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Evaluation of the historical records of lead pollution in the annual growth rings and bark pockets of a 250-year-old Quercus crispula in Nikko, Japan
The Science of the Total Environment 295 (2002) 91–100
Evaluation of the historical records of lead pollution in the annual growth rings and bark pockets of a 250-year-old Quercus crispula in Nikko, Japan David J. Bellisa,*, Kenichi Satakea, Masato Nodab, Naoki Nishimurac, Cameron W. McLeodd a National Institute for Environmental Studies, Onogawa 16-2, Tsukuba, 305-8506, Ibaraki, Japan Field Science Center for Northern Biosphere, Hokkaido University, Otoineppu, Nakagawa-gun, 098-2501, Hokkaido, Japan c Okayama Science University, Kamifukuda, Kawakami-mura, Maniwa-gun, 717-06, Okayama, Japan d Centre for Analytical Sciences, Department of Chemistry, University of Sheffield, Dainton Building, Brook Hill, Sheffield, S3 7HF, UK
b
Received 7 December 2001; accepted 29 January 2002
Abstract The annual growth rings and bark pockets of a 250-year-old Japanese oak (Quercus crispula), collected from the Nikko National Park, Japan in 2000 AD, were analysed by ICP mass spectrometry. The annual rings, sampled in 5year increments, recorded Pb concentrations from 0.01 to 0.1 mg kgy1 and there was no significant change in concentration with time. In contrast, bark pocket samples dating from 1875 to the present showed a progressive increase in Pb concentration with time, from approximately 0.1 to 10 mg kgy1. Shoots of epiphytic moss growing on the tree trunk contained 17 mg kgy1 Pb. The bark pockets recorded historical increases in airborne Pb pollution accompanying the industrialisation of Japan, which was initiated by the opening of Japan’s borders from 1854. This increase was not reflected by the annual rings. The 206Pby207Pb isotope ratio of the bark pockets decreased from approximately 1.18 to 1.16 from 1964 to the present, indicating changes in the sources of Pb pollution. The 206Pby 207 Pb isotope ratio of the moss shoots was similar to the current bark (1.16). The data showed bark pockets to be more effective than annual rings for recording historical change in airborne lead pollution. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Annual rings; Bark pockets; Historical monitoring; Pb pollution; Pb isotope ratio
1. Introduction Historical change in environmental pollution by trace elements, and in particular Pb, has been recorded in layered natural materials such as ice *Corresponding author. Tel.: q81-298-50-2447; fax: q81298-56-7170. E-mail address: [email protected] (D.J. Bellis).
cores, sediments and ombrotrophic peat bogs (Candelone et al., 1995; Shotyk et al., 1996; Weiss et al., 1999). Similarly, biomonitors such as herbarium collections (Herpin et al., 1997), the annual growth rings of trees (Watmough, 1999) and more recently, tree bark pockets (Satake et al., 1996; Bellis et al., in press), have provided historical specimens for analysis. The quality and accuracy
0048-9697/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 2 . 0 0 0 5 4 - 2
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of the record contained in such materials depends on a number of factors, including the level of accumulation from the environment, the rate of formation or growth, the post-depositional stability of the pollutant or material and the accuracy of the dating. Comparison and combination of these data, in conjunction with estimated changes in emissions from records of industrial history (Nriagu, 1996), provides information on how anthropogenic activities have altered natural biogeochemical cycles and increased the toxic load of the environment. Trees are one of the most available specimens for historical monitoring, having a wide geographical distribution in both human-influenced and natural environments. Their rate of growth is well suited to monitoring the large-scale environmental changes that have occurred since the industrial revolution and during the 20th Century, in particular. The use of annual rings as historical monitors has proved controversial, however, as it is unclear whether the relative concentration of trace elements accurately reflects relative changes in the environment (Hagemeyer, 1993; Nabais et al., 2001). Annual rings are formed by all trees in temperate and arctic regions, as differing seasonal growth rates form visible bands. To provide a historical record, a proportion of the pollutants present in the environment must be accumulated and stored within the currently forming annual ring in a reproducible fashion. Trees may accumulate environmental pollutants directly from the atmosphere, by deposition on the leaves or bark, or indirectly following deposition on the soil and subsequent root uptake (Lepp, 1975). In the case of foliar deposition, pollutants may be transported to the annual rings via the phloem whilst deposits on the bark may be transported by diffusion (Lepp and Dollard, 1974). Accumulation via the soil to the annual rings is strongly influenced by the soil chemistry and the solubility and form of the pollutant. There may be a substantial delay in time between deposition and uptake, depending on the rate of migration through the soil to the root depth. Elements dissolved in the soil solution are adsorbed by the root and may be transported to the annual rings in the transpiration stream, which may pass through more than
one annual ring depending on the species (Cutter and Guyette, 1993). Following deposition in the annual ring, radial translocation of elements across ring boundaries can occur, particularly via the rays (Symeonides, 1979). The radial distribution of elements has also been shown to change seasonally (Hagemeyer and Schafer, 1995) and following re-sampling after 10 years (Hagemeyer, 1995). The accumulation of environmental pollutants by annual growth rings is thus an indirect and complex process and highly dependent on water solubility, the tree species and other environmental factors (Cutter and Guyette, 1993). Furthermore, the concentration of many trace elements of environmental concern in the annual rings is relatively low with regard to their concentration in the environment, particularly those with low solubility. Analysis of annual rings has, however, provided valuable information, with a number of studies accurately reflecting known changes in environmental pollution (Guyette et al., 1991; Eklund, 1995; Watmough and Hutchinson, 1996). As noted above, tree bark accumulates trace elements (and other pollutants such as sulphur, nitrates and trace organic compounds (Schulz et al., 1999) directly from the atmosphere through wet or dry deposition and has thus been employed as a biomonitor of (recent) environmental pollution (Ward et al., 1974; Lotschert and Kohm, 1978; Walkenhorst et al., 1993). Deposits are accumulated on the surface of the outer bark and concentrations decrease exponentially with depth (Hammp and Holl, 1974; Satake et al., 1996), indicating that diffusion is limited. A secondary deposition may occur during rainfall following the ‘washout’ of pollutants accumulated at higher levels by the leaves or branches. Loss processes include washout and the shedding of bark. Relatively little is known, however, about these processes. The exposure time of bark to the atmosphere depends on its rate of formation and frequency of shedding, and will vary with the tree species and possibly the age of the trunk or branch (Borger, 1973). The exposure time is not, however necessarily the same as the ‘accumulation period’ and some kind of pseudo steady state concentration may be established. In addition to these processes,
