Fig. 3
Attribution of variation in NDVI calculated from model experiments with three machine learning algorithms. In panel (a), (b), (c), the effects of various factors on NDVI in the cropland zone, the natural vegetation zone and the whole region are shown respectively from 1982 to 2018, while panel (d), (e), (f) represent the attribution of NDVI variation in the two periods. The “early” notation indicates the period of 1982–1995, and “late” indicates 1995–2018. The drivers include irrigated cropland expansion (ES_ICE), nitrogen fertilization change (ES_NFER), elevated atmospheric CO2 concentration (ES_eCO2), photosynthetically active radiation change (ES_PAR), relative humidity change (ES_RH) and air temperature change (ES_Ta). ES_ALL notation indicates the result of model simulation with all factors changing over time. Error bars show the 95% confidence intervals of the trends. Two asterisks indicate that the trend is extremely significant (P < 0.01), and one asterisk indicates that the trend is significant (P < 0.05)