Evaluation of CO_2 fixation through dendrochronology for the wood grown in rain forest.

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Since the wood grown in a rain forest does not form annual rings because of lacking the definite change of the climate, it is difficult to estimate the age of the trees, therefore, the fixation of carbon dioxide at that area is difficult to know. To perform denderochronology, we measured the natural radioactivity of the wood as well as elemental distributions and water distribution in wood disks for both a Japanese tree (Cryptomeria japonica) and an Indonesian tree (Alstonia species) which was grown in a rain forest In the measurement of natural radioactivity, we set up the measuring system for a pure Ge detector. The ^<210>Pb radioactivity in a wood disk, across the annual rings, was reduces from outer to the center part. Though, the decreasing rate of the radioactivity was not exactly equal to that of the half-life of ^<210>Pb, for both wood species, the annual age of the Alstonia was estimated from the measurement of Cryptomeria japonica. In the case of elemental analysis, neutron activation analysis was performed. More than 20 elements were analyzed and was found that Mg was a promising element to estimate the age of the trees, for the Mg concentration was linearly changed within die wood disks. Especially for the Alstonia, the wood age was not shown from neutron Radiograph.