

been homogenized with time. Similarly, directionality washas been shown experimentally that Cs can be absorbed fromnot found in oaks (Q. petraea) contaminated by the ChNPP

bark surface and transferred to sapwood (Wang et al. 2016,

accident 24 years after the accident (Kilic 2012).

2018), we assumed that 137Cs absorption through bark had

Although there was no common directionality in the

not occurred dominantly since 2012 because dissolved 137Cs

Cs concentration among the trees, there was a spatial

on tree surfaces, which is thought to be absorbed by trees,

autocorrelation in the 137Cs concentration within a tree stem.

decreased drastically during 2011 (Nishikiori et al. 2015).

137

137

Cs concentrations between two adjacent

The lack of a correlation between the bark and the sapwood

directions tended to be smaller than between two opposite

appears to support this assumption. However, there was a weak

The difference in the

Bulletin of FFPRI, Vol.21, No.1, 2022

32

Shinta OHASHI et al.

between two samples in a tree

Mean difference in 137Cs ratio*

Bark

Sapwood

Heartwoodp < 0.01

1.00

Cedar

CypressOak0.75
p < 0.01

0.50

p < 0.01

0.25
0.00
Adjacent

Opposite

Adjacent

Opposite

Adjacent

Positional relationship within a tree stem

Opposite

x: Bark, y: Sapwood
to mean concentration in all directions

Ratio of 137Cs concentration in a direction

Fig. 3. The difference in 137Cs ratio* between two directions within a tree stem in 2015.
* Ratio of 137Cs concentration in a direction to mean concentration in all directions (the same variable with the y-axis of Fig. 2).
The trees of which stems were divided into eight directions were analyzed (n = 10). Each point shows a mean difference in the137
Cs ratio between two directions (eight combinations for adjacent position and four combinations for opposite position) in atree. p-values indicate the significance level of the left-tailed, paired t-test.
x: Bark, y: Heartwood

x: Sapwood, y: Heartwood

2.0

2.0

1.5

1.5

1.5

1.0

1.0

1.0

2.0

Cedar

CypressOak

0.5

R = 0.180, p ≥ 0.05

0.5

1.0

1.5

Ratio of

2.0
137

0.5

0.5

R = 0.340, p < 0.01

0.5

1.0

1.5

2.0

R = 0.748, p < 0.001

0.5

1.0

1.5

2.0

Cs concentration in a direction to mean concentration in all directions

Fig. 4. Correlation of the circumferential distribution patterns of the 137Cs concentration between the stem parts in 2015.
correlation between the distribution patterns in the bark andthose in the heartwood (R = 0.340, p < 0.01), implying that137

3.2 137Cs concentration observational error via partial
sampling

Cs might have been transferred intensively from the barkThe 137 Cs concentration in the sapwood determinedsurface to the stem wood in 2011 and remained somewhat invia partial sampling (core sampling) had a one-to-one

the heartwood. Such a “memory effect” in the heartwood wascorrespondence with that determined via bulk sampling

also suggested in our previous study comparing the 137Cs and(disk sampling) in 2016, regardless of the number of cores133

Cs distributions in tree stems collected in 2014 (Ohashi

collected from a tree (Fig. 5a). Because the ratio of the 137Cs

et al. 2020). However, the correlation between the bark andconcentration in the core to the disk was 1.00 on average (Fig.

the heartwood is thought to become weaker with time as the5b), we determined that there was no systematic error in theformation of new heartwood and the diffusion of 137Cs in the137heartwood proceeds. Lastly, there was a strong correlationin the heartwood was 1.10 on average and its 95% confidence

between the distribution patterns in the sapwood and those ininterval ranged to values greater than 1.00 (Fig. 5b), showing

the heartwood (R = 0.748, p < 0.001). This strong correlationthat there was a positive systematic error (overestimation)

Cs transfer in stem wood is likely to be morein the 137Cs concentration in the heartwood core. This result

dominant in the radial direction than in the circumferential

is contrary to our assumption that there can be a negative(tangential) direction, probably owing to the existence of ray

systematic error (underestimation) in the 137Cs concentration(Kuroda et al. 2020).

in the inner part (heartwood) of a wood core because the core

indicates that137

Cs concentration in the sapwood core. However, the ratio

contains too much wood near the pith, in which the 137Cs
森林総合研究所研究報告 第 21 巻 1 号, 2022

