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Accumulation of lead in tree trunk bark pockets as pollution time capsules
the Science of the Total Environment A”lm”“.,d!amll,~Sknur~-.=h ,“@ ,hC E”mmD~“, .n6i”I.!mo~.hip.uh *.n The Science of the Total Environment 181 (1996) 25-30
Accumulation of lead in tree trunk bark pockets as pollution time capsules Ken’ichi Satake*, Atushi Tanaka, Katsuhiko Kimura National
Institute
for Environmental
Studies,
Onogawa
16-2, Tsukuba.
Ibaraki
305, Japan
Received 13 March 1995; accepted 7 September 1995
Abstract
The concentration of leadin bark pocketsof tree trunkswasinvestigatedfor its potentialuseasan archival indicator of pollution. The bark pocketsinvestigatedwerethoseof a conifer, Cryptomeriu japonicn, formed around 1760-1780 (235-255 years ago) at Nikko about 100 km north of Tokyo, and around 1786-1809(186-209 years ago) on YakusbimaIsland, locatedin a remotesouthernregion of Japan.The leadconcentrationsin theseC.juponicu bark pockets,representingthe total lead accumulationduring a period of about 20 yearsat eachsite, were0.1 pg Pb g-’ at Nikko and 0.22 pg Pb g-’ at Yakushima.In contrast, the leadconcentrationin the outer bark of C. juponica at the presenttime is about 150pg Pb g-’ at Nikko (1990)and 1.4 pg Pb g-’ at Yakushima(1992).The useof leaded gasoline,the main sourceof lead in the atmosphere,wasinitiated in Japanin 1949and reacheda maximumduring 1960-1965.As the production of leadedgasolinewas stoppedin 1987,the lead concentrationin the outer bark represents the total for a period of about 40years.Therefore,theseresultssuggest an increasein leadpollution of about three ordersof magnitudeat Nikko, which is relatively closeto Tokyo, and one order of magnitudeat Yakushima, which is relatively remote. Keywork
Bark pocket; Lead; Heavy metal; Monitoring; Year ring
1. Introduction The level of pollution before the Industrial Revolution is of considerable interest for evaluating the present day situation. The Industrial Revolution began in the United Kingdom about 250 years ago, and was followed by the industrialization of other countries in Europe, North America and * Corresponding author. 0048-9697/96/$15.00 SSDI
Asia together with its accompanying environmental pollution. In Japan and China, which closed their borders during 1639-1858 and 1757-1842, respectively, industrialization began in the latter half of the 19th century. Many materials from the environment have been used for the historical monitoring of atmospheric pollutants transported as wet and dry deposition. Sediments[1,2], peat [3], polar ice [4,5] and tree rings [7-lo] are typical of thesematerials.
0 1996 Elsevier Science B.V. All rights reserved
0048-9697(95)04955-Z
26
K. Satake
et al. /The
Science of the Total
However, difftculties remain in finding historical specimens which hold the pollutants without dispersion, translocation or further contamination and which are suitable for dating. We have therefore adopted a new method for monitoring historical trends in air pollution covering the period from several hundred years ago to the present. Inside tree trunks, bark is often enclosed between annual rings, forming so-called bark pockets. Here we describe the initial results of a study on lead pollution recorded in bark pockets of trees before industrialization had occurred in Japan and China. Special interest was paid to these bark pockets because of their potential for showing the pollution level as pollution time capsules. The lead concentrations accumulated in the bark pockets were investigated to demonstrate the applicability of this approach. 2. Materials
ad methods
2.1. Field collection
Outer bark and bark pockets from individual trees of the conifer Cryptomeria japonica wefe collected at Nikko (Imaichi 36’43 ‘N 139O42‘E) in 1990 and Yakushima Island (30°20’N 130°30’E) in 1992. Nikko, a district located about 100 km north of Tokyo, is famous for the Toshogu Shrine, which was built about 360 years ago, when avenues of C. japonica were also planted. There are 13 Ooo individuals of C. japonica 350-360 years old in these avenues. Yakushima, about 25 km in diameter with a maximum altitude of 1935 m, is located in a remote southern region of Japan, about 800 km east of Shanghai, China (Fig. 1). This island includes a national park and world heritage site famous for its many C. japonica older than a thousand years. The bark pocket collected at Nikko was formed around 1760 to 1780 (235-255 years ago) in the trunk of a 350-year-old C. japonica. The bark pocket collected on Yakushima was formed around 1786 to 1809 (186-209 years ago), in the trunk of a 226-year-old C. japonica (Fig. 2a,b).
