δ13C response surface resolves humidity and temperature signals in trees

Stem cellulose of bean plants (Vicia faba) grown under controlled conditions exhibits inverse linear carbon-isotope reactions to changes in both relative humidity (RH) and temperature (T), readily mappable as a planar delta(13)C response surface in RH-T space. The analogous response surface for annual late-wood cellulose delta(13)C from a field calibration using fir trees (Abies alba) in the Black Forest, southern Germany, also supports resolution of independent delta-RH and delta-T effects. The response of cellulose delta(13)C to RH and T derived from this new calibration differs markedly from estimates based on univariate linear regression analysis: The sensitivity of delta(13)C to RH is stronger than that inferred previously (c. -0.17 parts per thousand/% vs. -0.12 parts per thousand/%, respectively), whereas the delta-T coefficient is weaker and reversed in sign (c. -0.15 parts per thousand/K vs. +0.36 parts per thousand/K). This new perspective on the coupled influence of moisture and temperature changes on tree-ring cellulose delta(13)C helps to unify divergent observations about carbon-isotope signals in trees, especially the broad range of apparent delta-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 delta(13)C data, coupling of the carbon-isotope response-surface 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. Copyright (C) 2000 Elsevier Science Ltd.