1. Introduction

Historical monitoring of environmental pollution, combined with present day monitoring, is essential for assessing pollution levels in the world which have different levels and types of human activity and different environmental histories.

Recently, bark enclosed in tree trunks, known as tree "bark pockets", has been shown in Japan to provide some of the most readily available historical specimens for monitoring air pollution1.Bark pockets are common in tree trunks. The phenomenon is well recognized by forestry technicians, saw millers, and dendrologists, because trunks with bark pockets often have problems with lack of strength, discoloration and microbial decomposition of the xylem layer.Bark pockets have not previously been identified as being useful parts of a tree. Their potential value for scientific research has previously been overlooked. Harvested trunks with bark pockets have mostly been discarded as waste. Therefore, many forest trees identified as containing bark pockets have been left standing and not harvested.

This paper describes the scientific use of bark pockets for environmental research as "pollution time capsules". This approach opens a new frontier in methods of historical monitoring in the world.

2. Tree "bark pockets" for historical monitoring of air pollution

Tree bark provides ideal specimens for directly measuring levels of local pollution. This is because a tree bark accumulates air pollutants directly onto its surface (Fig. 1a,b). However, despite this suitability for providing direct evidence of air pollution, bark has not been used to monitor historical trends in air pollutants because it is also exposed to polluted air emitted by modern sources, thereby confounding historical interpretation. The presence of bark pockets within the trunk removes this disadvantage. During tree growth bark can be incorporated into the trunk to form a "bark pocket" by many mechanisms which are described in the next section. One of the important aspects of using bark pockets as pollution time capsules is that the bark pockets are located between annual rings, thus allowing extremely accurate dating.

　Fig.1

3. Mechanisms of bark pocket formation

There are many physical and biological causes of the formation of bark pockets as pollution time capsules. The mechanisms are classified as follows:

3-1 Mechanism (1): Bark pocket - type 1, encapsulation of a wound

Fig. 2 shows a wound in a tree trunk caused by physical damage and the subsequent development of covering the wound with newly formed bark and xylem. Figs. 3 show bark pockets formed by this process.

Encapsulation of a wound is one of the main mechanisms of bark pocket formation. A wound in the process of encapsulation by surrounding bark is called a "bark pocket-to-be". The process of encapsulation of the polluted outer bark leads to the enclosure of pollutants into the trunk by the tree's annual rings, thus allowing accurate dating. In some cases, biological materials such as shoots of epiphytic bryophytes are found between the two bark layers in the bark pocket. Epiphytic bryophytes are potentially indicators of atmospheric pollution２. Information about the past environment can be obtained from species found in samples through analysis of the pollutants contained in the biological materials. In addition to this, the outermost xylem in the bark pocket (X1 in Fig 3) is available for samples which retain pollutants, provided microbial decomposition is limited.

Fig.2	Fig.3

3-2 Mechanism (2): bark pocket - type 2, joining of two tree trunks (branches)

Fig. 4a,b shows the initial contact between two tree trunks or branches and the radial section of bark pockets formed by this mechanism (Xm-Xn). Fig. 5 shows radial section of bark pocket. Fig. 6 shows a vertical section of the bark pocket (Y0-Yn).

This type of bark pocket provides at least three replicates of continuous time series data on the outer bark samples because the barks are formed symmetrically on both sides of the horizontal and vertical axes. The bark pockets cover long spans of successive years, from old to new, permitting monitoring of continuous, year-by-year, historical changes in pollution.

Fig.4

Fig.5	Fig.6
Fig.7	Fig.8

3-3 Mechanism (3): Bark pocket - type 3, encapsulation of a branch

Fig. 7a, b, c show the steps in the encapsulation of a branch. Shaping and/or pruning of branches are the main cause of this type of bark pocket, which is usually observed in trees planted on roadsides and in parks, gardens and artificial forests. The appearance of the outer bark formed in this encapsulation process differs from that of normal outer barks. This type looks like a "cat's eye" but it is a "tree' eye" for watching the environment (Fig. 8a, b). Tree's eye is an indicator of the location of bark pockets in a tree. Some tree's eyes in the process of bark pocket formation can be called "bark pockets-to-be" because they change to bark pockets after encapsulation.

The physical and chemical characteristics of outer bark differ markedly with different tree species. For example, the outer bark of black pine (Pinus thunbergii), red pine (P. densiflora) and white birch (Betula tauschii) peel off from the trunk. The barks of some species exfoliate with characteristic periodicity. In this case, it is difficult to estimate trends in air pollution using outer bark and bark pockets because the exposure period of the outer bark is shorter than the time estimated from the annual rings enclosing the bark pocket. However, the section of the cut end of the branch or wax rosin layer in the bark pocket remains available as an accumulator of air pollutants provided that the xylem of the section is not microbially decomposed. In this case, the annual rings of the cut end of the branch and the record of pruning help with correct dating of the section.

