Applications of Tropical Isotope Dendroclimatology in Thailand

Reconstructing the terrestrial expression of tropical climate variability over the last several centuries remains a challenge despite advances in the development of a variety of proxy records (e.g. corals, speleothems, ice cores). The use of traditional dendrochronological techniques has been hindered in the tropics because of invisible or indistinct banding in the wood. The seasonality in rainfall and relative humidity in the tropics, while not large enough to cause the creation of visibly distinct rings, may still generate seasonal signals in the oxygen and carbon isotopic composition of tree cellulose. Recent analytical advances in sample preparation, chemical extraction and continuous flow gas-source mass spectrometry have made possible the recovery of long term seasonal isotopic records in tropical trees. Previously, we have demonstrated that these techniques are capable of resolving seasonal climate signals on a variety of tropical tree species from Indonesia, including some for which no visible growth rings are present. We further explore this approach using trees from monsoonal northern Thailand in an attempt to characterize the most suitable species for this application. Analysis of δ ¹⁸O of cellulose from 10 trees coming from 4 different regions of northern Thailand highlights the difficulties in developing these new records as many species show very irregular seasonality. In contrast, some species show very regular and pronounced seasonality in δ ¹⁸O. In addition, seasonal variations in δ ¹³C can assist with chronology when seasonal variability in δ ¹⁸O is less distinct. The coherence between oxygen isotopic records extracted from different tree species growing in the same geographical region confirms the existence of a common environmental control. Comparison with instrumental records of precipitation is complicated by a high degree of spatial variability between sparsely distributed stations. This suggests the potential for creating an alternative index of regional meteorological conditions using a network of many isotope dendrochronological records.