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the concentration of trace elements in bark may also be affected by the texture of the bark, the height and orientation on the tree, the influence of epiphytic lichens and mosses, weather and climate and the immediate surroundings (Walkenhorst et al., 1993). The formation of bark pockets, bark located within the tree trunk, is a relatively common phenomenon. Bark may be enclosed by the trunk during tree growth following damage to the stem, by inclusion of a broken branch, at the junction of two branches or by the converging directions of growth of an irregularly shaped trunk (Satake, 1999, 2001; Bellis, 2000). The relative position of the bark pockets with respect to the annual rings allows their age, i.e. the date they were enclosed by the trunk, to be determined (Satake et al., 1996; Bellis et al., in press). It is assumed that the pollutant content of the bark pocket is representative of the composition of the bark at the time it was enclosed, which reflects the recent level of atmospheric pollution. Comparison of bark pockets of differing age, or of portions of bark pockets progressively enclosed over time, provides a record of historical change in pollution. Whereas historical monitoring with annual rings has been extensively studied there is, as yet, limited data from bark pockets and the two specimens have not been directly compared. In this study, we analyse the Pb concentration and 206Pby 207 Pb isotope ratio in the annual rings and bark pockets of a 250-year-old Japanese oak or ‘Mizunara’ (Quercus crispula) tree, collected from Nikko, Japan, and compare the results in terms of how they reflect historic change in pollution. Japan is currently the world’s second largest economy, but has a shorter industrial history than UK, mainland Europe and North America. Large-scale historical change in airborne Pb pollution may be expected reflecting this industrial development. 2. Experimental A section of trunk containing bark pockets was collected from a 250-year-old Japanese oak (Quercus crispula), felled in December 2000, from the grounds of the National Research Institute of Aqua-culture, at Chugushi in the Nikko National
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Park (latitude 368459, longitude 1398289, altitude 1291 m a.s.l.). Quercus crispula is commonly distributed throughout Japan. The bark was approximately 1 cm in thickness, the outermost part being thinly layered. The bark was partly covered by moss shoots. The Nikko National Park lies approximately 100 km to the north of Tokyo. It is a mountainous region with relatively few urban areas, little industry and is a popular tourist destination. The section of trunk was collected at approximately 1 m above ground level. The trunk contained a number of enclosed bark pockets along three radial axes, designated Bark pockets A, B and C, formed due to the converging directions of growth of the irregularly shaped trunk (Fig. 1). The years over which the bark pockets were enclosed by the trunk was established from the annual growth rings. A 1=1 cm2 section from the outside of the trunk to its centre (63 cm in length) was prepared to provide samples of annual rings. The annual rings were divided into 5-year increments, from which 200-mg samples (dry wt.) were obtained. 1-cm-thick radial slices of the trunk containing the bark pockets were prepared. The outer bark of the bark pockets was sampled at 1-cm intervals, with respect to the radius of the trunk. The samples were 1–2 mm in thickness, providing approximately 100 mg of sample. Similar samples of the current bark were taken from the ‘mouth’ of the bark pocket. A 100-mg sample of moss shoots was also taken from the mouth of Bark Pocket C. The moss shoots were identified to contain both Herpetineuron toccoae (Sull. et Lesq.) Card and Rauiella fujisana (Paris) Reimers. Herpetineuron toccoae is widely distributed in Asia, Africa and South America, growing on tree and rock surfaces. Rauiella fujisana is distributed in Japan and Siberia, growing on tree surfaces. The samples were oven dried for 12 h at 60 8C and digested using Teflon bombs at 140 8C for 4 h with 1-ml high purity concentrated nitric acid per 100 mg sample and diluted with Millipore 18 V high purity water (Kojima and Iida, 1986). Elemental and isotopic analysis of Pb was performed by inductively coupled plasma mass spectrometry (ICP-MS) (HP4500, Yokogawa) at the National Institute for Environmental Studies,
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Fig. 1. Bark pockets in the irregularly shaped trunk of a 250-year-old Japanese oak (Quercus crispula).
Japan. Calibration was provided by standard Pb solutions and a solution containing the Pb isotope standard reference material 982 (NIST). 3. Results The concentration of Pb in the annual rings (1750–2000) was approximately 0.1 mg kgy1
(Table 1). Several samples were lower than the (blank limited) limit of detection of 0.01 mg kgy1. The proximity of the values to the limit of detection most likely reduces the accuracy of the determination and the low concentration increases the risk of error due to contamination during sample preparation. The concentration of Pb in the bark pockets (1875–2000) varied from approxi-
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Table 1 Pb concentration (mg kgy1) in the annual growth rings Years
Pb (mg kgy1)
Years
Pb (mg kgy1)
years
Pb (mg kgy1)
1996–2000 1991–1995 1986–1990 1981–1985 1976–1980 1971–1975 1966–1970 1961–1965 1956–1960 1951–1955 1946–1950 1941–1945 1936–1940 1931–1935 1926–1930 1921–1925 1916–1920
0.023 0.037 0.032 0.056 0.014 -0.010 0.015 -0.010 0.061 -0.010 -0.010 0.012 0.014 0.010 0.010 -0.010 0.016
1911–1915 1906–1910 1901–1905 1896–1900 1891–1895 1886–1890 1881–1885 1876–1880 1871–1875 1866–1870 1861–1865 1856–1860 1851–1855 1846–1850 1841–1845 1836–1840 1831–1835
-0.010 0.024 0.019 0.037 0.086 0.014 0.013 0.017 0.032 0.040 0.040 0.023 0.086 0.027 0.016 0.012 0.033
1826–1830 1821–1825 1816–1820 1811–1815 1806–1810 1801–1805 1796–1800 1791–1795 1786–1790 1781–1785 1776–1780 1771–1775 1766–1770 1761–1765 1756–1760 1750–1755
0.060 0.022 0.020 0.046 0.017 0.012 -0.010 -0.010 -0.010 0.012 -0.010 0.023 0.037 0.034 0.061 0.019
mately 0.1 to 10 mg kgy1 (Table 2). No samples were below detection limits. Compared to the bark pockets, the concentration of Pb in the annual rings was low and showed little change over time (Fig. 2). There was some scatter in the data, which is attributable to the relatively low Pb concentrations and natural variability. The Pb concentration Table 2 Pb concentration (mg kgy1) and
of the bark pockets, however, increased exponentially over 2 orders of magnitude from the earliest available samples (from 1875), which had a similar, but generally higher concentration, than the corresponding annual rings, to the present day bark. The moss shoots contained 17 mg kgy1 Pb (Table 2), somewhat higher than the present bark.