Environment
181 (19%)
25-30
Fig. I. Locations of Nikko and Yakushima Island in Japan.
2.2. Analytical procedure
The samples were sub-divided into thin sections of outer bark, inner bark, xylem layer and bark pockets with a thickness ranging from 0.15 to 0.76 mm using a Japanese fiat plane and/or razor. These subsections were dried at 60°C for more than 24 h and weighed. The specimens for ICP-MS analysis were digested with a Teflon double-vessel bomb using nitric acid vapor [l 11. The amount of nitric acid in the outer Teflon vessel was 1 ml per 50 mg of sample in the inner vessel. The nitric acid vapor produced at 140% for 4 h was suflicient for digesting the sample in the inner vessel. The digested samples were dissolved in purified water prepared with a Mill&Q water purification system. The dried specimens of bark pockets for SSAAS analysis [12] were analyzed directly. 3. Results The bark pockets of C. japonica collected at Yakushima were formed by damage to the tree trunk followed by covering of the wound with newly produced bark and xylem. This is one of the main mechanisms of bark pocket formation. The
K. Satake
et al. / The Science of the Total Environment
181 (19%)
25-30
a
bark pocket
Fig. 2. A bark pocket in a 226year-old C. japonica tree trunk collected from Yakushima Island in 1992 (a). Physical damage of the tree trunk is the first step. The tree was damaged 206 years ago (1786) when it was 20 years old (b). Then new bark and xylem were produced by the vascular cambium around the wound. The newly produced bark on both sides of the wound come into mutual contact, followed by conjunction of the vascular cambium layer. It took 23 years for the wound to be covered by newly produced annual rings, forming the bark pocket. The rings covering the bark pocket have been produced annually by the conjugated vascular cambium since the tree was 43 years old.
21
28
K. Satake
et al. / The Science of the Total Environment
process of encapsulation of the polluted bark leads to the enclosure of past pollution in the trunk by the tree’s annual rings. The lead concentration in the outer modern bark of the C. japonicu specimen from Nikko, analyzed by both ICP-MS and SSAAS, was about 150 rg Pb g-’ in the outermost sample and decreased to 0.06 pg Pb g-’ (Fig. 3a) in the innermost one. The lead concentration in a radial section of the xylem layer ranged between 0.1 and 0.9 pg Pb g-’ (n = 14, analyzed according to annual rings). The lead concentration in the bark pocket enclosed within the tree trunk at Nikko 235-255
I81 (19%)
25-30
years ago was about 0.1 hg Pb g-‘. In contrast, the lead concentration in the outermost part of the bark from Yakushima which reflects the modern pollution level on the island, was about 1.4 pg Pb ii!-I, two orders of magnitude lower than that in the Nikko sample (Fig. 3b). Furthermore the lead concentration in the bark pocket enclosed within the tree trunk at Yakushima 186-209 years ago was about 0.22 rg Pb g-’ on the outermost side of the outer bark and decreased exponentially to as low as 0.02 c(g g-‘, about one order of magnitude higher than the detection limit for SSAAS analysis (Fig. 3~). 4. Discussion
a outer bark
(Nikko)
,
i
100
. :
J
t
50
I
.
i J
i
I’-’
1.’ b outer bark
(Yakushlma) E
.
. . 0 1
,
.
,
,.
, .*
,-, . -c.
c bark pockzt
. . . 0 /~~“,““,““1~“~1”‘~1”“~“11 0
0.5
1
.
15
2
Depth
(mm)
,
(Yakushlma)
. 2.5
,-,
[
F
1 3
3.5
Fig. 3. Radial distribution of lead concentration in the outer bark of C. japonica collected at Nikko 4 m above the ground (a) and that in the outer bark (b) and bark pocket (c) collected on Yakushima Island (9 & determined by ICP-MS; 0, determined by SSAAS (solid sample atomic absorption spectrometry)).
