delta C-13 response surface resolves humidity and temperature signals in trees

Stem cellulose of bean plants (Vicia faba) grown under controlled condition s exhibits inverse linear carbon-isotope reactions to changes in both relat ive humidity (RH) and temperature (T), readily mappable as a planar delta(1 3)C response surface in RH-T space. The analogous response surface for annu al late-wood cellulose delta(13)C from a field calibration using fir trees (Abies alba) in the Black Forest, southern Germany, also supports resolutio n of independent delta-RH and delta-T effects. The response of cellulose de lta(13)C to RH and T derived from this new calibration differs markedly fro m estimates based on univariate linear regression analysis: The sensitivity of delta(13)C to RH is stronger than that inferred previously (c. -0.17 pa rts per thousand/% vs. -0.12 parts per thousand/%, respectively), whereas t he delta-T coefficient is weaker and reversed in sign (c. -0.15 parts per t housand/K vs. +0.36 parts per thousand/K). This new perspective on the coup led influence of moisture and temperature changes on tree-ring cellulose de lta(13)C helps to unify divergent observations about carbon-isotope signals in trees, especially the broad range of apparent delta-T relations obtaine d in calibration studies, which are often used as paleoclimate transfer fun ctions. Although this highlights the large potential uncertainties surround ing paleoclimate reconstruction based solely on delta(13)C data, coupling o f the carbon-isotope response-surface approach with equivalent response sur faces for hydrogen or oxygen isotopes may afford new opportunities for inve stigating the nature of past climate variability and change from tree-ring sequences. Copyright (C) 2000 Elsevier Science Ltd.