Reconciling intrinsic and ecosystem water-use efficiency reveals physiological mechanisms underlying carbon–water coupling across biological scales

Abstract Understanding how carbon–water coupling scales from leaves to ecosystems remains a major challenge because water-use efficiency (WUE) metrics integrate physiological, hydrological, and ecological processes. We integrated tree-ring width, triple stable isotopes (δ¹³C, δ 18 O and δ²H), eddy covariance (EC) observations, and a process-based model to investigate the drivers of intrinsic ( i WUE) and ecosystem water-use efficiency (WUE eco ) across biological scales in a mature Fagus sylvatica forest between 2004 and 2023. Modelled and isotope-derived i WUE were strongly correlated (r = 0.61, p eco derived from EC was lower than model estimates and exhibited a positive trend not reproduced by the model. This divergence primarily reflected differences in biological representation and hydrological integration, as the single-species model underestimated ecosystem evapotranspiration by excluding co-occurring deciduous and evergreen species together with understory vegetation. Triple-isotope analyses showed that the long-term increase in i WUE was driven by enhanced photosynthetic assimilation rather than sustained stomatal closure, whereas δ 18 O and δ²H variability was mainly controlled by source-water dynamics. These findings provide a mechanistic framework for reconciling leaf- and ecosystem-scale WUE and improving predictions of forest carbon–water coupling under climate change.