Lead has been used since ancient times [ 131, and there are many natural inputs of lead to the atmosphere including volcanoes, the sea, and elsewhere [ 141. However, environmental lead pollution increased during 19th and 20th centuries because of the drastic increase in lead consumption due to human activities. The use of leaded gasoline has been mainly responsible for the lead pollution load during the 20th century in urban areas, and long-range transport of lead has been reported [ 151. In the case of Japan, the use of leaded gasoline was initiated in 1949 and reached a maximum level during 1960- 1965. The production of leaded gasoline was finally stopped in 1987. The concentration of lead in the bark pockets of C. japonica at Nikko and Yakushima represent the total lead accumulation during a period of about 20 years at each site before encapsulation of the bark occurred. The lead concentration in modern outer bark represents the total of at least 40 years of the period of use of leaded gasoline ( 1949- 1987) in Japan, although lead pollution is a worldwide phenomenon. These data show that the concentration of lead in tree bark at Nikko (150 pg Pb g-l) is about three orders of magnitude higher than it was 235-255 years ago (0.1 pg g-‘) and at Yakushima 186-209 years ago (0.22 rg g-l). Other studies of the lead concentration in C. juponicu outer bark from Yokohama, near Tokyo, have indicated a maximum of 200 pg Pb g-’ and a value near the highway of 310 rg Pb g-’ [ 16,17).
K. Satake
et al. /The
Science
of the Total
These data are consistent with the lead concentrations in the outer bark obtained at Nikko, and suggest a drastic increase of lead pollution in Japan. There have been many reports [g,lO] of lead concentrations in annual tree rings, which have been used to monitor environmental pollutants. However, the lead concentrations in xylem layers, including annual rings, are very low compared with those in outer bark. The annual rings of a tree are enclosed by the inner and outer bark. Most of a tree’s accumulation of lead from air pollution is deposited in the outer bark (Fig. 2b) and radial transportation through the bark to the xylem is limited [18]. Most of the lead in tree rings originates from the soil. Therefore, it is difficult to obtain direct information on atmospheric lead pollution from annual rings. The lead transported to the xylem layer is initially water-soluble, and thus lateral movement of lead may occur between adjacent rings [19]. Thus, tree ring data may not accurately reflect historical trends in pollution. The biological role of the outer bark, which is in contact with the atmosphere, differs from that of the xylem layer. The outer bark’s role is protection of the inner bark, and cambium and xylem layers from physical biological and chemical damage resulting from, among other things, the accumulation of pollutants. Despite the suitability of bark for providing direct evidence of atmospheric pollution, it has not been commonly used to monitor historical trends in atmospheric pollutants because it is exposed to modern levels of pollution. However, the presence of bark pockets inside the trunk changes this perspective. Bark pockets are located between annual rings, which date them just like a clock. Translocation of pollutants from bark pockets after encapusulation is considered to be unlikely because both of the outer surfaces of the bark in the pockets are in contact with each other, thus preventing translocation (see bark bl and bark b2 in Fig. 2b). In this study, lead pollution was used as a representative type of heavy metal pollution. Similar evaluations are possible for other inorganic and organic pollutants and stable [20] and radioactive isotopes using bark pockets as pollution time capsules.
Environment
181 (19%)
25-30
29
Acknowledgments We Nikko, Island, Uchida
thank S. Kakinuma of Toshogu Shrine, and S. Ichikawa of Y-NAC, Yakushima for their help in Sample collection and T. for his help in sample preparation.
References 111 D.N. Eddington and J.A. Robbins, Records of lead deposition in Lake Michigan sediments since 1800. Environ. Sci. Technol., 10 (1976) 266-274. 121 M. Paezel, H. Scraber and K. Bjerkli, Do decreased trace metal concentrations in surticial Skagerrak sediments over the last 15-30 years indicate decreased pollution? Environ. Pollut., 84 (1994) 213-226. [31 E.L. Livett, J.A. Lee and J.H. Tallis, Lead, zinc and copper of British blanket peats. J. Ecol., 67 (1979) 865-891. 141 M. Murozumi, T.J. Chow and CC. Patterson, Chemical concentrations of pollutant lead aerosols, terrestrial dusts and sea salts in Greenland and Antarctic snow strata. Geochim. Cosmochim. Acta, 33 (1969) 1247-1294. 151CF. Boutron, and CC. Patterson, Lead concentration changes in Antarctic ice during the Wisconsin/Holocene transition. Nature, 323 (1986) 222-225. 