Diurnally asymmetric impacts of heatwaves on vegetation net primary production across China

Heatwaves increasingly threaten vegetation productivity and terrestrial carbon sinks, yet whether daytime, nighttime, and compound heatwaves induce distinct vegetation responses remains poorly understood. Understanding these differences is crucial for assessing ecosystem vulnerability under intensified and diurnally asymmetric warming and for identifying the hydroclimatic conditions that regulate post-heatwave responses. Using daily net primary production (NPP), meteorological, soil moisture, and other environmental datasets, we investigated the spatiotemporal evolution of daytime heatwaves (DHW), nighttime heatwaves (NHW), and compound heatwaves (CHW), and quantified post-heatwave NPP responses across China during the growing seasons of 2001–2024. NHW and CHW increased significantly in frequency, duration, and intensity (P < 0.01), whereas DHW showed weak overall changes, with a significant decline in frequency. CHW produced the longest response lags and durations, while NHW exhibited the highest mean negative response intensity, particularly in summer, when the cumulative standardized anomaly magnitude reached 37.55. DHW was associated with a net negative effect of −0.07, whereas NHW showed a net positive effect of +0.24, revealing pronounced daytime–nighttime asymmetry in overall productivity responses. Stronger and more persistent responses were concentrated in the temperate arid and semi-arid region and the Tibetan Plateau region, with grasslands generally showing greater sensitivity than forests. NPP responses were significantly more negative under dry-background than non-dry-background conditions across all three heatwave types ( P < 0.01). Random forest and SHAP analyses identified SPEI, precipitation, vapor pressure deficit, soil moisture, and surface downward solar radiation (SSRD) as leading predictors of NPP responses, with SSRD consistently ranking among the top three predictors of negative response intensity across all three heatwave types. These findings deepen our understanding of the diurnally asymmetric effects of heatwaves on vegetation productivity and provide valuable insights for improving terrestrial carbon-sink vulnerability assessments under changing hydroclimatic conditions.