3-4 Mechanism (4): Bark pocket - type 4, the joining of uneven trunk

Fig. 9 shows the bark pocket formed in the process of growing on an uneven trunk. This type of bark pocket is often observed in beech (Fagus japonica, Fagus sylvatica) which has an uneven trunk. This type of bark pocket in the trunk forms in the same compass direction as the continuously jointed outer bark. Bark pockets arranged in the same compass direction in the trunk offer an advantage for understanding historical changes in pollution because there is a directional relationship between the source of pollutants and the tree which accumulates the pollutants.

Fig.9

　3-5 Other mechanisms (5): Bark pocket - type 5

There are other occasional causes and characteristic mechanisms of bark pocket formation. Artificial materials such as wire, nails and fences can damage a tree and become encapsuled to form bark pockets as the tree grows. Wood fire, thunder attack and cracks formed in the trunks by freezing in cold winters are other causes of characteristic bark pocket formation.

4. Characteristics of bark pockets as "pollution time capsules"

Natural and artificial materials have been used for historical monitoring of pollutants transported into terrestrial and aquatic ecosystems (Fig. 10). Typical of these materials are lake sediments, polar ice, herbarium specimens, peat, tree rings, bones, freshwater shells and corals. However, these materials are not always fit for historical monitoring of air pollution. Difficulties often exist in the use of such specimens for monitoring.

Fig.10

Major problems include: (1) dispersal and translocation of the pollutants; (2) contamination during natural and/or artificial preservation periods; (3) difficulty in sourcing materials with a suitable time scale from inhabited monitoring sites such as urban areas, and also from remote and background areas; (4) incorrect dating. Corresponding examples of these problems are: (1) human activities such as fishing by net and bioturbation by bottom fauna often disturb the laminar structure of lake sediments formed by the deposition of particulates; (2) herbarium specimens are in contact with the current atmosphere during their storage. Exchange of pollutants between bones and soil or contamination by soil may occur; (3) polar ice from the Antarctic may provide information on background areas or global pollution, but it is difficult to get information on pollution in populated urban or rural areas; (4) evaluation and application of dating methods suitable for the selected materials are required.

One of the practical dating methods usually used is Pb-210 dating. The half-life of Pb-210 is about 22.2 years. Therefore dating is limited to between several and 150 years and the resolution depends on the accuracy of analysis. Another method is the C-14 method. The half-life of C-14 is about 5600 years. Thus the dating is from several thousand years to 20-30 thousand years. There are some other dating methods such as chronological methods using annual rings in tree trunks, shells or corals. This dating, especially the use of a tree's annual rings, is possible to more than several thousand years3. Consequently, if we want to know about changes in pollution which occurred during and after The Industrial Revolution, dendrochronological dating is best fitted for this purpose.

6. Difference between bark pockets and annual rings as pollution time capsules

There are many reports of heavy metals that accumulate in tree rings corresponding to historical changes in pollution4. Trees form a growth ring every year. Many publications have analyzed the pollutants in annual rings and calculated the age of the rings, but this presents problems because most atmospheric pollutants that accumulate in tree rings are limited to water-soluble compounds transported by way of soil and roots. Also, there are time lags, sometimes more than ten years, between actual soil pollution and the count of annual rings.

Most air pollutants in the form of wet and dry depositions on trees accumulate directly on the outer bark. The concentration of pollutants in the annual rings is very low compared with that of the outer bark because of the dispersal which occurs in the process of pollutant transportation through soils and trees. Furthermore, lateral movement of pollutants may occur between adjacent rings 5.

Therefore, it is difficult to obtain direct information on air pollution from annual rings, although some historical trends in pollution are reflected in the concentration of pollutants in the annual rings.

In the case of bark pockets, there are no time lags, and dispersal of pollutants and translocation of pollutants from bark pockets after encapsulation is considered to be limited, because the outermost parts of the two barks in contact with each other consist of the dead tissue in the bark pockets and the bark pocket is not a passageway for water from the roots to the leaves.

7. Examples of historical monitoring using bark pockets

For our research using bark pockets for historical monitoring, a 350-year-old Japanese cedar (Cryptomeria japonica) that had suffered typhoon damage and a 226-year-old Japanese cedar were collected, respectively, from Nikko in 1990 and Yakushima Island in 1992. Nikko, located about 100 km north of Tokyo, is famous for its Toshogu shrine (a World Heritage site) which was built about 360 years ago. Also at this time, avenues of C. japonica were planted along the roadside. Yakushima Island, famous for many C. japonica trees older than a thousand years, includes a national park and is also a World Heritage site.

