Hydroclimatic Gradients Shape Differential Responses of Vegetation Photosynthesis in Croplands and Typical Natural Vegetation to Atmospheric Evaporative Demand and Soil Moisture

Vegetation photosynthesis is jointly regulated by precipitation supply, atmospheric water demand, and vertically stratified soil moisture, but their relative roles across vegetation types and hydroclimatic backgrounds remain unclear. Using monthly satellite solar-induced chlorophyll fluorescence (SIF), ERA5-Land hydroclimatic variables, and vegetation-type data across China from 2000 to 2024, this study combined monthly anomaly analysis, lagged partial correlation, and XGBoost–SHAP to quantify moisture-response patterns in cropland, forest, grassland, and shrubland along an arid–humid gradient. Vegetation SIF increased overall from 2000 to 2024, with a clear northwest–southeast gradient and higher multiyear mean SIF in forests and shrublands than in croplands and grasslands. XGBoost–SHAP results showed that vapor pressure deficit (VPD) was the largest single contributor to SIF anomalies at the national scale, accounting for 31.8% of total moisture-factor contribution, followed by precipitation, shallow soil moisture, and middle- plus deep-layer soil moisture. The VPD contribution differed among vegetation types, reaching 43.3% in grassland, 32.5% in cropland, 30.4% in shrubland, and 27.5% in forest. Lagged responses also varied with soil depth and hydroclimatic background. Precipitation and shallow soil moisture mainly produced concurrent or 1-month lagged responses, whereas middle- and deep-layer soil moisture generated 2–3-month delayed effects in some arid and semi-arid cropland and woody vegetation. Across the arid–humid gradient, dominant moisture constraints shifted from deeper soil-water regulation in arid regions toward VPD control in transitional zones and precipitation or shallow-soil-moisture control in humid ecosystems. These findings demonstrate that vegetation SIF anomalies are shaped by coupled atmospheric demand, precipitation input, and soil-water storage, with vegetation traits and hydroclimatic background jointly determining the dominant moisture-response pathway.