206
Pby207Pb isotope ratios in the bark pockets and moss
Bark pocket
Years
Pb (mg kgy1)
206
Pby 207Pb
A
2000 1997–2000 1994–1996 1968–1973 1964–1967 1944–1955 1930–1943 1922–1929 1913–1921 1904–1912 1900–1903 1894–1899 1890–1893 1884–1889 1880–1883 1875–1879
2.6 5.7 2.0 4.2 2.0 0.50 0.62 0.93 0.82 0.21 0.20 0.060 0.088 0.092 0.093 0.061
1.152 1.163 1.164 1.171 1.167 1.171 1.170 1.174 1.179 1.173 1.172 1.176 1.180 1.186 1.175 1.179
206
s (ns10)
6.3 5.0 1.0 1.2
1.163 1.172 1.173 1.174
0.004 0.004 0.007 0.008
2000 2000 1995–2000 1992–1994 1986–1991 1979–1985 1974–1978 1969–1973 1964–1968
7.3 10 6.1 7.2 4.2 2.7 2.8 2.4 3.2
1.156 1.160 1.162 1.169 1.172 1.171 1.174 1.177 1.177
0.006 0.003 0.004 0.006 0.004 0.006 0.005 0.005 0.006
2000
17
1.157
0.006
s (ns10)
Bark pocket
Years
0.010 0.004 0.006 0.006 0.008 0.020 0.012 0.004 0.012 0.021 0.033 0.037 0.031 0.026 0.032 0.025
B
2000 1993–2000 1970–1972 1926–1938
C
Moss shoots
Pb (mg kgy1)
Pby 207Pb
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Fig. 2. Pb concentration in the annual growth rings, bark pockets and moss shoots of a 250-year-old Japanese oak from Nikko, Japan.
The three sections of bark pockets contained discrete bark pockets covering different periods of time (Table 2). Bark pocket A was continuously enclosed from 1875 to 1955, 1964 to 1973, and from 1994 to the present (2000). Between these times no bark was enclosed. Similarly, Bark pocket B was enclosed from 1926 to 1938, 1970 to 1972, and 1994 to 2000, while Bark pocket C was continuous from 1964 to 2000. There is some difference in the Pb concentration of samples of the same age from the different sections and in the current bark, but this is small compared to the change with time, and the overall trend in Pb concentration in the different bark pockets is consistent. The low concentration of Pb in the annual rings prevented accurate and precise determination of the 206Pby 207Pb isotope ratio, and analysis was thus restricted to the bark pocket and moss shoots (Table 2). Bark pocket samples formed prior to
ca.1960 consistently recorded 206Pby 207Pb isotope ratios between 1.17 and 1.18, though the precision of individual measurements was relatively poor due to the low concentrations. Samples enclosed after ca.1960 recorded lower 206Pby 207Pb. The trend was particularly clear in ‘Bark pocket C (Fig. 3), which showed a progressive decrease in 206 Pby 207Pb from approximately 1.18 to 1.16, accompanying increasing concentrations. The moss shoots recorded a similar 206Pby207 Pb to the current bark. 4. Discussion The concentration of Pb in the annual growth rings in Quercus crispula from Nikko was relatively low compared to the majority of studies in the literature, which mostly sampled locations close to significant sources such as smelters or urban environments (Hagemeyer, 1993; Wat-
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Fig. 3.
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206
Pby207Pb isotope ratios (mean"S.D., ns10) in Bark pocket C and moss shoots.
mough, 1999). Similarly, the Pb concentration of the current bark, 3–10 mg kgy1, was also relatively low compared to other studies employing bark as a biomonitor (approximate range 1–10 000 mg kgy1, Nyangababo and Ichikuni, 1986; Walkenhorst et al., 1993; Bellis et al., 2001). The low concentration of Pb reflects the relatively low-level of pollution in the Nikko National Park. The concentration of Pb was higher in the bark pockets than annual rings, reflecting a higher level of accumulation from the environment. The Pb concentration of the annual rings from 1750–2000 remained low (-0.1 mg kgy1) over time, providing no evidence of historic change in the level of pollution. In contrast, the bark pockets showed an approximately 100-fold increase in Pb concentration from 1875 to 2000. No bark pocket samples formed prior to 1875 were available. It is unlikely that the concentration of bark during this period was lower than the annual rings. The lowest
concentration of Pb was recorded in the oldest bark pockets (ca. 1875). The concentrations, of approximately 0.1 mg kgy1, are similar to those recorded in bark pockets formed between 1760– 1780 (0.10 mg kgy1 Pb) and 1789–1809 (0.22 mg kgy1 Pb) at other sites in Japan by Satake et al. (1996). This level of Pb concentration in bark is probably representative of the natural background level, as there was little industrial activity in Japan at this time, apart from localised metal workings. It is likely that the natural background concentration in vegetation in industrialised countries like Japan can only be determined from historic samples. The concentration of Pb in the bark pockets rises exponentially over time from ca. 1875, indicating large increases in atmospheric Pb inputs. Increasing atmospheric Pb pollution is strongly linked to industrialisation (Nriagu, 1996). The industrialisation of Japan began in the latter half
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Table 3 206 Pby207Pb isotope ratios of selected materials in Japan (Mukai et al., 1993) Material
206 Pby 207Pb isotope ratioa
Pb oresb Petrol Coals and soils Waste incineration (fly ash) Aerosol
1.15–1.20 1.05–1.21 1.18–1.20 1.15 1.14–1.19
a
Original data 207Pby 206Pb. b Imported and domestic.
