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

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.

KEYWORDS:

• Elevated CO₂
• gas exchange
• Δ¹³c
• intrinsic water use efficiency
• tree ring

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by National Key R&D Program of China (2021YFD1500705) and the National Natural Foundation of China (31670476).