The tree-ring stable isotope technique is an effective approach to climate reconstruction and to establishment of tree responses to environmental factors. In this study, the tree-ring samples of Abies faxoniana from a sub-alpine forest of Wolong in Western Sichuan were used to develop the tree-ring width chronology and stable carbon isotope series based on the standard dendrochronology methods and the stable carbon isotope techniques. The Wolong Nature Reserve in Western Sichuan is located in a transition zone from the Chengdu plain to the Tibeten Platau, with a warm and moist summer and cold and dry winter. Abies faxoniana is the dominant tree species of the sub-alpine forests at the elevation of 2600-3200 m. Tree ring data were collected in 2010 along an elevation gradient (at 2750, 2950, 3150 m). Climate data were obtained from two nearby national climate stations, Dujiangyan (103°34'E,30°59'N) and Xiaojin (102°21'E,31°N). Isotopic analyses were done at the Chinese Academy of Forestry.
With the data and analyses, we established responses of Abies faxoniana tree-ring carbon stable isotope composition to several climatic factors (e.g., precipitation, mean month temperature, and mean month relative humidity). The main results of this study are as follows: 1) The carbon stable isotope series of Abies faxoniana (1904-2009) varied from -23.33‰ to -26.31‰ with the average value of -24.91‰, with the coefficient of variation ranging from -0.011 to -0.038. The carbon stable isotope series showed strong first-order autocorrelation (i.e., 0.651, 0.936, and 0.333 along the elevation gradient).2) The tree-ring carbon stable isotope composition showed similar trends at the same altitude, while the isotope composition displayed different trends as altitudes change. However, the relationships between the tree-ring carbon stable isotope composition and climatic factors were complicated. The tree-ring detrend series (DS) values from the lower altitude showed a significant positive correlation to the mean monthly temperature of the December of the current year and the mean monthly relatively humidity of the August in the previous year (P < 0.05). The tree-ring DS values from the higher altitude showed a significant positive correlation to the mean monthly temperature of the April of the current year and the mean monthly relatively humidity of the August in the previous year (P < 0.05). The tree-ring DS values from the mid-altitude showed a significant positive correlation to the mean monthly temperature of the February and November of the current year and the January and November in the previous year (P < 0.05). The most sensitive variable to the DS values was the winter temperature with a strong "lag effect".
The results indicated that tree ring growth of Abies faxoniana in sub-alpine is mainly constrained by temperature with little dependence on rainfall. Thus, a warming climate should help improve the radial growth rate and the survival capability of Abies faxoniana, and this in turn should increase the stability of sub-alpine dark forests in Western Sichuan. The results should help alleviate the inadequacies of doing research in Qinghai-Tibeat plateau with fewer meteorological stations and shorter observation periods, and provide baseline data for studying responses of conifer trees to climate change in the region.