of the 19th Century. The arrival of the American fleet of Captain Perry in 1853 led to the opening of Japans borders the following year and ultimately to the Meiji restoration of 1968, which established Tokyo as the capital of Japan. The following period saw the introduction of many western industries and with them the start of widespread pollution in the Japan, some 125 years after The Industrial Revolution began in the UK (1750). It appears that the level of airborne Pb at this location was still increasing in recent years. Smelting and fossil fuel combustion are likely to have been major sources of Pb throughout the industrial period. In particular, copper smelting in the neighbouring district of Ashio (1877–1973) may have contributed significant amounts. Pb additives in petrol (gasoline) and waste incineration are likely to have become significant in the latter half of the 20th Century. Table 3 shows the 206Pby 207 Pb isotope ratios of various materials in Japan (Mukai et al., 1993). The 206Pby 207Pb of the bark pockets prior to ca.1960 (1.17–1.18) is broadly consistent with emissions from smelting, with a possible contribution from coal combustion. The reduction in 206Pby 207Pb recorded in the bark pockets from ca.1960 indicates a change in the sources of Pb or in their relative contribution. In the UK and mainland Europe, large reductions in the 206Pby 207Pb isotope ratio have been recorded, resulting from the use of Pb additives in petrol (Weiss et al., 1999). Pb additives in petrol were employed in Japan from 1949 to 1987, with maximum usage during 1960–65 (Mukai et al., 1993). The 206Pby 207Pb isotope ratio of the addi-
tives varied widely (1.05–1.21), however, and this source is not distinguished. The reduction in 206Pby 207Pb may have resulted, in part, from waste incineration (206Pby 207Pbs 1.15). The current bark and moss shoots have similar 206Pby 207Pb isotope ratios to typical values reported in atmospheric aerosol and the values reported for fly-ash from waste (refuse) incinerators (Table 3). The relatively wide range and similarity of the 206Pby 207Pb isotope ratios of the likely sources makes them difficult to distinguish, particularly if no individual source is dominant. It is likely that the Pb contained in the bark and bark pockets is a mix of multiple sources. The similarity of the Pb concentration and the 206Pby 207Pb isotope ratio of the current bark and moss is significant, as mosses are known to be effective monitors of recent airborne pollution (Bruning and Kreeb, 1993). In particular, the near identical 206Pby 207Pb suggest a similar accumulation time for bark and moss, which would be relatively short compared to the time period covered by the bark pocket. Moss shoots may also be preserved within bark pockets (Satake et al., 1995). Bark pockets appear to provide a reliable measure of historic change in airborne Pb pollution, and may potentially provide information on other trace elements and environmental pollutants. In particular, bark pockets appear to be more effective historical monitors than annual growth rings. A recent extensive review of nearly 200 published studies, dating from 1957, concluded that annual ring analysis does not provide a reliable record of historical change in pollution, principally as the radial distribution of elements is not stable over time (Nabais et al., 2001). Unlike annual rings, bark pockets accumulate pollutants directly from the atmosphere, resulting in higher element concentrations, particularly for insoluble elements (Bellis, 2000). There should also be time-lag due to delayed accumulation, (i.e. from the soil), while dispersal and translocation of elements following encapsulation within the bark pocket is considered to be limited (Satake, 2001). Bark pockets open new possibilities in the field of historical monitoring and should find widespread application in the future.
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5. Conclusions Trees provide information on historical change in environmental pollution. This study showed that bark pockets are more effective monitors of airborne Pb pollution than annual rings, as they accumulate Pb directly from the atmosphere. The bark pockets recorded a 100-fold increase in Pb concentration from ca. 1875 to the present, illustrating the impact of industrialisation on the environment in Japan. The annual rings had lower Pb concentrations and did not reflect increasing levels of pollution, but provided an effective dating. Acknowledgments The authors would like to express their gratitude to Kazumasa Ikuta (National Research Institute of Aqua-culture, Japan), Kazuo Kawakami (Kawakami Green Environment, Japan), Hideaki Shibata (Hokkaido University, Japan), Tetsuo Noguchi and Kyouko Takada (National Institute of Environmental Studies, Japan) for valuable contributions to sample collection, preparation and analysis. D.J. Bellis would like to acknowledge the receipt of a Fellowship from the Science and Technology Agency, Japan. The study was supported by a Grant-in-Aid for Scientific Research (B) from The Ministry of Education, Culture, Sports, Science and Technology, Japan. References Bellis DJ. Monitoring airborne trace elements in past and present environments with tree bark. PhD Thesis. University of Sheffield, UK, 2000:89–137. Bellis DJ, McLeod CW, Satake K. The potential of elemental and isotopic analysis of tree bark for discriminating sources of airborne lead contamination in the UK. J Environ Monit 2001;3:198 –201. Bellis DJ, McLeod CW, Satake K. Pb and 206Pby207Pb isotopic analysis of a tree bark pocket near Sheffield, UK recording historical change in airborne pollution during the 20th Century. Sci Total Environ, in press. Bruning F, Kreeb KH. Mosses as biomoniotors of heavy metal contamination within urban areas. In: Markert B, editor. Plants as Biomonitors-Indicators for Heavy Metals in the Terrestrial Environment. Weinheim: VCH, 1993. p. 395 – 401. Borger GA. Development and shedding of tree bark. In: Kozlowski TT, editor. Development and Shedding of Plant Parts. New York: Academic Press, 1973. p. 205 –237.
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Candelone JP, Hong S, Pellone C, Boutron CF. Post-industrial revolution changes in large-scale atmospheric pollution of the northern hemisphere by heavy metals as documented in central Greenland snow. J Geophys Res 1995;100:16605 – 16616. Cutter BE, Guyette RP. Anatomical, chemical and ecological factors affecting tree species choice in dendrochemical studies. J Environ Qual 1993;22:611 –619. Eklund M. Cadmium and lead deposition around a Swedish battery plant as recorded in oak tree rings. J Environ Qual 1995;24:126 –131. Guyette RP, Cutter BE, Henderson GS. Long-term relationships between mining activity and levels of lead and cadmium in tree-rings of eastern red cedar. J Environ Qual 1991;20:146 – 150. Hagemeyer J. Monitoring trace metal pollution with tree rings: a critical reassessment. In: Markert B, editor. Plants as biomonitors—indicators for heavy metals in the terrestrial environment. Weinheim: VCH, 1993. p. 541 –563. Hagemeyer J. Radial distributions of Cd in stems of oak trees (Quercus robur L.) re-analysed after 10 years. Trees 1995;9:200 –203. Hagemeyer J, Schafer H. Seasonal variations in concentrations and radial distribution patterns of Cd, Pb and Zn in stem wood of beech trees (Fagus sylvatica L.). Sci Total Environ 1995;166:77 –87. Hammp R, Holl W. Radial and axial gradient of lead concentration in hardwoods. Arch Environ Cont Tox 1974;2:143 – 151. Herpin U, Markert B, Wechert V, Berlekamp J, Siewers U, Leith H. Retrospective analysis of heavy metal concentrations at selected locations in the Federal Republic of Germany using moss material from a herbarium. Sci Total Environ 1997;205:1 –12. Kojima I, Iida C. Vapour phase digestion of botanical samples with acids in sealed polytetrafluoroethlene bomb. Anal Sci 1986;2:567 –570. Lepp NW. The potential of tree rings for monitoring heavy metal pollution patterns. Environ Pollut 1975;9:49 –61. Lepp NW, Dollard GJ. Studies on lateral movement of 210Pb in woody stems: patterns observed in dormant and nondormant stems. Oecologia 1974;15:179 –184. Lotschert W, Kohm H-J. Characteristics of tree bark as an indicator in high-immission areas II. Contents of Heavy Metals. Oecologia 1978;37:121 –132. Mukai H, Furuta N, Fujii T, Ambe Y, Sakamoto K, Hashimoto Y. Characterisation of sources of lead in the urban air of Asia using ratios of stable lead isotopes. Environ Sci Technol 1993;27:1347 –1356. Nabais C, Freitas H, Hagemeyer J. Tree rings and dendroanalysis. In: Prasad MNV, editor. Metals in the Environment: Analysis by Biodiversity. New York: Marcel Decker, 2001. p. 367 –400. Nriagu JO. A history of global metal pollution. Science 1996;272:246 –248.
