Nonlinear threshold responses of agroecosystem water use efficiency to increasing vapor pressure deficit

Increasing vapor pressure deficit (VPD) alters ecosystem carbon and water exchange, but agroecosystem water use efficiency (WUE) may respond nonlinearly to rising atmospheric evaporative demand. The VPD levels at which these nonlinear transitions occur, their spatial consistency, and the factors associated with spatial variation in breakpoint magnitude remain poorly understood. We therefore examined nonlinear WUE responses to increasing VPD and assessed how breakpoint magnitude varied across croplands in relation to environmental and agricultural characteristics. Using multisource observations from 111 sub-basins of the Songhua River Basin during 2002–2024 and independent eddy covariance observations from cropland sites, we identified a nonlinear WUE–VPD relationship. The first derivative of the generalized additive model fit indicated a point of maximum WUE sensitivity to VPD at approximately 0.27 kPa, whereas piecewise linear fitting identified the primary basin-scale breakpoint at approximately 0.41 kPa. Repeated subsampling indicated that the basin-scale breakpoint was robust to sampling variation, while local breakpoint estimates varied across sub-basins. Statistically supported breakpoints were detected in 59 of the 111 sub-basins, and site-level screening retained breakpoint estimates for 20 of the 28 evaluated cropland flux sites. The mean breakpoint estimate across the 20 retained flux sites was approximately 0.43 kPa. Sub-basin breakpoint magnitude was negatively associated with leaf area index, soil bulk density, and mean growing-season precipitation total, and positively associated with C4 crop proportion. Above the basin-scale breakpoint, the sensitivity of gross primary productivity to VPD increased significantly, whereas the sensitivity of evapotranspiration showed no significant change, indicating an asymmetric carbon–water response. At the retained flux sites, higher Bowen ratios and lower evaporative fractions above the site-specific breakpoints were consistent with a relative shift in energy partitioning toward sensible heat. The approximately 0.41-kPa breakpoint characterizes the basin-scale WUE response to increasing VPD but does not represent a universal local threshold. Spatial variation in breakpoint magnitude was associated with climate, vegetation structure, soil properties, and crop composition, although the observational design precludes direct causal attribution to individual management factors. These findings clarify spatial variation in nonlinear agroecosystem WUE responses to increasing VPD and underscore the importance of local environmental context when interpreting carbon–water responses to atmospheric drying.