No altitude-dependent effects of climatic signals are recorded in Smith fir tree-ring 18O on the southeastern Tibetan Plateau, despite a shift in tree growth

To reveal the influence of current warming on tree growth and 18O in a sensitive high-latitude region that is undergoing rapid climate change, we examined tree width and the earlywood and latewood 18O at two sites with a 400-m elevation difference in the Sygera Mountains of the southeastern Tibetan Plateau. The study period was from 1950 to 2011. The mean tree-ring index at the low site was higher than that at the high site during the study period. The climatic responses of earlywood and latewood 18O at both sites were similar. Earlywood 18O was mainly influenced by the June to August temperature and total cloud cover, whereas latewood 18O was mainly controlled by relative humidity from July to August. Spatial correlations with CRU TS 3.1 regional data suggest that our 18O chronologies can represent climatic changes over large regions. The high offset between earlywood 18O at the two sites (2.3 parts per thousand higher at the low site) was mainly influenced by the high temperature lapse rate as a function of altitude during the earlywood growing season. Furthermore, meltwater with lower 18O values might have affected earlywood 18O at the high site, and thereby increased the earlywood 18O offset between two sites. The low latewood 18O offset between the two sites (0.4 parts per thousand higher at the low site) was not significant, but appears to have been primarily influenced by the low precipitation 18O lapse rate as a function of altitude during the latewood growing season. Earlywood 18O of Smith fir suitable for reconstructing past temperatures and latewood 18O suitable for reconstructing past relative humidity on the southeastern Tibetan Plateau were identified.