No widespread decline in canopy conductance under elevated atmospheric CO2

Leaf stomatal conductance and transpiration rates have been commonly presumed to decline under elevated CO₂ concentrations (eCO₂) via partial stomatal closure. While this has great implications for the terrestrial carbon and hydrological balances, eCO₂ effects on stomatal conductance and transpiration at the ecosystem scale are highly debatable. Here, we used global ecosystem-level measurements from 78 eddy covariance sites to study long-term trends in canopy conductance (G_c). An empirical canopy conductance model was also used to quantify the separate contributions of CO₂, gross primary production (GPP), and vapor pressure deficit (D) to the trends in G_c (dG_c/dt). We found that the majority of the 78 sites did not have a significant trend in G_c. Only 15 sites exhibited significant dG_c/dt trends, while the direction of the trends was not consistent across these sites. GPP contributed the most to the change in G_c. D played an essential role in regulating G_c, and favorable climates and low D increased G_c even under eCO₂. Leaf ambient CO₂ concentration (C_a) had a consistent and relatively weak yet negative effect on G_c at most sites. Moreover, a state-of-the-art land surface model (CLM5.0) systematically underestimated G_c for these 78 sites and the model also exhibited a stronger role for CO₂ but a weaker role for D in regulating G_c. Our results reveal the lack of widespread effects of eCO₂ on G_c, and a state-of-the-art land surface model is unable to accurately capture the G_c trends. Our results indicate that the stomatal suppression of evapotranspiration in response to eCO₂ may have been overestimated by these earth system models at large scales. Our findings can help improve models and better project future changes in G_c, evapotranspiration, and runoff in the context of rising CO₂ and climate change.