Gas exchange and 13C-based estimates of intrinsic water-use efficiency show different responses to CO2 enrichment: a global meta-analysis of experimental studies

Abstract

ABSTRACT Tree intrinsic water-use efficiency (iWUE) is a key characteristic of ecosystem functions central to the global cycles of carbon, water, and energy. Gas exchange and stable carbon isotope compositions have been widely used to assess iWUE. However, it remains unclear to what extent the iWUE estimates obtained using different approaches are comparable, particularly in combination with changes in temperature, precipitation, and experimental duration. Herein, the responses of different estimators of iWUE (as A/g s or through isotopic composition of leaves or tree rings) to elevated CO2 concentrations (eCO2) were assessed through a meta-analysis of 361 observations from 98 independent experiments. Our results show that (1) different proxies for iWUE provide inconsistent estimates of the response of iWUE to eCO2. (2) Throughout the growing season, the average response of g s to eCO2 was smaller than the single-point values estimated by gas exchange and the eCO2-induced reduction in g s was stronger for evergreen species than for deciduous species. (3) Precipitation, rather than temperature, significantly affected the responses of 13C-based estimates of iWUE to eCO2. (4) There was no significant response of leaf 13C-based iWUE estimates to eCO2. (5) The importance of photorespiration in elevated CO2 enhancement responses may be overestimated in evergreen species.