Multi-Type Dynamic Trajectories of Vegetation Water Use Efficiency and Carbon–Water Asynchrony Patterns in the Yangtze River Basin (2001–2020)

Vegetation water use efficiency (WUE = GPP/ET) quantifies the carbon–water exchange efficiency of terrestrial ecosystems. Most previous studies rely on linear trends and thus fail to capture nonlinear dynamics or to compare systematically with net primary productivity (NPP) trajectories. Using MODIS GPP and ET data for 2001–2020, we applied a multi-type dynamic trajectory framework—competitive fitting of null, linear, quadratic, and piecewise models selected by AIC and validated by bootstrap—to classify WUE time series of each pixel in the Yangtze River Basin into eight standard trajectory types, and compared them with contemporaneous NPP trajectories. Positive WUE trajectories occupied only 40.03% of the basin, substantially less than the 60.00% for NPP; no abrupt trajectories were detected, consistent with the lower sensitivity of the ratio-based WUE indicator to abrupt changes than NPP. Approximately 21.3% of the basin showed carbon–water asynchrony (positive NPP concurrent with negative or neutral WUE). In contiguous evergreen needleleaf plantations, the asynchrony rate reached 83.3% (n = 24; 95% CI: 64.1–93.3%; preliminary given the small sample). The inflection of positive WUE trajectories preceded that of NPP by an average of 1.2 years, providing a potential early-warning indicator of carbon-sink reversal, pending independent prospective validation. Human activity intensity, soil moisture, and vapor–pressure deficit (VPD; dual nonlinear turning points at 0.85 and 1.12 kPa) were the dominant correlates of positive WUE trends, whereas drought-stress indices dominated negative trends. Directional responses of WUE to ENSO matched those of NPP, but the response magnitude was amplified relative to NPP (~2.5× for positive trajectories), with no significant phase dependence. These results indicate that vegetation greening and WUE improvement are asynchronous in the Yangtze River Basin, supporting a shift in ecological-engineering evaluation from carbon-sink quantity alone toward carbon–water synergy.