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Climate signals in tree-ring δ18O from different tree species in the Bhutan Himalaya
Paper ID : 1049-ADA2013
Authors:
Masaki Sano *1, Phuntsho Tshering2, Koji Fujita3, Jiro Komori3, Takeshi Nakatsuka3
1Graduate School of Environmental Studies, Nagoya University, Furo-Cho, Chikusa-ku, Nagoya 464-8601, Japan
2Department of Geology and Mines
3Graduate School of Environmental Studies, Nagoya University
Abstract:
In contrast to the quantity and quality of the ring-width-based climate reconstructions from the western Himalaya, those from the eastern Himalaya are rather few, due at least partially to reduced climate sensitivity resulting from relatively abundant rainfall. We describe the dendroclimatic potential of oxygen isotope composition (δ18O) derived from three species growing in the Bhutan Himalaya. Sampling was carried out at two sites with different altitudes and ecological conditions in northern Bhutan. The sampling sites are 30 km distant from each other. One (LNN) is located near the alpine treeline of 4400 m above sea level (masl), in which a wide-open forest is dominated by juniper (Juniperous indica). The other (WCH) is located in an altitude of 3500 masl, in which a semi-open forest is composed of larch (Larix griffithii) and spruce (Picea spinulosa). Collected core samples were visually cross-dated using standard methodology, resulting in the absolute assignment of calendar years to every growth ring. We selected three cores (trees) for each species (a total of 9 trees) for isotopic analysis. Oxygen isotope ratios were individually determined for each core over the last 50 years (1962–2011).

The variations in tree-ring cellulose δ18O are in good agreement not only among intra-species but also among the three species. As expected, climatic responses of tree-ring δ18O are generally consistent among different species. Specifically, the tree-ring δ18O chronologies derived from juniper and spruce trees are correlated negatively with June and July precipitation, and positively with July temperature, even though the sampling sites differ from each other. The larch chronology originating from the same site for spruce also shows negative correlations with June and July precipitation, but no significant signal seen with temperatures. In addition, the larch data show a negative correlation with precipitation in September. In order to identify the most important climatic factor and season, each chronology was correlated with seasonalized precipitation and temperatures. It turned out, first of all, that precipitation primarily controls variations in tree-ring δ18O for all the three species. The negative correlations of tree-ring δ18O with precipitation can be explained by the ‘amount effect’, an inverse relationship observed between δ18O of precipitation and the amount of precipitation at lower latitudes. Second, the most important season contributing to tree-ring δ18O varies among species: June-July for juniper, May-September for larch, and May-July for spruce. Overall, the climatic responses are consistent with previous findings obtained from the Nepal Himalaya. Spatial correlation analyses with global sea surface temperatures indicate El Niño and La Niña phases generally result in dry and wet conditions, respectively, in the study region. We conclude that tree-ring δ18O from the Bhutan Himalaya is a promising proxy to reconstruct monsoon rainfall and to investigate the geographical influence of ENSO before the instrumental period. At the Conference we will present an extended version of the tree-ring δ18O record that dates back to the mid-18th century.
Keywords:
Oxygen isotope ratios, larch, juniper, spruce, Bhutan, monsoon, ENSO
Status : Abstract Accepted