161L. Kardell and J. Larsson, Lead and cadmium in oak tree rings (Quercus robur L). Ambio, 7 (1978) 117-121. [71 C. Symeonides, Tree ring analysis for tracing the history of pollution: application to a study in Northern Sweden. J. Environ. Qual., 8 (1979) 483-486. 181 CF. Baes III and H.L. Ragsdale, Age-specific lead distribution in xylem rings of tree genera in Atlanta, Georgia Environ. Pollut. Ser. B, 2 (1981) 21-35. [91 H.L. Ragsdale and C.W. Berish, The decline of lead in tree rings of Carya spp. in urban Atlanta, GA, USA. Biogeochemistry, 6 (1988) 21-29. 1101C. Stewart, D.A. Norton and J.E. Fergusson, Historical monitoring of heavy metals in kahikatea ring wood in Christchurch, New Zealand. Sci. Total Environ., 105 (1991) 171-190. Ill1 I. Kojima and C. Iida, Vapor phase digestion of botanical samples with acids in sealed polytetrafluoroethlene bomb. Anal. Sci., 2 (1986) 567-570. 1121D.C. Baxter and W. Frech, On the direct analysis of solid samples by graphite furnace atomic absorption spectrometry. Fresenius J. Anal. Chem., 337 (1990) 253-263. 1131 GA. Drasch, Lead burden in prehistorical, historical and modem human bones. Sci. Total Environ., 24 (1982) 199-231. I141 A. Ng and C. Patterson, Natural concentrations of lead in ancient arctic and antarctic ice. Geochim. Cosmochim. Acta, 45 (1981) 2109-2121. 1151H. Mukai, A. Tanaka, T. Hujii and M. Nakao, Lead isotope ratios of airborne particulate matter as tracers of
30
K. Satake et al. / The Science of the Total Environment 181 (19%) 25-30
long-range transport of air pollutants around Japan. J. Geophys. Res., 99 (1994) 3717-3726. [la] J.T. Nyangababo and M. Ichikuni, The use of cedar bark in the study of heavy metal contamination in the Nagatsuta area, Japan. Environ. Pollut. Ser. B, 11 (1986) 21 l-229. [ 171 M. Tsurumi, Sources of elements in cedar bark collected near a highway (Japan). Sci. Total Environ., 86 (1989) 239-250. 1181 D. Barnes, M.A. Hamadah and J.M. Ottaway, The lead,
copper and zinc content of tree rings and bark. Sci. Total Environ., 5 (1976) 63-67. [l9] J.R. Donnelly, J.B. Shane and P.G. Schaberg, Lead mobility within the xylem of red spruce seedlings: implications for the development of pollution histories. J. Environ. Qua]., 19 (1990) 268-271. [ZO] H. Mukai, N. Furuta, T. Hujii, Y. Ambe, S. Sakamoto and Y. Hashimoto, Characterization of sources of lead in the urban air of Asia using ratios of stable lead isotopes. Environ Sci. Technol., 27 (1993) 1347-1356.
Accumulation of lead in tree trunk bark pockets as pollution time capsules Ken’ichi Satake*, Atushi Tanaka, Katsuhiko Kimura National
Institute
for Environmental
Studies,
Onogawa
16-2, Tsukuba.
Ibaraki
305, Japan
Received 13 March 1995; accepted 7 September 1995
Abstract
The concentration of leadin bark pocketsof tree trunkswasinvestigatedfor its potentialuseasan archival indicator of pollution. The bark pocketsinvestigatedwerethoseof a conifer, Cryptomeriu japonicn, formed around 1760-1780 (235-255 years ago) at Nikko about 100 km north of Tokyo, and around 1786-1809(186-209 years ago) on YakusbimaIsland, locatedin a remotesouthernregion of Japan.The leadconcentrationsin theseC.juponicu bark pockets,representingthe total lead accumulationduring a period of about 20 yearsat eachsite, were0.1 pg Pb g-’ at Nikko and 0.22 pg Pb g-’ at Yakushima.In contrast, the leadconcentrationin the outer bark of C. juponica at the presenttime is about 150pg Pb g-’ at Nikko (1990)and 1.4 pg Pb g-’ at Yakushima(1992).The useof leaded gasoline,the main sourceof lead in the atmosphere,wasinitiated in Japanin 1949and reacheda maximumduring 1960-1965.As the production of leadedgasolinewas stoppedin 1987,the lead concentrationin the outer bark represents the total for a period of about 40years.Therefore,theseresultssuggest an increasein leadpollution of about three ordersof magnitudeat Nikko, which is relatively closeto Tokyo, and one order of magnitudeat Yakushima, which is relatively remote. Keywork
Bark pocket; Lead; Heavy metal; Monitoring; Year ring
1. Introduction The level of pollution before the Industrial Revolution is of considerable interest for evaluating the present day situation. The Industrial Revolution began in the United Kingdom about 250 years ago, and was followed by the industrialization of other countries in Europe, North America and * Corresponding author. 0048-9697/96/$15.00 SSDI
Asia together with its accompanying environmental pollution. In Japan and China, which closed their borders during 1639-1858 and 1757-1842, respectively, industrialization began in the latter half of the 19th century. Many materials from the environment have been used for the historical monitoring of atmospheric pollutants transported as wet and dry deposition. Sediments[1,2], peat [3], polar ice [4,5] and tree rings [7-lo] are typical of thesematerials.
