Spatio-Temporal Changes and Contribution of Human and Meteorological Factors to Grassland Net Primary Productivity in the Three-Rivers Headwater Region from 2000 to 2019

Abstract

Since the beginning of the 21st century, the net primary productivity (NPP) of grassland in the Three-Rivers Headwaters (TRH) region has changed significantly. In this study, NPP was assessed by the Carnegie-Ames-Stanford approach (CASA) model in TRH from 2000 to 2019. The abrupt changes of NPP and meteorological factors were analyzed by cumulative departure, MK test, and Pettitt test. The contributions of meteorological and human factors to changes in grassland NPP were quantitatively assessed using the scenario simulation method. The obtained results showed that: (1) From 2000 to 2019, the NPP of grasslands increased from 135.72 to 141.16 gC/m−2a−1. However, the overall growth trend was not significant, and the proportion of significant growth was only 31.45%; (2) An abrupt increase in meteorological factors occurred around 2005, while an abrupt increase in NPP occurred around 2008, which showed that 2008 was the year when human factors, such as ecological projects and policies, began to show a significant impact on the growth of NPP; and (3) The contribution of human factors to the abrupt increase in NPP was significantly greater than the contribution of meteorological factors. The contribution of human factors exceeded 70% in 93.68% of the studied area, reaching 98% in general, while the contribution of meteorological factors was less than 2%. Among them, the human contributions to the Yangtze River Source, the Yellow River Source, and the Lancang River Source all exceeded 95%. The negative effects of meteorological factors on the growth of NPP in the abovementioned three regions were as high as 47.35%, 48.66%, and 36.91%, respectively. Human factors have contributed greatly to the increase in NPP in most of the source areas of the Yellow River, the eastern part of Tanggulashan County, the southeastern part of Zhiduo County, and the western part of Zaduo County at the source of the Yangtze River.