CO2 fertilization is spatially distinct from stomatal conductance reduction in controlling ecosystem water-use efficiency increase

It is well known that global ecosystem water-use efficiency (EWUE) has noticeably increased over the last several decades. However, it remains unclear how individual environmental drivers contribute to EWUE changes, particularly from CO2 fertilization and stomatal suppression effects. Using a satellite-driven water-carbon coupling model-Penman-Monteith-Leuning version 2 (PML-V2), we quantified individual contributions from the observational drivers (atmospheric CO2, climate forcing, leaf area index (LAI), albedo and emissivity) across the globe over 1982-2014. The PML-V2 was well-calibrated and showed a good performance for simulating EWUE (with a determination coefficient (R (2)) of 0.56) compared to observational annual EWUE over 1982-2014 derived from global 95 eddy flux sites from the FLUXNET2015 dataset. Our results showed that global EWUE increasing trend (0.04 +/- 0.004 gC mm(-1) H2O decade(-1)) was largely contributed by increasing CO2 (51%) and LAI (20%), but counteracted by climate forcing (-26%). Globally, the CO2 fertilization effect on photosynthesis (23%) was similar to the CO2 suppression effect on stomatal conductance (28%). Spatially, the fertilization effect dominated EWUE trend over semi-arid regions while the stomatal suppression effect controlled over tropical forests. These findings improve understanding of how environmental factors affect the long-term change of EWUE, and can help policymakers for water use planning and ecosystem management.

Automated summary

This study quantified the individual contributions of CO2 fertilization and stomatal suppression effects to the global change in ecosystem water-use efficiency (EWUE). The results showed that the increasing trend in global EWUE was mainly driven by the increase in CO2 and leaf area index (LAI), but counteracted by climate forcing. The fertilization effect of CO2 on photosynthesis was similar to the suppression effect on stomatal conductance. Spatially, the fertilization effect dominated in semi-arid regions, while the stomatal suppression effect controlled over tropical forests.