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.