0 1996 Elsevier Science B.V. All rights reserved
0048-9697(95)04955-Z
26
K. Satake
et al. /The
Science of the Total
However, difftculties remain in finding historical specimens which hold the pollutants without dispersion, translocation or further contamination and which are suitable for dating. We have therefore adopted a new method for monitoring historical trends in air pollution covering the period from several hundred years ago to the present. Inside tree trunks, bark is often enclosed between annual rings, forming so-called bark pockets. Here we describe the initial results of a study on lead pollution recorded in bark pockets of trees before industrialization had occurred in Japan and China. Special interest was paid to these bark pockets because of their potential for showing the pollution level as pollution time capsules. The lead concentrations accumulated in the bark pockets were investigated to demonstrate the applicability of this approach. 2. Materials
ad methods
2.1. Field collection
Outer bark and bark pockets from individual trees of the conifer Cryptomeria japonica wefe collected at Nikko (Imaichi 36’43 ‘N 139O42‘E) in 1990 and Yakushima Island (30°20’N 130°30’E) in 1992. Nikko, a district located about 100 km north of Tokyo, is famous for the Toshogu Shrine, which was built about 360 years ago, when avenues of C. japonica were also planted. There are 13 Ooo individuals of C. japonica 350-360 years old in these avenues. Yakushima, about 25 km in diameter with a maximum altitude of 1935 m, is located in a remote southern region of Japan, about 800 km east of Shanghai, China (Fig. 1). This island includes a national park and world heritage site famous for its many C. japonica older than a thousand years. The bark pocket collected at Nikko was formed around 1760 to 1780 (235-255 years ago) in the trunk of a 350-year-old C. japonica. The bark pocket collected on Yakushima was formed around 1786 to 1809 (186-209 years ago), in the trunk of a 226-year-old C. japonica (Fig. 2a,b).
Environment
181 (19%)
25-30
Fig. I. Locations of Nikko and Yakushima Island in Japan.
2.2. Analytical procedure
The samples were sub-divided into thin sections of outer bark, inner bark, xylem layer and bark pockets with a thickness ranging from 0.15 to 0.76 mm using a Japanese fiat plane and/or razor. These subsections were dried at 60°C for more than 24 h and weighed. The specimens for ICP-MS analysis were digested with a Teflon double-vessel bomb using nitric acid vapor [l 11. The amount of nitric acid in the outer Teflon vessel was 1 ml per 50 mg of sample in the inner vessel. The nitric acid vapor produced at 140% for 4 h was suflicient for digesting the sample in the inner vessel. The digested samples were dissolved in purified water prepared with a Mill&Q water purification system. The dried specimens of bark pockets for SSAAS analysis [12] were analyzed directly. 3. Results The bark pockets of C. japonica collected at Yakushima were formed by damage to the tree trunk followed by covering of the wound with newly produced bark and xylem. This is one of the main mechanisms of bark pocket formation. The
K. Satake
et al. / The Science of the Total Environment
181 (19%)
25-30
a
bark pocket
Fig. 2. A bark pocket in a 226year-old C. japonica tree trunk collected from Yakushima Island in 1992 (a). Physical damage of the tree trunk is the first step. The tree was damaged 206 years ago (1786) when it was 20 years old (b). Then new bark and xylem were produced by the vascular cambium around the wound. The newly produced bark on both sides of the wound come into mutual contact, followed by conjunction of the vascular cambium layer. It took 23 years for the wound to be covered by newly produced annual rings, forming the bark pocket. The rings covering the bark pocket have been produced annually by the conjugated vascular cambium since the tree was 43 years old.