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Nyangababo JT, Ichikuni M. The use of Cedar bark in the study of heavy metal contamination in the Nagasuta area, Japan. Environ Pollut (B) 1986;11:211 –229. Satake K. Bark pockets record the historical change of atmospheric pollution (in Japanese). Scias, Dec. 1999: 60–61. Satake K. Tree bark. Tree ‘bark pockets’ as pollution time capsules for historical monitoring. In: Prasad MNV, editor. Metals in the Environment: Analysis by Biodiversity. New York: Marcel Decker, 2001. p. 353 –366. Satake K, Kimura K, Tanaka A. Two-hundred-years old hoots of the epiphytic moss Brotherella henonii preserved in a bark pocket of the conifer Cryptomeria japonica. J Bryol 1995;18:815 –832. Satake K, Tanaka A, Kimura K. Accumulation of lead in tree trunk bark pockets as pollution time capsules. Sci Total Environ 1996;181:25 –30. Schulz H, Popp P, Huhn H, Stark H-J, Schuurmann G. Biomonitoring of airborne inorganic and organic pollutants by means of pine tree barks. I. Temporal and spatial variations. Sci Total Environ 1999;232:49 –58. Shotyk W, Norton SA, Farmer JG. Summary of the workshop on peat bog archives of atmospheric metal deposition. Water Air Soil Pollut 1996;100:213 –219.
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Evaluation of the historical records of lead pollution in the annual growth rings and bark pockets of a 250-year-old Quercus crispula in Nikko, Japan David J. Bellisa,*, Kenichi Satakea, Masato Nodab, Naoki Nishimurac, Cameron W. McLeodd a National Institute for Environmental Studies, Onogawa 16-2, Tsukuba, 305-8506, Ibaraki, Japan Field Science Center for Northern Biosphere, Hokkaido University, Otoineppu, Nakagawa-gun, 098-2501, Hokkaido, Japan c Okayama Science University, Kamifukuda, Kawakami-mura, Maniwa-gun, 717-06, Okayama, Japan d Centre for Analytical Sciences, Department of Chemistry, University of Sheffield, Dainton Building, Brook Hill, Sheffield, S3 7HF, UK
b
Received 7 December 2001; accepted 29 January 2002
Abstract The annual growth rings and bark pockets of a 250-year-old Japanese oak (Quercus crispula), collected from the Nikko National Park, Japan in 2000 AD, were analysed by ICP mass spectrometry. The annual rings, sampled in 5year increments, recorded Pb concentrations from 0.01 to 0.1 mg kgy1 and there was no significant change in concentration with time. In contrast, bark pocket samples dating from 1875 to the present showed a progressive increase in Pb concentration with time, from approximately 0.1 to 10 mg kgy1. Shoots of epiphytic moss growing on the tree trunk contained 17 mg kgy1 Pb. The bark pockets recorded historical increases in airborne Pb pollution accompanying the industrialisation of Japan, which was initiated by the opening of Japan’s borders from 1854. This increase was not reflected by the annual rings. The 206Pby207Pb isotope ratio of the bark pockets decreased from approximately 1.18 to 1.16 from 1964 to the present, indicating changes in the sources of Pb pollution. The 206Pby 207 Pb isotope ratio of the moss shoots was similar to the current bark (1.16). The data showed bark pockets to be more effective than annual rings for recording historical change in airborne lead pollution. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Annual rings; Bark pockets; Historical monitoring; Pb pollution; Pb isotope ratio
1. Introduction Historical change in environmental pollution by trace elements, and in particular Pb, has been recorded in layered natural materials such as ice *Corresponding author. Tel.: q81-298-50-2447; fax: q81298-56-7170. E-mail address: [email protected] (D.J. Bellis).
