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ABSTRACT
To examine whether tree rings can be used to detect or assess local historical 90Sr or 137Cs fallout, such as that resulting from the Hiroshima atomic bomb, radial distribution of 90Sr and 137Cs in trees was examined. We studied a gymnosperm [Japanese cedar, Cryptomeria japonica (L. f.) D. Don] and an angiosperm (Japanese persimmon, Diospyros kaki Thunb.) tree species from the vicinity of the atomic bomb hypocenter, and from other locations in Japan. A significant amount of 137Cs was detected in tree rings formed before 1945, indicating lateral migration of Cs. In contrast, the specific activity of 90Sr in the Hiroshima Japanese cedar showed the highest level in 1945, due to relatively immobile characteristics of Sr compared with Cs. Strontium-90 and Sr analyses in tree rings helped identify and distinguish between residual 90Sr activity from the Hiroshima atomic bomb and the atmospheric nuclear testing. This indicates the possibility of detecting or assessing previous local 90Sr pollution through with treering analysis.
TREE RINGS have been used as biomonitors of various environmental parameters such as temperature, precipitation, and heavy metal pollution (Schweingruber, 1996; Epstein and Krishnamurthy, 1990; Hagemeyer, 1993; Katayama et al., 1993). Some elements in tree rings have been related to soil acidification (Dewalle et al., 1991, 1999; McClenahen et al., 1989; Guyette et al., 1992). Cutter and Guyette (1993) suggested species suitable for dendrochemical study. The advantage of using tree rings as a biomonitor is the prospect of exact dating and the tree's potentially great age, which makes historical reconstruction of environmental information feasible.
MATERIALS AND METHODS
Tree Sampling
A gymnosperm tree and an angiosperm tree, Japanese cedar and Japanese persimmon, were collected from the area of "black rain" (Fig. 1; Roesch, 1987). Both tree species grow over a wide region in Japan and Japanese cedar is the longestliving tree species in Japan (up to 1000 yr old). Long-lived tree species with a large geographic distribution are considered desirable for dendrochemical study (Cutter and Guyette, 1993). However, due to the unavailability of tree species living from before 1945 in this region and time-consuming analysis of wood 90Sr (1 man-month for 10 data points), we sampled (cut down) one tree of each species. The Japanese cedar and the Japanese persimmon trees were situated...