21
28
K. Satake
et al. / The Science of the Total Environment
process of encapsulation of the polluted bark leads to the enclosure of past pollution in the trunk by the tree’s annual rings. The lead concentration in the outer modern bark of the C. japonicu specimen from Nikko, analyzed by both ICP-MS and SSAAS, was about 150 rg Pb g-’ in the outermost sample and decreased to 0.06 pg Pb g-’ (Fig. 3a) in the innermost one. The lead concentration in a radial section of the xylem layer ranged between 0.1 and 0.9 pg Pb g-’ (n = 14, analyzed according to annual rings). The lead concentration in the bark pocket enclosed within the tree trunk at Nikko 235-255
I81 (19%)
25-30
years ago was about 0.1 hg Pb g-‘. In contrast, the lead concentration in the outermost part of the bark from Yakushima which reflects the modern pollution level on the island, was about 1.4 pg Pb ii!-I, two orders of magnitude lower than that in the Nikko sample (Fig. 3b). Furthermore the lead concentration in the bark pocket enclosed within the tree trunk at Yakushima 186-209 years ago was about 0.22 rg Pb g-’ on the outermost side of the outer bark and decreased exponentially to as low as 0.02 c(g g-‘, about one order of magnitude higher than the detection limit for SSAAS analysis (Fig. 3~). 4. Discussion
a outer bark
(Nikko)
,
i
100
. :
J
t
50
I
.
i J
i
I’-’
1.’ b outer bark
(Yakushlma) E
.
. . 0 1
,
.
,
,.
, .*
,-, . -c.
c bark pockzt
. . . 0 /~~“,““,““1~“~1”‘~1”“~“11 0
0.5
1
.
15
2
Depth
(mm)
,
(Yakushlma)
. 2.5
,-,
[
F
1 3
3.5
Fig. 3. Radial distribution of lead concentration in the outer bark of C. japonica collected at Nikko 4 m above the ground (a) and that in the outer bark (b) and bark pocket (c) collected on Yakushima Island (9 & determined by ICP-MS; 0, determined by SSAAS (solid sample atomic absorption spectrometry)).
Lead has been used since ancient times [ 131, and there are many natural inputs of lead to the atmosphere including volcanoes, the sea, and elsewhere [ 141. However, environmental lead pollution increased during 19th and 20th centuries because of the drastic increase in lead consumption due to human activities. The use of leaded gasoline has been mainly responsible for the lead pollution load during the 20th century in urban areas, and long-range transport of lead has been reported [ 151. In the case of Japan, the use of leaded gasoline was initiated in 1949 and reached a maximum level during 1960- 1965. The production of leaded gasoline was finally stopped in 1987. The concentration of lead in the bark pockets of C. japonica at Nikko and Yakushima represent the total lead accumulation during a period of about 20 years at each site before encapsulation of the bark occurred. The lead concentration in modern outer bark represents the total of at least 40 years of the period of use of leaded gasoline ( 1949- 1987) in Japan, although lead pollution is a worldwide phenomenon. These data show that the concentration of lead in tree bark at Nikko (150 pg Pb g-l) is about three orders of magnitude higher than it was 235-255 years ago (0.1 pg g-‘) and at Yakushima 186-209 years ago (0.22 rg g-l). Other studies of the lead concentration in C. juponicu outer bark from Yokohama, near Tokyo, have indicated a maximum of 200 pg Pb g-’ and a value near the highway of 310 rg Pb g-’ [ 16,17).
K. Satake
et al. /The
Science
of the Total
These data are consistent with the lead concentrations in the outer bark obtained at Nikko, and suggest a drastic increase of lead pollution in Japan. There have been many reports [g,lO] of lead concentrations in annual tree rings, which have been used to monitor environmental pollutants. However, the lead concentrations in xylem layers, including annual rings, are very low compared with those in outer bark. The annual rings of a tree are enclosed by the inner and outer bark. Most of a tree’s accumulation of lead from air pollution is deposited in the outer bark (Fig. 2b) and radial transportation through the bark to the xylem is limited [18]. Most of the lead in tree rings originates from the soil. Therefore, it is difficult to obtain direct information on atmospheric lead pollution from annual rings. The lead transported to the xylem layer is initially water-soluble, and thus lateral movement of lead may occur between adjacent rings [19]. Thus, tree ring data may not accurately reflect historical trends in pollution. The biological role of the outer bark, which is in contact with the atmosphere, differs from that of the xylem layer. The outer bark’s role is protection of the inner bark, and cambium and xylem layers from physical biological and chemical damage resulting from, among other things, the accumulation of pollutants. Despite the suitability of bark for providing direct evidence of atmospheric pollution, it has not been commonly used to monitor historical trends in atmospheric pollutants because it is exposed to modern levels of pollution. However, the presence of bark pockets inside the trunk changes this perspective. Bark pockets are located between annual rings, which date them just like a clock. Translocation of pollutants from bark pockets after encapusulation is considered to be unlikely because both of the outer surfaces of the bark in the pockets are in contact with each other, thus preventing translocation (see bark bl and bark b2 in Fig. 2b). In this study, lead pollution was used as a representative type of heavy metal pollution. Similar evaluations are possible for other inorganic and organic pollutants and stable [20] and radioactive isotopes using bark pockets as pollution time capsules.
