Decoupling of Tree-Ring Cellulose δO and δH Highlighted by Their Contrasting Relationships to Climate and Tree Intrinsic Variables.

Oxygen (δO) and hydrogen (δH) 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 δO and δH and tree-ring width (TRW), tree crown volume, tree age and climate in silver fir and Douglas-fir and found substantial differences between δO and δH. Overall, δO-δH correlations were weak to absent but became significantly negative under high summer vapour pressure deficit (VPD). δO and δH 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 δO but positive in δH. Tree crown volume and δO or δH had no significant relationships. Most strikingly, δO strongly depended on spring climate (precipitation and VPD), whereas δH depended on summer climate (temperature and VPD) for both species. Our study shows that the δO-δH 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).