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Age‐Related Climate Response of Tree‐Ring δ13C and δ18O From Spruce in Northwestern China, With Implications for Relative Humidity Reconstructions

Understanding varying climate responses in tree‐ring data across tree ages is important, but little is known about tree‐age effects on climate responses in tree‐ring stable isotopes. To detect whether age differences in tree‐ring δ13C and δ18O could lead to differing climate responses, we measured tree‐ring cellulose δ13C and δ18O (1901–2010) from Schrenk spruce (Picea schrenkiana) trees in northwestern China with ages ranging from 110 to 470 years, which we binned into three age groups. Tree‐ring δ13C (pin‐corrected) and δ18O exhibited similar year‐to‐year variability between age groups and did not feature age‐related trends. δ13C series from old trees (270–470 years) showed stronger legacy effects, reflecting influences from the antecedent period (due to carbohydrate reserves and climate), compared to young trees (110–125 years). Both tree‐ring δ13C and δ18O values decreased with increasing relative humidity (RH) and precipitation and with decreasing mean and maximum temperatures during the main growing season (May–August). δ13C and δ18O exhibited age‐dependent climate responses: Young trees had a stronger climate response in δ13C but a weaker or similar climate response in δ18O compared to old trees. We developed multiple growing‐season RH reconstructions based on composite chronologies using δ13C and δ18O series from different age groups. In particular, we found that including δ13C from young trees improved the skill of RH reconstructions because of the age‐specific mechanisms driving the δ13C‐climate relationship, but that caution is warranted with regard to extreme values. We therefore suggest that young trees should be considered when using stable isotopes, particularly in δ13C, for climate reconstruction.