Decoupling of Tree-Ring Cellulose δ¹⁸O and δ²H Highlighted by Their Contrasting Relationships to Climate and Tree Intrinsic Variables

Oxygen (δ¹⁸O) and hydrogen (δ²H) stable isotope ratios are tightly coupled in precipitation and, albeit damped, in leaf water, but are often decoupled in tree-ring cellulose. The environmental and physiological conditions in which this decoupling occurs are not yet well understood. We investigated the relationships between δ¹⁸O and δ²H and tree-ring width (TRW), tree crown volume, tree age and climate in silver fir and Douglas-fir and found substantial differences between δ¹⁸O and δ²H. Overall, δ¹⁸O-δ²H correlations were weak to absent but became significantly negative under high summer vapour pressure deficit (VPD). δ¹⁸O and δ²H had positive and negative nonlinear relationships with TRW, respectively, with clear relationships at the site and tree levels for silver fir and, to a lesser extent, for Douglas-fir. Age trends for silver fir were weakly negative in δ¹⁸O but positive in δ²H. Tree crown volume and δ¹⁸O or δ²H had no significant relationships. Most strikingly, δ¹⁸O strongly depended on spring climate (precipitation and VPD), whereas δ²H depended on summer climate (temperature and VPD) for both species. Our study shows that the δ¹⁸O-δ²H decoupling in tree-ring cellulose in two temperate conifer species could be highlighted by their contrasting relationships to climate and tree intrinsic variables (TRW, age).