Divergent patterns and drivers of water use efficiency across leaf and ecosystem scales in the Horqin Sandy Land

Water use efficiency (WUE) is crucial in the fragile Horqin Sandy Land, yet how global change and ecosystem traits influence its patterns remains unclear.

This study focused on two ecosystems in the Horqin Sandy Land-semi-mobile dunes with Artemisia halodendron (SMAH) and a meadow wetland dominated by Phragmites australis (MPA). Using plant carbon isotope ratios and eddy covariance data from the 2022-2023 growing seasons, we revealed the distribution patterns of leaf-scale (WUE_leaf ) and ecosystem-scale (WUE_eco ) WUE and identified their environmental regulatory mechanisms.

The results showed that the variation trend of WUE_leaf in SMAH closely tracked soil moisture dynamics, whereas WUE_eco lagged behind WUE_leaf , demonstrating that a low T/ET (transpiration to evapotranspiration ratio) plays a key "attenuator" role. In MPA, WUE_eco variation aligned with groundwater level and temperature. Overall, WUE_leaf and WUE_eco responded oppositely to key drivers, but were both abiotic-dominated. WUE_leaf was influenced mainly by soil temperature (with an optimum of 22.8°C) and soil organic carbon, while WUE_eco was driven primarily by net radiation and vegetation cover.

WUE_eco variation arose from vegetation-environment interactions rather than vegetation type alone. Thus, WUE_eco cannot be directly extrapolated from WUE_leaf . These results underscore aligning WUE metric selection with research objectives in water-carbon coupling studies, highlighting the role of ecosystem structure and functional traits in regulating water-carbon dynamics.