Climate signals in tree-ring δ18O and δ13C from southeastern Tibet: insights from observations and forward modelling of intra- to interdecadal variability

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 delta C-13 and delta O-18 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 delta O-18 and delta C-13, is primarily to variations in relative humidity, but by different processes. In delta O-18, the response is via source water delta O-18 but also arises from leaf water delta O-18 enrichment. In delta C-13, the response is via changes in stomatal conductance but is modified by carry-over effects from prior periods. We conclude that tree-ring delta O-18 may be a more robust climate proxy than delta C-13, and delta C-13 may be more suited to studies of site-related physiological responses to the local environment.