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

Abstract

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 CO _2 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 CO _2 , 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 H _2 O decade ^−1 ) was largely contributed by increasing CO _2 (51%) and LAI (20%), but counteracted by climate forcing (−26%). Globally, the CO _2 fertilization effect on photosynthesis (23%) was similar to the CO _2 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.

Keywords

• ecosystem water-use efficiency
• CO2 fertilization
• stomatal conductance
• climate change
• earth greening