Dating of dipterocarp tree rings: establishing a record of carbon cycling and climatic change in the tropics

Abstract

In a first step to obtain a proxy record of past climatic events (including the El Ni (n) over tildeo-Southern Oscillation) in the normally a seasonal tropical environment of Sabah, a radial segment from a recently fallen dipterocarp (Shorea Superba) was radiocarbon dated and subjected to carbon isotope analysis. The high-precision radiocarbon results fell into the ambiguous modern plateau where several calibrated dates can exist for each sample. Dating was achieved by wiggle matching using a Bayesian approach to calibration. Using the defined growth characteristics of Shorea superba, probability density distributions were calculated and improbable dates rejected. It was found that the tree most likely started growing around AD 1660-1685. A total of 173 apparent growth increments were measured and, therefore, it could be determined that the tree formed one ring approximately every two years. Stable carbon isotope values were obtained from resin-extracted whole wood from each ring. Carbon cycling is evident in the 'juvenile effect', resulting from the assimilation of respired carbon dioxide and lower light levels below the canopy, and in the 'anthropogenic effect' caused by increased industrial activity in the late-nineteenth and twentieth centuries. This study demonstrates that palaeoenvironmental information can be obtained from trees growing in a seasonal environment, where climatic conditions prevent the formation of well-defined annual rings.