Interdecadal Variation of Normalized Differential Vegetation Index Response to Climate Warming and Humidification in the Qinghai Plateau during the Growing Season

Herein, the variation characteristics of the growing season (May–September) vegetation on the Qinghai Plateau and their responses to climate change are studied based on the normalized differential vegetation index (NDVI) dataset obtained from the Advanced Very-High-Resolution Radiometer of the National Oceanic and Atmospheric Administration, as well as the high-resolution near-surface air temperature and surface precipitation rate of the China meteorological forcing dataset from 1982 to 2018. Statistical methods used in this study include linear tendency estimation, Mann–Kendall (M–K) test, partial correlation, and analysis of variance. The results show the following: (1) during the growing season of the Qinghai Plateau from 1982 to 2018, the vegetation exhibits a spatial distribution feature that gradually decreases from southeast to northwest, and there is a considerable increasing trend overall with a climate tendency rate of 0.01 (10 a)⁻¹. However, the trend of vegetation has a bipolar development feature in space; conversely, while the vegetation in the eastern and southwestern parts of the plateau considerably improves (57.3% of the total area), the vegetation of the Qaidam Basin in the northwestern part of the plateau exhibits a deterioration trend (14.8% of the total area); (2) during 1982–2018, the climate of the Qinghai Plateau is characterized by warming and humidification as a whole, but its trend considerably changed around 1998, and there are differences in the regions and intensities of climate warming and humidification in different periods. The warming in period P1 (1982–1998) was stronger than that in period P2 (1998–2018), and the humidification in period P2 was stronger than that in period P1. The trend of NDVI in the growing season of the plateau fluctuates with no obvious trend before 1998, but it exhibits a considerably increasing trend after 1998, which is consistent with the trend of surface precipitation rate in the plateau’s vegetation; (3) there is an interdecadal variation in the response of surface vegetation on the Qinghai Plateau to climate warming and humidification. In period P1, when the water is relatively insufficient, the surface vegetation has a negative feedback on the thermal factor (air temperature), and warming is not conducive to the growth of surface vegetation, particularly in the northern part of the source area of the Yellow River and near the Qinghai Lake. However, in period P2, under the conditions of relatively sufficient water, the surface vegetation mainly has a positive feedback on both the water and heat factors, crucially contributing to the extensive improvement of surface vegetation on the plateau in the growing season.