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Incursion of the Indian Monsoon flow onto the Qinghai–Tibetan Plateau is an important regional moisture-delivery mechanism, but the causes of intra-seasonal and longer-term variations are poorly understood. We observed a clear intra-seasonal cycle in the oxygen isotope composition ( δ 18 O) of Smith fir ( Abies forrestii var. georgei) growing at two different altitudes (high-elevation site: 29°39′N, 94°42′E, 4200 m a.s.l.; low-elevation site: 29°39′N, 94°43′E, 3800 m a.s.l.) on a SE-facing slope in the Sygera Mountains, southeastern Qinghai–Tibetan Plateau. Mechanistic modeling suggests that δ 18 O of precipitation, which in turn depends on relative humidity and precipitation amount, is important in controlling the intra-seasonal tree-ring isotopic cycle, and we speculate that the same controls also operate on interannual and longer time scales. If so, multi-century δ 18 O records with intra-seasonal resolution may enable us to infer long-term variations in the incursion of the Indian Monsoon onto the Qinghai–Tibetan Plateau. [ABSTRACT FROM AUTHOR]

To reveal the influence of current warming on tree growth and δ18O in a sensitive high-latitude region that is undergoing rapid climate change, we examined tree width and the earlywood and latewood δ18O 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 δ18O at both sites were similar. Earlywood δ18O was mainly influenced by the June to August temperature and total cloud cover, whereas latewood δ18O was mainly controlled by relative humidity from July to August. Spatial correlations with CRU TS 3.1 regional data suggest that our δ18O chronologies can represent climatic changes over large regions. The high offset between earlywood δ18O at the two sites (2.3‰ 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 δ18O values might have affected earlywood δ18O at the high site, and thereby increased the earlywood δ18O offset between two sites. The low latewood δ18O offset between the two sites (0.4‰ higher at the low site) was not significant, but appears to have been primarily influenced by the low precipitation δ18O lapse rate as a function of altitude during the latewood growing season. Earlywood δ18O of Smith fir suitable for reconstructing past temperatures and latewood δ18O suitable for reconstructing past relative humidity on the southeastern Tibetan Plateau were identified. [ABSTRACT FROM AUTHOR]