Environment
181 (19%)
25-30
29
Acknowledgments We Nikko, Island, Uchida
thank S. Kakinuma of Toshogu Shrine, and S. Ichikawa of Y-NAC, Yakushima for their help in Sample collection and T. for his help in sample preparation.
References 111 D.N. Eddington and J.A. Robbins, Records of lead deposition in Lake Michigan sediments since 1800. Environ. Sci. Technol., 10 (1976) 266-274. 121 M. Paezel, H. Scraber and K. Bjerkli, Do decreased trace metal concentrations in surticial Skagerrak sediments over the last 15-30 years indicate decreased pollution? Environ. Pollut., 84 (1994) 213-226. [31 E.L. Livett, J.A. Lee and J.H. Tallis, Lead, zinc and copper of British blanket peats. J. Ecol., 67 (1979) 865-891. 141 M. Murozumi, T.J. Chow and CC. Patterson, Chemical concentrations of pollutant lead aerosols, terrestrial dusts and sea salts in Greenland and Antarctic snow strata. Geochim. Cosmochim. Acta, 33 (1969) 1247-1294. 151CF. Boutron, and CC. Patterson, Lead concentration changes in Antarctic ice during the Wisconsin/Holocene transition. Nature, 323 (1986) 222-225. 161L. Kardell and J. Larsson, Lead and cadmium in oak tree rings (Quercus robur L). Ambio, 7 (1978) 117-121. [71 C. Symeonides, Tree ring analysis for tracing the history of pollution: application to a study in Northern Sweden. J. Environ. Qual., 8 (1979) 483-486. 181 CF. Baes III and H.L. Ragsdale, Age-specific lead distribution in xylem rings of tree genera in Atlanta, Georgia Environ. Pollut. Ser. B, 2 (1981) 21-35. [91 H.L. Ragsdale and C.W. Berish, The decline of lead in tree rings of Carya spp. in urban Atlanta, GA, USA. Biogeochemistry, 6 (1988) 21-29. 1101C. Stewart, D.A. Norton and J.E. Fergusson, Historical monitoring of heavy metals in kahikatea ring wood in Christchurch, New Zealand. Sci. Total Environ., 105 (1991) 171-190. Ill1 I. Kojima and C. Iida, Vapor phase digestion of botanical samples with acids in sealed polytetrafluoroethlene bomb. Anal. Sci., 2 (1986) 567-570. 1121D.C. Baxter and W. Frech, On the direct analysis of solid samples by graphite furnace atomic absorption spectrometry. Fresenius J. Anal. Chem., 337 (1990) 253-263. 1131 GA. Drasch, Lead burden in prehistorical, historical and modem human bones. Sci. Total Environ., 24 (1982) 199-231. I141 A. Ng and C. Patterson, Natural concentrations of lead in ancient arctic and antarctic ice. Geochim. Cosmochim. Acta, 45 (1981) 2109-2121. 1151H. Mukai, A. Tanaka, T. Hujii and M. Nakao, Lead isotope ratios of airborne particulate matter as tracers of
30
K. Satake et al. / The Science of the Total Environment 181 (19%) 25-30
long-range transport of air pollutants around Japan. J. Geophys. Res., 99 (1994) 3717-3726. [la] J.T. Nyangababo and M. Ichikuni, The use of cedar bark in the study of heavy metal contamination in the Nagatsuta area, Japan. Environ. Pollut. Ser. B, 11 (1986) 21 l-229. [ 171 M. Tsurumi, Sources of elements in cedar bark collected near a highway (Japan). Sci. Total Environ., 86 (1989) 239-250. 1181 D. Barnes, M.A. Hamadah and J.M. Ottaway, The lead,
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