cores, sediments and ombrotrophic peat bogs (Candelone et al., 1995; Shotyk et al., 1996; Weiss et al., 1999). Similarly, biomonitors such as herbarium collections (Herpin et al., 1997), the annual growth rings of trees (Watmough, 1999) and more recently, tree bark pockets (Satake et al., 1996; Bellis et al., in press), have provided historical specimens for analysis. The quality and accuracy
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of the record contained in such materials depends on a number of factors, including the level of accumulation from the environment, the rate of formation or growth, the post-depositional stability of the pollutant or material and the accuracy of the dating. Comparison and combination of these data, in conjunction with estimated changes in emissions from records of industrial history (Nriagu, 1996), provides information on how anthropogenic activities have altered natural biogeochemical cycles and increased the toxic load of the environment. Trees are one of the most available specimens for historical monitoring, having a wide geographical distribution in both human-influenced and natural environments. Their rate of growth is well suited to monitoring the large-scale environmental changes that have occurred since the industrial revolution and during the 20th Century, in particular. The use of annual rings as historical monitors has proved controversial, however, as it is unclear whether the relative concentration of trace elements accurately reflects relative changes in the environment (Hagemeyer, 1993; Nabais et al., 2001). Annual rings are formed by all trees in temperate and arctic regions, as differing seasonal growth rates form visible bands. To provide a historical record, a proportion of the pollutants present in the environment must be accumulated and stored within the currently forming annual ring in a reproducible fashion. Trees may accumulate environmental pollutants directly from the atmosphere, by deposition on the leaves or bark, or indirectly following deposition on the soil and subsequent root uptake (Lepp, 1975). In the case of foliar deposition, pollutants may be transported to the annual rings via the phloem whilst deposits on the bark may be transported by diffusion (Lepp and Dollard, 1974). Accumulation via the soil to the annual rings is strongly influenced by the soil chemistry and the solubility and form of the pollutant. There may be a substantial delay in time between deposition and uptake, depending on the rate of migration through the soil to the root depth. Elements dissolved in the soil solution are adsorbed by the root and may be transported to the annual rings in the transpiration stream, which may pass through more than
one annual ring depending on the species (Cutter and Guyette, 1993). Following deposition in the annual ring, radial translocation of elements across ring boundaries can occur, particularly via the rays (Symeonides, 1979). The radial distribution of elements has also been shown to change seasonally (Hagemeyer and Schafer, 1995) and following re-sampling after 10 years (Hagemeyer, 1995). The accumulation of environmental pollutants by annual growth rings is thus an indirect and complex process and highly dependent on water solubility, the tree species and other environmental factors (Cutter and Guyette, 1993). Furthermore, the concentration of many trace elements of environmental concern in the annual rings is relatively low with regard to their concentration in the environment, particularly those with low solubility. Analysis of annual rings has, however, provided valuable information, with a number of studies accurately reflecting known changes in environmental pollution (Guyette et al., 1991; Eklund, 1995; Watmough and Hutchinson, 1996). As noted above, tree bark accumulates trace elements (and other pollutants such as sulphur, nitrates and trace organic compounds (Schulz et al., 1999) directly from the atmosphere through wet or dry deposition and has thus been employed as a biomonitor of (recent) environmental pollution (Ward et al., 1974; Lotschert and Kohm, 1978; Walkenhorst et al., 1993). Deposits are accumulated on the surface of the outer bark and concentrations decrease exponentially with depth (Hammp and Holl, 1974; Satake et al., 1996), indicating that diffusion is limited. A secondary deposition may occur during rainfall following the ‘washout’ of pollutants accumulated at higher levels by the leaves or branches. Loss processes include washout and the shedding of bark. Relatively little is known, however, about these processes. The exposure time of bark to the atmosphere depends on its rate of formation and frequency of shedding, and will vary with the tree species and possibly the age of the trunk or branch (Borger, 1973). The exposure time is not, however necessarily the same as the ‘accumulation period’ and some kind of pseudo steady state concentration may be established. In addition to these processes,
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the concentration of trace elements in bark may also be affected by the texture of the bark, the height and orientation on the tree, the influence of epiphytic lichens and mosses, weather and climate and the immediate surroundings (Walkenhorst et al., 1993). The formation of bark pockets, bark located within the tree trunk, is a relatively common phenomenon. Bark may be enclosed by the trunk during tree growth following damage to the stem, by inclusion of a broken branch, at the junction of two branches or by the converging directions of growth of an irregularly shaped trunk (Satake, 1999, 2001; Bellis, 2000). The relative position of the bark pockets with respect to the annual rings allows their age, i.e. the date they were enclosed by the trunk, to be determined (Satake et al., 1996; Bellis et al., in press). It is assumed that the pollutant content of the bark pocket is representative of the composition of the bark at the time it was enclosed, which reflects the recent level of atmospheric pollution. Comparison of bark pockets of differing age, or of portions of bark pockets progressively enclosed over time, provides a record of historical change in pollution. Whereas historical monitoring with annual rings has been extensively studied there is, as yet, limited data from bark pockets and the two specimens have not been directly compared. In this study, we analyse the Pb concentration and 206Pby 207 Pb isotope ratio in the annual rings and bark pockets of a 250-year-old Japanese oak or ‘Mizunara’ (Quercus crispula) tree, collected from Nikko, Japan, and compare the results in terms of how they reflect historic change in pollution. Japan is currently the world’s second largest economy, but has a shorter industrial history than UK, mainland Europe and North America. Large-scale historical change in airborne Pb pollution may be expected reflecting this industrial development. 2. Experimental A section of trunk containing bark pockets was collected from a 250-year-old Japanese oak (Quercus crispula), felled in December 2000, from the grounds of the National Research Institute of Aqua-culture, at Chugushi in the Nikko National
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Park (latitude 368459, longitude 1398289, altitude 1291 m a.s.l.). Quercus crispula is commonly distributed throughout Japan. The bark was approximately 1 cm in thickness, the outermost part being thinly layered. The bark was partly covered by moss shoots. The Nikko National Park lies approximately 100 km to the north of Tokyo. It is a mountainous region with relatively few urban areas, little industry and is a popular tourist destination. The section of trunk was collected at approximately 1 m above ground level. The trunk contained a number of enclosed bark pockets along three radial axes, designated Bark pockets A, B and C, formed due to the converging directions of growth of the irregularly shaped trunk (Fig. 1). The years over which the bark pockets were enclosed by the trunk was established from the annual growth rings. A 1=1 cm2 section from the outside of the trunk to its centre (63 cm in length) was prepared to provide samples of annual rings. The annual rings were divided into 5-year increments, from which 200-mg samples (dry wt.) were obtained. 1-cm-thick radial slices of the trunk containing the bark pockets were prepared. The outer bark of the bark pockets was sampled at 1-cm intervals, with respect to the radius of the trunk. The samples were 1–2 mm in thickness, providing approximately 100 mg of sample. Similar samples of the current bark were taken from the ‘mouth’ of the bark pocket. A 100-mg sample of moss shoots was also taken from the mouth of Bark Pocket C. The moss shoots were identified to contain both Herpetineuron toccoae (Sull. et Lesq.) Card and Rauiella fujisana (Paris) Reimers. Herpetineuron toccoae is widely distributed in Asia, Africa and South America, growing on tree and rock surfaces. Rauiella fujisana is distributed in Japan and Siberia, growing on tree surfaces. The samples were oven dried for 12 h at 60 8C and digested using Teflon bombs at 140 8C for 4 h with 1-ml high purity concentrated nitric acid per 100 mg sample and diluted with Millipore 18 V high purity water (Kojima and Iida, 1986). Elemental and isotopic analysis of Pb was performed by inductively coupled plasma mass spectrometry (ICP-MS) (HP4500, Yokogawa) at the National Institute for Environmental Studies,
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Fig. 1. Bark pockets in the irregularly shaped trunk of a 250-year-old Japanese oak (Quercus crispula).
