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Stable isotopes in tree rings are increasingly used as proxies for climatic and ecophysiological changes. However, uncertainties remain about the strength and consistency of their response to environmental variation at different temporal (i.e. seasonal to inter-decadal) scales., We developed 5 yr of intra-seasonal and 62 yr of early- and late-wood δ13C and δ18O series of Smith fir ( Abies georgei var. smithii) on the southeastern Tibetan Plateau, and used a process-based forward model to examine the relative importance of environmental and physiological controls on the isotopic data., In this temperate high-altitude region, the response, both δ18O and δ13C, is primarily to variations in relative humidity, but by different processes. In δ18O, the response is via source water δ18O but also arises from leaf water 18O enrichment. In δ13C, the response is via changes in stomatal conductance but is modified by carry-over effects from prior periods., We conclude that tree-ring δ18O may be a more robust climate proxy than δ13C, and δ13C may be more suited to studies of site-related physiological responses to the local environment. [ABSTRACT FROM AUTHOR]