Tree height strongly affects estimates of water-use efficiency responses to climate and CO₂ using isotopes

Various studies report substantial increases in intrinsic water-use efficiency (W _i ), estimated using carbon isotopes in tree rings, suggesting trees are gaining increasingly more carbon per unit water lost due to increases in atmospheric CO₂. Usually, reconstructions do not, however, correct for the effect of intrinsic developmental changes in W _i as trees grow larger. Here we show, by comparing W _i across varying tree sizes at one CO₂ level, that ignoring such developmental effects can severely affect inferences of trees' W _i . W _i doubled or even tripled over a trees' lifespan in three broadleaf species due to changes in tree height and light availability alone, and there are also weak trends for Pine trees. Developmental trends in broadleaf species are as large as the trends previously assigned to CO₂ and climate. Credible future tree ring isotope studies require explicit accounting for species-specific developmental effects before CO₂ and climate effects are inferred.Intrinsic water-use efficiency (W _i ) reconstructions using tree rings often disregard developmental changes in W _i as trees age. Here, the authors compare W _i across varying tree sizes at a fixed CO₂ level and show that ignoring developmental changes impacts conclusions on trees' W _i responses to CO₂ or climate.