The bark pocket collected at Nikko was formed around 1760 to 1780 (240-220 years ago based on 2000) and the bark pocket collected at Yakushima was formed around 1786-1809 (214-191 years ago). The concentrations of lead as a pollutant in the bark pockets and outer bark were determined, since the use of leaded gasoline as a fuel for automobiles has increased drastically since World War II. Many countries still use it and lead pollution is still spreading, although manufacturing and sales of leaded gasoline are already banned in Japan. The results of the analysis showed a marked difference between the amount of lead contained in the bark pockets from the Edo era and that in the outer bark which reflected modern lead pollution. In the case of the cedar trees from the roadside in Nikko, the concentration of lead in modern samples was about 1000 times higher than that of the Edo era (Fig. 11). In the case of Yakushima Island, a remote island located in the southern part of Japan, the concentration of lead in modern samples was about 10-20 times higher than that of the Edo era 1.

In addition to these samples, trees containing bark pockets were collected in the precincts of Muro Temple in Nara prefecture in Japan, about 60 km south of Kyoto and about 37 km south of Nara. The analysis of bark pockets of C. japonica from Muro Temple using laser-ablation ICP-MS revealed clearly the historical changes in mercury and lead pollution over a period from 140 to 70 years ago and up to the present time6. The respective concentrations of lead and mercury in the modern outer bark were about 40 and 4 times higher than those of 140 years ago.

Fig.11

8. New project concerning atmospheric changes in the world

While environmental pollution and destruction of nature expanding on a global scale,it is becoming more and more important to grasp the influence of human activities on a global environment, especially in the world. In the historical monitoring using bark pockets, heavy metals were analyzed as pollutants. However, similar monitoring are possible for other inorganic and organic pollutants. Therefore, we are planning to propose new project for historical monitoring concerning atmospheric changes, and in addition to this, we are planning to select the forests for historical monitoring (Time Capsule Forest) in the world. The Nakagawa experimental forest, Hokkaido University, located at Latitude 44o44'44" is the first Time Capsule Forest for this purpose.

References
1 K Satake, A Tanaka, K Kimura. Accumulation of lead in tree trunk bark pockets as pollution time capsules. Sci. Total Environ., 181: 25-30, 1996.

2 K Satake, K. Kimura, A Tanaka, V Virtanen. Two-hundred-years old shoots of the epiphytic moss Brotherella henonii preserved in a bark pocket of the conifer Cryptomeria japonica. J. Bryology 18: 815-832, 1995.

3 FH Schweingruber. Tree Rings. Kluwer Academic Publishers, Dordrecht, Holland, 1988.

4 CF Baes III, HL Ragsdale. Age-specific lead distribution in xylem rings of tree genera in Atlanta, Georgia. Environ. Pollut. Ser. B., 2: 21-35, 1981.

5 JR Donnelly, JB Shance, PG Schaberg. Lead mobility within the xylem of red spruce seedlings: Implications for the development of pollution histories. J. Environ. Qual., 19: 268-271, 1990.

6 K Satake, R Idegawa, M Obata, N Furuta, N. Historical Environmental Monitoring

using bark pockets as pollution time capsules. Proc. 5th Intern. Conf. On The Biogeochem. Of Trace Elements; Vienna 99, Vol. II: 1074-1075, 1999.

Figures

Fig. 1a,b Accumulation of air pollutants on the surface of tree.
Fig. 2 A wound in a tree trunk caused by physical damage and the subsequent development of covering the wound with newly formed bark and xylem.
Fig. 3 Bark pocket type 1 in a tree trunk (Cryptomeria japonica).
Fig. 4a,b Initial contact between two tree trunks or branches (a) and radial section of bark pockets formed by this mechanism (Xm-Xn)(b).
Fig. 5 Radial section of bark pocket formed by joining two branches (x-m - xm)
Fig. 6 Vertical section of the bark pocket formed by joining two branches (y0 -yn).
Fig. 7a,b,c Steps in the encapsulation of a branch after pruning.
Fig. 8a,b Cat's eye (=Tree's eye = bark pocket to-be) formed on the outer bark after pruning the branch.
Fig. 9 Bark pocket formed in the process of growing on an uneven trunk.
Fig. 10 Typical natural and artificial materials for historical monitoring of pollutants transported into terrestrial and aquatic ecosystems (Examples are lake sediments, polar ice, herbarium specimens, peat, corals and tree rings ).
Fig. 11 Relative concentration of lead in the bark pockets collected from Nikko and Yakushima.