Japan. Calibration was provided by standard Pb solutions and a solution containing the Pb isotope standard reference material 982 (NIST). 3. Results The concentration of Pb in the annual rings (1750–2000) was approximately 0.1 mg kgy1
(Table 1). Several samples were lower than the (blank limited) limit of detection of 0.01 mg kgy1. The proximity of the values to the limit of detection most likely reduces the accuracy of the determination and the low concentration increases the risk of error due to contamination during sample preparation. The concentration of Pb in the bark pockets (1875–2000) varied from approxi-
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Table 1 Pb concentration (mg kgy1) in the annual growth rings Years
Pb (mg kgy1)
Years
Pb (mg kgy1)
years
Pb (mg kgy1)
1996–2000 1991–1995 1986–1990 1981–1985 1976–1980 1971–1975 1966–1970 1961–1965 1956–1960 1951–1955 1946–1950 1941–1945 1936–1940 1931–1935 1926–1930 1921–1925 1916–1920
0.023 0.037 0.032 0.056 0.014 -0.010 0.015 -0.010 0.061 -0.010 -0.010 0.012 0.014 0.010 0.010 -0.010 0.016
1911–1915 1906–1910 1901–1905 1896–1900 1891–1895 1886–1890 1881–1885 1876–1880 1871–1875 1866–1870 1861–1865 1856–1860 1851–1855 1846–1850 1841–1845 1836–1840 1831–1835
-0.010 0.024 0.019 0.037 0.086 0.014 0.013 0.017 0.032 0.040 0.040 0.023 0.086 0.027 0.016 0.012 0.033
1826–1830 1821–1825 1816–1820 1811–1815 1806–1810 1801–1805 1796–1800 1791–1795 1786–1790 1781–1785 1776–1780 1771–1775 1766–1770 1761–1765 1756–1760 1750–1755
0.060 0.022 0.020 0.046 0.017 0.012 -0.010 -0.010 -0.010 0.012 -0.010 0.023 0.037 0.034 0.061 0.019
mately 0.1 to 10 mg kgy1 (Table 2). No samples were below detection limits. Compared to the bark pockets, the concentration of Pb in the annual rings was low and showed little change over time (Fig. 2). There was some scatter in the data, which is attributable to the relatively low Pb concentrations and natural variability. The Pb concentration Table 2 Pb concentration (mg kgy1) and
of the bark pockets, however, increased exponentially over 2 orders of magnitude from the earliest available samples (from 1875), which had a similar, but generally higher concentration, than the corresponding annual rings, to the present day bark. The moss shoots contained 17 mg kgy1 Pb (Table 2), somewhat higher than the present bark.
206
Pby207Pb isotope ratios in the bark pockets and moss
Bark pocket
Years
Pb (mg kgy1)
206
Pby 207Pb
A
2000 1997–2000 1994–1996 1968–1973 1964–1967 1944–1955 1930–1943 1922–1929 1913–1921 1904–1912 1900–1903 1894–1899 1890–1893 1884–1889 1880–1883 1875–1879
2.6 5.7 2.0 4.2 2.0 0.50 0.62 0.93 0.82 0.21 0.20 0.060 0.088 0.092 0.093 0.061
1.152 1.163 1.164 1.171 1.167 1.171 1.170 1.174 1.179 1.173 1.172 1.176 1.180 1.186 1.175 1.179
206
s (ns10)
6.3 5.0 1.0 1.2
1.163 1.172 1.173 1.174
0.004 0.004 0.007 0.008
2000 2000 1995–2000 1992–1994 1986–1991 1979–1985 1974–1978 1969–1973 1964–1968
7.3 10 6.1 7.2 4.2 2.7 2.8 2.4 3.2
1.156 1.160 1.162 1.169 1.172 1.171 1.174 1.177 1.177
0.006 0.003 0.004 0.006 0.004 0.006 0.005 0.005 0.006
2000
17
1.157
0.006
s (ns10)
Bark pocket
Years
0.010 0.004 0.006 0.006 0.008 0.020 0.012 0.004 0.012 0.021 0.033 0.037 0.031 0.026 0.032 0.025
B
2000 1993–2000 1970–1972 1926–1938
C
Moss shoots
Pb (mg kgy1)
Pby 207Pb
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Fig. 2. Pb concentration in the annual growth rings, bark pockets and moss shoots of a 250-year-old Japanese oak from Nikko, Japan.
The three sections of bark pockets contained discrete bark pockets covering different periods of time (Table 2). Bark pocket A was continuously enclosed from 1875 to 1955, 1964 to 1973, and from 1994 to the present (2000). Between these times no bark was enclosed. Similarly, Bark pocket B was enclosed from 1926 to 1938, 1970 to 1972, and 1994 to 2000, while Bark pocket C was continuous from 1964 to 2000. There is some difference in the Pb concentration of samples of the same age from the different sections and in the current bark, but this is small compared to the change with time, and the overall trend in Pb concentration in the different bark pockets is consistent. The low concentration of Pb in the annual rings prevented accurate and precise determination of the 206Pby 207Pb isotope ratio, and analysis was thus restricted to the bark pocket and moss shoots (Table 2). Bark pocket samples formed prior to
ca.1960 consistently recorded 206Pby 207Pb isotope ratios between 1.17 and 1.18, though the precision of individual measurements was relatively poor due to the low concentrations. Samples enclosed after ca.1960 recorded lower 206Pby 207Pb. The trend was particularly clear in ‘Bark pocket C (Fig. 3), which showed a progressive decrease in 206 Pby 207Pb from approximately 1.18 to 1.16, accompanying increasing concentrations. The moss shoots recorded a similar 206Pby207 Pb to the current bark. 4. Discussion The concentration of Pb in the annual growth rings in Quercus crispula from Nikko was relatively low compared to the majority of studies in the literature, which mostly sampled locations close to significant sources such as smelters or urban environments (Hagemeyer, 1993; Wat-
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Fig. 3.
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206
Pby207Pb isotope ratios (mean"S.D., ns10) in Bark pocket C and moss shoots.
