Event-specific gross primary productivity anomaly responses to abrupt dry–wet hydrological transitions

Rapid alternation between dry and wet hydrological conditions represents an increasingly important compound hydroclimatic disturbance, yet how such short-term water-state shifts are translated into ecosystem productivity anomalies remains poorly understood. Here, we developed an event-scale framework to examine the transmission of hydrological dry–wet abrupt alternation to gross primary productivity (GPP) anomalies across the Yangtze River Basin from 1982 to 2018. Runoff and GPP series were separately detrended and standardized for each intra-annual 8-day period to identify runoff transitions and associated GPP anomaly responses. We identified 369,814 hydrological transitions, of which 60.6% were D2W (Dry to Wet) and 39.4% were W2D (Wet to Dry). Among 341,501 events with complete GPP response windows, 66.6% were followed by at least one newly emerging GPP anomaly, defined by a shift from within ±1 standard deviation to beyond this range, while 39.1% exhibited a GPP response consistent with the direction of hydrological change, namely a positive anomaly after D2W or a negative anomaly after W2D. Matched pseudo-event experiments showed that both probabilities exceeded seasonally and temporally matched background levels, supporting an event-specific association between abrupt hydrological transitions and GPP anomalies beyond random temporal coincidence. The environmental controls were strongly nonlinear and differed between D2W and W2D responses. Moderate antecedent shortwave radiation was associated with a higher probability of direction-consistent GPP anomalies, whereas excessive radiation reduced this probability; higher radiation shortened D2W response lags, while moderate antecedent precipitation shortened W2D lags. D2W peak anomalies strengthened with increasing VPD up to moderate levels, whereas W2D peak anomalies increased under high antecedent radiation; D2W peak responses were also larger overall, while resistance, resilience, and response non-detection differed little between transition directions. These results show that ecosystem productivity responses to abrupt dry–wet transitions depend on the combined effects of hydrological change and atmospheric constraints.