137

33

Cs observational errors via partial samplingab
Sapwood2

y = 1.204x0.966
R 2 = 0.963

y = 0.919x1.030
R 2 = 0.903

MAPE = 15%

MAPE = 14%137

y = 1.163x0.974
R 2 = 0.991

y = 0.870x1.041
R 2 = 0.942

MAPE = 8%

MAPE = 12%

102

103
137

102

1.00 ± 0.10
(0.92–1.08)

1.10 ± 0.13
(1.00–1.20)

Sapwood

Heartwood

1.25
1.00
0.75

1.75
1.50
1.25
1.00
0.75

103

Mean of two directions

10

2

Mean of two directions

103

1.10 ± 0.17
(1.02–1.18)

1.50

Ratio of 137Cs concentration (core/disk)

Oak1.00 ± 0.21
(0.90–1.10)

Single direction

10

1.75

Cypress

Single direction

Cs activity concentration in wood core (Bq kg−1)

Cedar

103

Mean ± SD(95% CI)

Heartwood−1

Cs activity concentration in wood disk (Bq kg )Fig. 5. Comparison of the 137Cs concentrations in the wood disk and core samples (single direction or the mean of two
directions) in 2016.
The disk and core samples were chipped using a cutting mill with 6-mm and 2-mm sieves, respectively.
MAPE mean absolute percentage error, SD standard deviation, CI confidence interval
concentration is relatively low in many cases (e.g., Mahara eterror in the 137Cs concentration in the heartwood cores inal. 2014, Ohashi et al. 2014, 2020). However, there are somethe period of 2017–2019 (Fig. 6). Unfortunately, the species

Cs concentration in wood near the pith isdependency of the systematic error is difficult to discuss in thishigher than that on the outer side (Ogawa et al. 2016, Ohashi

study because of the small sample size and the uncertainty inet al. 2020). Therefore, we might have collected samples fromestimating the 137Cs concentration in heartwood from the partial

such trees in 2016 by chance because there was no systematic

sampling remains. Nevertheless, because the radial distribution

cases in which the

137

137

b
Sapwood
ChestnutPine103

y = 1.653x0.926

10

y = 1.489x0.927

22

R = 0.967

R 2 = 0.941

MAPE = 14%

MAPE = 10%

102

103
137

Mean ± SD(95% CI)

Heartwood

102

103

Ratio of 137Cs concentration (core/disk)

Cs activity concentration in wood core (Bq kg−1)

a

1.75

1.06 ± 0.15
(0.94–1.18)

1.03 ± 0.14
(0.92–1.14)

Sapwood

Heartwood

1.50
1.25
1.00
0.75

−1

Cs activity concentration in wood disk (Bq kg )Fig. 6. Comparison of the 137Cs concentrations in the wood disk and core samples (mean of two directions) in the period of
2017–2019
The disk samples were chipped using a cutting mill with a 6-mm sieve, and the core samples were roughly chipped using
pruning shears.
MAPE mean absolute percentage error, SD standard deviation, CI confidence interval
Bulletin of FFPRI, Vol.21, No.1, 2022

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Shinta OHASHI et al.

of the 137Cs concentration in heartwood is presumed to becomelarger than those in the concentrations determined from two

homogeneous with time following an accident (Ogawa et al.

normally chipped cores (8%–14%) but smaller than those in2016), we expect that the uncertainty is smaller or negligible

the concentrations determined from a single normally chippedin recent samples (e.g., samples collected 10 years after thecore (16%–18%). Therefore, this relatively simple method

accident).

using two roughly chipped cores appears to be reasonable for137

Cs concentration in the sapwood andpractical use because it provides a larger amount of sample forheartwood determined from a single core were 14%–15%,

radioactivity measurement and smaller observational errors

and the observational errors were estimated to be 16%–18%.

than using a single normally chipped core, as well as taking

These errors likely arose from the variability of the 137Cs

less time for sample processing than chipping cores with aconcentration in the circumferential direction because they

cutting mill.

The MAPEs of theare similar to the mean RSD of the 137Cs concentration among

The 137Cs concentration in the bark determined from thethe directions (12%–14%, as described in Section 3.1). Whenpartial sampling (piece sampling) also had a one-to-one

averaging the 137Cs concentrations in two cores (directions), thecorrespondence with that determined from the bulk sampling

observational errors decreased to 8%–14%. This is consistent(sampling from the entire circumference), and no systematicwith the theoretical error in the mean value for two directions,error was observed in the period of 2017–2020 (Fig. 7);

approximately 12%, which can be calculated using the law of

however, the observational error was estimated to be 38%,

error propagation and by assuming an observational error of

showing that the random error is relatively large. This error is

17% (16%–18%, as mentioned above) (i.e., √172 + 172 ÷ 2 ).

