Disentangle the individual effects of eCO2 on ET across vegetation types

Evapotranspiration (ET) is experiencing profound shifts in response to elevated CO₂ (eCO₂), with critical implications for hydrological cycles and ecosystems. It is widely recognized that CO₂ enrichment modulates ET components through three distinct pathways: radiative forcing (intensifying the greenhouse effect), physiological effects (regulating stomatal conductance), and structural effects (enhancing vegetation productivity and leaf area index). However, these changes in the coupled biosphere-atmosphere processes and their interactions, which potentially compensate for or amplify one another, make them difficult to disentangle and assess individually. Consequently, there is little consensus on the individual and combined effects of eCO₂ on ET through radiative climate change, plant physiological changes and structural changes, not to mention the variability among vegetation types.

In this work, we establish a "bottom-up" attribution framework based on counterfactual sensitivity experiments, and employed an optimized Shuttleworth-Wallace dual-source model to decouple the specific impacts of eCO₂ on plant transpiration, soil evaporation, and precipitation interception. The research primarily addresses two pivotal questions: (1) What are the isolated and the combined effects of eCO₂ on ET across different propagation pathways? (2) How do these effects vary across different vegetation types?

Preliminary results indicate that the negative physiological effects and positive radiative effects largely offset each other, with the absolute magnitude of their individual contributions far exceeding the positive structural effects. The ET response exhibits significant inter-biome heterogeneity, and physiological effects dominate the response magnitude across all vegetation types, with the exception of croplands and deciduous broadleaf forests. These findings suggest that further increases in CO₂ concentrations may intensify physiological regulation to a threshold that triggers a regime shift in ET from an increasing to a decreasing trend. These findings allows us to project the impact of futureCO₂ concentrations on the interaction processes of water between the biosphere and atmosphere.