Background, aim, and scope Dendroclimatology is one of the important approaches in paleoclimate studies because of tree-ring's high temporal resolution and accuracy of dating. Traditional dendrcoalimatology employed tree-ring width to investigate climate variations. To obtain more climate signals, other tree-ring parameters such as density, stable isotope ratios are employed during recently years. Tree-ring stable oxygen isotope ratios have the advantage that the physiological controls are well understood and relatively simple in comparison to many factors controlling tree-ring width. The tree-ring stable oxygen isotope ratios may keep the memory of past changes in the stable oxygen isotope ratio of precipitation because tree roots absorb soil water that are from precipitation. In addition, relative humidity is known as the other factor impacting the tree-ring stable oxygen isotope ratios by affecting the enrichment of stable oxygen isotope ratios in leaf water. Generally, it is a necessary step to remove age-related trend in tree-ring widths studies. However, time series of tree-ring stable oxygen isotope are not necessary to carry out the detrending process because of rarely juvenile effect, consequently tree-ring stable oxygen isotope series could preserve more low-frequency climate signals. In Europe and North America, tree-ring stable oxygen isotope study has developed over the past half century. Comparing with tree-ring widths studies, tree-ring stable oxygen isotope studies are very rare in China, especially in some ecologically fragile regions such as Tibetan Plateau, where climate variations are very important to global climate change. To investigate the climate potential of tree-ring stable oxygen isotope in southern Tibetan Plateau, we employed three tree-ring samples of growing Tsuga chinesis Pritz from Yadong County, Tibetan Plateau to carry out the analysis of climate responses in this study. Materials and methods There are missing rings or false rings in most trees in Tibetan Plateau because of critical hydrological conditions. In order to obtain the exact calendar year of the samples, we performed cross dating using the Skeleton Plot method. Annual tree-ring width was measured using a LINTAB system that has a precision of 0.01 mm. Quality control was carried out using the COFECHA program. Tree-ring cores, 11C, 13C and 20C, were selected to carry out cellulose stable oxygen isotope analysis in this study. The pith year of the samples are unclear because all samples may be fractured during store. Annual wood material of the three cores without any missing rings, were separated from each tree-ring core. Most of the rings is very narrow, and there is indistinct boundary between earlywood and latewood. To avoid separation errors, we used whole annual rings to do the isotopic analyses. We usesd a razor blade to separate annual sample carefully under a binocular microscope. The wood material of the annual ring was put to a labeled small bottle. Then the cellulose of annual ring was extracted from annual wood material by organic solvent and sodium hydroxide. About 0.13 - 0.17 mg of homogeneous cellulose was loaded into a silver capsule, (in duplicate for each sample) and then determined the cellulose stable oxygen isotope ratio with a continuous flow system with a pyrolysis-type elemental analyzer (Finnigan TC/EA) and an isotope ratio mass spectrometer (Thermo Delta V Advantage) in Research Institute for Humanity and Nature, Japan. We calculated cellulose stable oxygen isotope ratio by a comparison with an isotope ratio that was predetermined using commercial cellulose (Merck KGaA, Darmstadt, Germany) which was inserted frequently during the measuring process. The oxygen isotope ratios were expressed as delta18O, which represents the per mil deviation relative to the Vienna Standard Mean Ocean Water (VSMOW).
1.中国科学院地球环境研究所, 黄土与第四纪地质国家重点实验室 2.全球变化研究协同创新中心, 西安, 陕西 710061, 中国 3.Research Institute for Humanity and Nature, Kyoto 603-8047, Japan 4.中国科学院地理科学与资源研究所, 北京 100101, 中国