Abstract
Various studies report substantial increases in intrinsic water-use efficiency (Wi), 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 Wi as trees grow larger. Here we show, by comparing Wi across varying tree sizes at one CO2 level, that ignoring such developmental effects can severely affect inferences of trees’ Wi. Wi 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.
Highlights
Various studies report substantial increases in intrinsic water-use efficiency (Wi), estimated using carbon isotopes in tree rings, suggesting trees are gaining increasingly more carbon per unit water lost due to increases in atmospheric CO2
The comparison reveals that for the three broadleaf species, time trends in Wi derived from dominant trees obtained from literature and complemented with new data collected from the four sites in this study are of a similar or lesser magnitude than the observed change in Wi with tree age
We show here that Wi increases strongly with tree age in the three broadleaf species Quercus, Fagus and Cedrela and weakly in Pinus
Summary
Various studies report substantial increases in intrinsic water-use efficiency (Wi), estimated using carbon isotopes in tree rings, suggesting trees are gaining increasingly more carbon per unit water lost due to increases in atmospheric CO2. A popular method of studying plant water use efficiency is to use the carbon isotope composition of tree rings (δ13Cplant, see Methods section) It is attractive because it provides long-term annual records. From tree ring δ13C and historical records of atmospheric δ13C, plant isotope discrimination (Δ13Cplant) can be calculated (see Methods section), which provides an estimate of changes in the ratio between assimilation and stomatal conductance for water vapour (A/gw). This ratio is called intrinsic plant water use efficiency (Wi).
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