EGU23-6684, updated on 25 Feb 2023
EGU General Assembly 2023
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Overestimated water-use efficiency responses to rising CO2 revealed by tree-ring 13C record

Xiao Ying Gong1,2, Wei Ting Ma1, Yong Zhi Yu1, and Xuming Wang1,2
Xiao Ying Gong et al.
  • 1Fujian Normal University, School of Geographical Sciences, China (
  • 2Key Laboratory for Humid Subtropical Eco-geographical Processes of the Ministry of Education, China

Water-use efficiency (WUE) is a key determinant of carbon and water fluxes at scales ranging from individual plants to continents and thus a key driver of hydro-climatic changes and carbon models. Multiple lines of evidence suggest that WUE of plants increases with atmospheric CO2, pointing to potential changes in physiological forcing of global carbon and hydrological cycles. Although the increase in forest WUE with atmospheric CO2 is widespread, declines in tree growth have been observed in different forests. These controversial results highlight the need to re-evaluate the historical trend of forest WUE.

13C signatures (i.e., δ13C) of plant organic matter is a useful tool for estimating WUE at temporal scales ranging from days to centuries. This estimation relies on photosynthetic 13C discrimination models that have different assumptions about the components of isotope discrimination. Mesophyll conductance is a key uncertainty in estimated WUE owing to its influence on diffusion of CO2 to sites of carboxylation.

We developed a 13C-based WUE model that takes into account the effect of mesophyll conductance and tested its performance with experimental data. We applied this model to 464 δ13C chronologies in tree-rings of 143 species spanning global biomes. Adjusted for mesophyll conductance, mean sensitivity of WUE to atmospheric CO2 (0.15 ppm ppm-1) was considerably smaller than those estimated from conventional modelling (0.23-0.30 ppm ppm-1). Our results showed a 10-ppm gain in WUE during the 20th century. Ratios of internal-to-atmospheric CO2 (ci/ca) in leaves maintained constant over time but differed among biomes and plant taxa. Our results also suggest that ratios of chloroplastic-to-atmospheric CO2 (cc/ca) are constrained over time, but this result is associated with the sensitivity of gs /gm ratio to CO2 which need further evaluation.

Over the last century, a general increase in forest WUE across the globe has been confirmed. However, our study shows that forest WUE may have not increased as much as previously suggested and that projections of CO2 fertilization may need to be adjusted accordingly.

How to cite: Gong, X. Y., Ma, W. T., Yu, Y. Z., and Wang, X.: Overestimated water-use efficiency responses to rising CO2 revealed by tree-ring 13C record, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-6684,, 2023.

Supplementary materials

Supplementary material file