considerably larger than the theoretical error in the mean value

Therefore, the reduction in the137

Cs concentration

observational error when increasing the number of cores wasdemonstrated well in this study. In the case of increasing thenumber of cores to 4, 8, and 16, the observational error is
expected to be reduced to 9%, 6%, and 4%, respectively (Table
1).
There was no systematic error in the sapwood andheartwood 137Cs concentrations determined from the two coresthat were roughly chipped using pruning shears (Fig. 6). The
observational errors were estimated to be 12%–17%, slightly

Part
Bark
Wood

Number of partial samples per tree
2
4
8
16
54%
38%
27%
19%
12%
9%
6%
4%

1
76%
17%

The estimates were calculated using the law of error propagation
and assuming that observational error via sampling bark pieces (3cm × 3 cm) from four directions is 38% and that via sampling awood core (diameter 12 mm) from a single direction is 17%.

b
Cedar

104

Ratio of 137Cs concentration (piece/bulk)

collected from four directions (Bq kg−1)

137

Cs activity concentration in bark piecesaTable 1. Estimates of the observational error in the 137Csconcentration determined via partial sampling ofbark and wood from a tree stem.

Cypress
Oak

ChestnutPine103

y = 1.132x0.980
R 2 = 0.958

102

MAPE = 31%

10
137

2

10

3

10

4

1.05 ± 0.49
(0.88–1.22)

Mean ± SD(95% CI)

3

2

1

0
Bark

Cs activity concentration in bark sample

collected from the entire circumference (Bq kg−1)

Fig. 7. Comparison of the 137Cs concentrations in the bark samples collected from the entire circumference and from four
directions of the tree stems in the period of 2017–2020.
The samples collected from the entire circumference were chipped using a cutting mill with a 6-mm sieve, and the samples
collected from four directions were cut into approximately 5-mm pieces in the axial and tangential directions using pruning
shears.
MAPE mean absolute percentage error, SD standard deviation, CI confidence interval
森林総合研究所研究報告 第 21 巻 1 号, 2022

137

35

Cs observational errors via partial sampling

Table 2. Sample size (number of sample trees) required to achieve the same relative standard error of the 137Cs concentrationin an investigation in which the observational error in the 137Cs concentration is 5% and the sample size is 3.
RSD among trees1) (%)RSE2) (%)
Observational error in 137Cs concentration
10%
15%
20%
30%
40%
0
3
12
27
48
108
192
5
4
7
15
25
55
98
10
6
5
8
12
24
41
20
12
4
4
6
9
14
30
18
3
4
4
6
8
40
23
3
3
4
5
6
50

29

3

3

3

4

5

1) Relative standard deviation of the Cs concentration among trees within a forest stand.
2) Relative standard error of the mean value of the 137Cs activity concentration within a forest stand when the observational error in the137
Cs concentration is 5% and the sample size is 3.
137

for four directions, 17%, calculated by assuming that the RSD

et al. 2017), we assumed a sample size of the bulk samplingof the 137Cs concentration in the bark among the directionsof three, an observational error of the bulk sampling of 5%

was 34%, from the results in Section 3.1 (i.e., √342 × 4 ÷ 4 ).

(the error caused by the counting error of the radioactivity

to increase the size of the bark pieces collected. If it is difficult

sampling of the sapwood and heartwood from two directions)to increase the size, increasing the number of bark pieces

in this case was estimated to be 3–4 (Table 2). This is nearly

will also reduce the observational error. For example, thethe same sample size with bulk sampling, meaning that theobservational error is estimated to be reduced to 27% whenpartial sampling of stem wood is acceptable for estimating thebark pieces are collected from eight directions (Table 1).

mean value of the 137Cs concentration in a forest stand without

This gap is most likely due to the small piece sizes (3 cm × 3

measurement), and a RSD among the trees of roughly 30%–

137

Cs concentration observational

40% (Fig. S1). The sample size required for an observational

error in bark determined via partial sampling, it is necessary

error of 10%–20% (assuming the error caused by the partialcm). Therefore, to reduce theincreasing the sample size compared with bulk sampling in

3.3 Sampling strategy

this case. For another example, the sample size required for an

The partial sampling of bark and wood from a standing

observational error of 40% (assuming the error caused by thetree was demonstrated to increase the observational error

partial sampling of the bark) was estimated to be 6–8 (Table

137

Cs

2). Therefore, even though the observational error caused

concentration compared with bulk sampling from a felled tree.

by partial sampling is relatively large, the mean value of the

Whether this increase in the observational error is acceptable

137

or not depends on the objective of a study. For example, in aaccuracy as bulk sampling by increasing the sample size in acase where the objective is to estimate the 137Cs concentration

realistic range.

