| Abstract: |
Stem cellulose of bean plants (Vicia faba) grown under controlled conditions exhibits inverse linear carbonisotope reactions to changes in both relative humidity (RH) and temperature (T), readily mappable as a planar d13C response surface in RHT space. The analogous response surface for annual late-wood cellulose d13C from a field calibration using fir trees (Abies alba) in the Black Forest, southern Germany, also supports resolution of independent d-RH and d-T effects. The response of cellulose d13C to RH and T derived from this new calibration differs markedly from estimates based on univariate linear regression analysis: The sensitivity of d13C to RH is stronger than that inferred previously (c. 20.17/% vs. 20.12/%, respectively), whereas the d-T coefficient is weaker and reversed in sign (c. 20.15/K vs. 10.36/K). This new perspective on the coupled influence of moisture and temperature changes on tree-ring cellulose d13C helps to unify divergent observations about carbonisotope signals in trees, especially the broad range of apparent d-T relations obtained in calibration studies, which are often used as paleoclimate transfer functions. Although this highlights the large potential uncertainties surrounding paleoclimate reconstruction based solely on d13C data, coupling of the carbonisotope responsesurface approach with equivalent response surfaces for hydrogen or oxygen isotopes may afford new opportunities for investigating the nature of past climate variability and change from tree-ring sequences.
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