mough, 1999). Similarly, the Pb concentration of the current bark, 3–10 mg kgy1, was also relatively low compared to other studies employing bark as a biomonitor (approximate range 1–10 000 mg kgy1, Nyangababo and Ichikuni, 1986; Walkenhorst et al., 1993; Bellis et al., 2001). The low concentration of Pb reflects the relatively low-level of pollution in the Nikko National Park. The concentration of Pb was higher in the bark pockets than annual rings, reflecting a higher level of accumulation from the environment. The Pb concentration of the annual rings from 1750–2000 remained low (-0.1 mg kgy1) over time, providing no evidence of historic change in the level of pollution. In contrast, the bark pockets showed an approximately 100-fold increase in Pb concentration from 1875 to 2000. No bark pocket samples formed prior to 1875 were available. It is unlikely that the concentration of bark during this period was lower than the annual rings. The lowest
concentration of Pb was recorded in the oldest bark pockets (ca. 1875). The concentrations, of approximately 0.1 mg kgy1, are similar to those recorded in bark pockets formed between 1760– 1780 (0.10 mg kgy1 Pb) and 1789–1809 (0.22 mg kgy1 Pb) at other sites in Japan by Satake et al. (1996). This level of Pb concentration in bark is probably representative of the natural background level, as there was little industrial activity in Japan at this time, apart from localised metal workings. It is likely that the natural background concentration in vegetation in industrialised countries like Japan can only be determined from historic samples. The concentration of Pb in the bark pockets rises exponentially over time from ca. 1875, indicating large increases in atmospheric Pb inputs. Increasing atmospheric Pb pollution is strongly linked to industrialisation (Nriagu, 1996). The industrialisation of Japan began in the latter half
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Table 3 206 Pby207Pb isotope ratios of selected materials in Japan (Mukai et al., 1993) Material
206 Pby 207Pb isotope ratioa
Pb oresb Petrol Coals and soils Waste incineration (fly ash) Aerosol
1.15–1.20 1.05–1.21 1.18–1.20 1.15 1.14–1.19
a
Original data 207Pby 206Pb. b Imported and domestic.
of the 19th Century. The arrival of the American fleet of Captain Perry in 1853 led to the opening of Japans borders the following year and ultimately to the Meiji restoration of 1968, which established Tokyo as the capital of Japan. The following period saw the introduction of many western industries and with them the start of widespread pollution in the Japan, some 125 years after The Industrial Revolution began in the UK (1750). It appears that the level of airborne Pb at this location was still increasing in recent years. Smelting and fossil fuel combustion are likely to have been major sources of Pb throughout the industrial period. In particular, copper smelting in the neighbouring district of Ashio (1877–1973) may have contributed significant amounts. Pb additives in petrol (gasoline) and waste incineration are likely to have become significant in the latter half of the 20th Century. Table 3 shows the 206Pby 207 Pb isotope ratios of various materials in Japan (Mukai et al., 1993). The 206Pby 207Pb of the bark pockets prior to ca.1960 (1.17–1.18) is broadly consistent with emissions from smelting, with a possible contribution from coal combustion. The reduction in 206Pby 207Pb recorded in the bark pockets from ca.1960 indicates a change in the sources of Pb or in their relative contribution. In the UK and mainland Europe, large reductions in the 206Pby 207Pb isotope ratio have been recorded, resulting from the use of Pb additives in petrol (Weiss et al., 1999). Pb additives in petrol were employed in Japan from 1949 to 1987, with maximum usage during 1960–65 (Mukai et al., 1993). The 206Pby 207Pb isotope ratio of the addi-
tives varied widely (1.05–1.21), however, and this source is not distinguished. The reduction in 206Pby 207Pb may have resulted, in part, from waste incineration (206Pby 207Pbs 1.15). The current bark and moss shoots have similar 206Pby 207Pb isotope ratios to typical values reported in atmospheric aerosol and the values reported for fly-ash from waste (refuse) incinerators (Table 3). The relatively wide range and similarity of the 206Pby 207Pb isotope ratios of the likely sources makes them difficult to distinguish, particularly if no individual source is dominant. It is likely that the Pb contained in the bark and bark pockets is a mix of multiple sources. The similarity of the Pb concentration and the 206Pby 207Pb isotope ratio of the current bark and moss is significant, as mosses are known to be effective monitors of recent airborne pollution (Bruning and Kreeb, 1993). In particular, the near identical 206Pby 207Pb suggest a similar accumulation time for bark and moss, which would be relatively short compared to the time period covered by the bark pocket. Moss shoots may also be preserved within bark pockets (Satake et al., 1995). Bark pockets appear to provide a reliable measure of historic change in airborne Pb pollution, and may potentially provide information on other trace elements and environmental pollutants. In particular, bark pockets appear to be more effective historical monitors than annual growth rings. A recent extensive review of nearly 200 published studies, dating from 1957, concluded that annual ring analysis does not provide a reliable record of historical change in pollution, principally as the radial distribution of elements is not stable over time (Nabais et al., 2001). Unlike annual rings, bark pockets accumulate pollutants directly from the atmosphere, resulting in higher element concentrations, particularly for insoluble elements (Bellis, 2000). There should also be time-lag due to delayed accumulation, (i.e. from the soil), while dispersal and translocation of elements following encapsulation within the bark pocket is considered to be limited (Satake, 2001). Bark pockets open new possibilities in the field of historical monitoring and should find widespread application in the future.
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5. Conclusions Trees provide information on historical change in environmental pollution. This study showed that bark pockets are more effective monitors of airborne Pb pollution than annual rings, as they accumulate Pb directly from the atmosphere. The bark pockets recorded a 100-fold increase in Pb concentration from ca. 1875 to the present, illustrating the impact of industrialisation on the environment in Japan. The annual rings had lower Pb concentrations and did not reflect increasing levels of pollution, but provided an effective dating. Acknowledgments The authors would like to express their gratitude to Kazumasa Ikuta (National Research Institute of Aqua-culture, Japan), Kazuo Kawakami (Kawakami Green Environment, Japan), Hideaki Shibata (Hokkaido University, Japan), Tetsuo Noguchi and Kyouko Takada (National Institute of Environmental Studies, Japan) for valuable contributions to sample collection, preparation and analysis. D.J. Bellis would like to acknowledge the receipt of a Fellowship from the Science and Technology Agency, Japan. The study was supported by a Grant-in-Aid for Scientific Research (B) from The Ministry of Education, Culture, Sports, Science and Technology, Japan. References Bellis DJ. Monitoring airborne trace elements in past and present environments with tree bark. PhD Thesis. University of Sheffield, UK, 2000:89–137. Bellis DJ, McLeod CW, Satake K. The potential of elemental and isotopic analysis of tree bark for discriminating sources of airborne lead contamination in the UK. J Environ Monit 2001;3:198 –201. Bellis DJ, McLeod CW, Satake K. Pb and 206Pby207Pb isotopic analysis of a tree bark pocket near Sheffield, UK recording historical change in airborne pollution during the 20th Century. Sci Total Environ, in press. Bruning F, Kreeb KH. Mosses as biomoniotors of heavy metal contamination within urban areas. In: Markert B, editor. Plants as Biomonitors-Indicators for Heavy Metals in the Terrestrial Environment. Weinheim: VCH, 1993. p. 395 – 401. Borger GA. Development and shedding of tree bark. In: Kozlowski TT, editor. Development and Shedding of Plant Parts. New York: Academic Press, 1973. p. 205 –237.
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