(which is considered random and unsystematic) of theCs concentration in a forest can be estimated with the samein the bark or wood of a single tree, the concentration should

However, there are also limitations in partial sampling

be determined from bulk sampling with a felled tree, or if it isas well as bulk sampling. Frequent sampling of bark anddifficult to fell the tree, determined via partial sampling with awood from the same tree reduces transport pathways oflarge number of bark pieces or wood cores from the tree (Table

water, photosynthate, and minerals, and might alter the 137Cs

1).

concentration in the remaining stem parts, or results in tree

In contrast, in a case where the objective is to estimate

death in the worst case. It is uncertain how many samples wethe mean value of the 137Cs concentration in a forest stand,can collect without affecting the tree significantly. Therefore, tothe estimation error (i.e., the standard error) increased bysustain a monitoring program for decades by partial sampling,

partial sampling from the trees can be canceled or even

it would be better to change sample trees every year within abecome smaller by increasing the number of sample trees (theforest stand and assign enough time interval to trees for barksample size). The sample size for partial sampling required

regeneration after sampling.

to achieve the same relative standard error as bulk sampling
depends on the sample size, the observational error of the 137Cs

Acknowledgments

concentration in the bulk sampling, and the RSD of the 137Cs

We would like to thank Dr. Akira Kagawa and Dr. Koichi

concentration among the trees in the forest stand. Here, based

Yamamoto for their great assistance with sampling and sample

on the monitoring survey conducted by FFPRI from 2011

preparations. We also thank the Center for Forest Restoration

to 2016 (Kuroda et al. 2013, Imamura et al. 2017b, Ohashi

and Radioecology of FFPRI for performing the radioactivity

Bulletin of FFPRI, Vol.21, No.1, 2022

36

Shinta OHASHI et al.

measurement. This study was financially supported by thecesium of the sixth airborne monitoring survey andForestry Agency of Japan.

airborne monitoring survey outside 80 km from the
Fukushima Dai-ichi NPP, https://emdb.jaea.go.jp/emdb/

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Shinta OHASHI et al.

福島原発事故で汚染された樹幹からの部分的なサンプリングによって生じる
137
樹皮および材中の Cs 濃度の観測誤差
大橋 伸太 1) *、黒田 克史 1)、藤原 健 2)、高野 勉 3)
要旨
福島第一原子力発電所事故によって汚染した樹皮や材の放射性セシウム (137Cs) 濃度を長期的にモニタリングするためには、試料は樹木を伐倒せずに立木から部分的に採取することが望ましい。しかし、樹皮や材中の 137Cs の分布は均一ではないと想定されるため、部分的なサンプリングによる観測誤差を明らかにし、その妥当性を確認する必要がある。本研究では、樹皮および材の 137Cs 濃度の樹幹円周方向の分布を明らかにするとともに、部分的なサンプリングによる樹皮および材の 137Cs 濃度の観測誤差を明らかにすることを目的とした。2015 年に伐採した樹幹を 8 方位 ( 一部 4 方位 ) に分割して求めた 137Cs 濃度の円周方向分布には、樹皮と材でそれぞれ平均 34% と 13% の相対標準偏差があったが、特定の方位への偏りや樹種依存性は見られなかった。2016 年以降に樹幹から部分的にサンプリングした試料 ( 樹皮は 3cm × 3 cm のピース× 4 方向、材は直径 12 mm のコア× 1–2 方向 ) から求めた 137Cs 濃度には、樹皮と材でそれぞれ約 38% と約 8–18% の観測誤差があると推計され、いずれも偶然誤差だと考えられた。例
えばこの部分的なサンプリングによって 137Cs 濃度の林分平均値を伐倒サンプリング ( 観測個体数 3) と同程度の精度で推定するためには、樹皮の場合は観測個体数を 6–8 に増やす必要があることがわかった。
キーワード：福島第一原子力発電所事故、放射性セシウム、円周方向分布、成長錐コア

原稿受付：令和 3 年 6 月 28 日 原稿受理：令和 3 年 10 月 7 日
1) 森林総合研究所 木材加工・特性研究領域
2) 森林総合研究所 森林バイオ研究センター
3) 森林総合研究所 震災復興・放射性物質研究拠点* 森林総合研究所 木材加工・特性研究領域 〒 305-8687 茨城県つくば市松の里１
森林総合研究所研究報告 第 21 巻 1 号, 2022