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The Arctic is experiencing rapid climatic and environmental changes, which could alter diets of nonmigratory herbivores both within and between populations. Here, we used carbon and nitrogen stable isotope analysis (C-13 and N-15) in hair to infer summer dietary differences of Rangifer tarandus platyrhynchus (nonmigratory reindeer) in Nordenskioldland, Svalbard and used regression models to predict how reindeer dietary intake depend on climatic variables. In linear regression models, current-year and last-year July temperatures positively correlated with both the C-13 and N-15 of reindeer hair, and were stronger predictors of isotopic signatures than July precipitation. C-13 and N-15 analysis indicated robust dietary differences between populations separated by less than 50km and within these valley systems in the proportions of nonmycorrhizal vascular plants and mosses. Remote sensing (NDVI) and isotopic data together suggested an early-season switch from moss to vascular plant dependence in valleys closer to the sea, suggesting both high site fidelity and localized differences in forage availability related to the onset of the growing season. C-13 photosynthetic discrimination increased with the rising atmospheric pCO(2), indicating that Svalbard plants are already responding physiologically to rising CO2 concentrations. Calves were higher in C-13 and N-15 than adults, consistent with a weak trophic effect. We also determined how hair C:N correlated with C-13 and N-15 patterns. Overall, these data provide evidence that C-13 and N-15 measurements can detect dietary shifts over short time periods and fine spatial scales, indicating that isotopic studies can help monitor herbivory in the changing Arctic.