Quantifying impacts of climate and human activities on the grassland in the Three-River Headwater Region after two phases of Ecological Project

The Three-River Headwater Region (TRHR) of China is a typical representative of the alpine environment in the Central Asian plateau and the alpine grassland in the world. Grassland degradation is one of its serious ecological problems. The purpose of this study is to quantify the joint impacts of climate and human activities on grassland changes in TRHR after two phases of Ecological Conservation and Construction Project (Ecological Project). Grassland vegetation coverage is selected as an indicator for analyzing grassland changes. We adopt Sen+Mann-Kendall trend analysis, residual trend analysis and correlation analysis methods to analyze the trends in spatial-temporal changes and driving factors of grassland in TRHR from 2000 to 2019. The results show that: (1) The grassland has been mainly restored, and the degraded grassland area only accounts for 1.66% of TRHR. After the implementation of the first phase of the Ecological Project, the percentage of restored grassland area has significantly increased from 8.82% to 24.57%, and slightly decreased during the second phase. (2) The establishment of national nature reserves and the implementation of the Ecological Project have changed the situation that “the grassland inside the reserve is worse than that outside the reserve”. (3) Grassland restoration is mainly affected by the joint effects of climate and human activities. Nevertheless, grassland degradation is mainly affected by human activities such as overgrazing and grassland reclamation. All of these findings can enrich our understanding of grassland restoration in TRHR. Artificial measures have certain limitations in promoting grassland restoration. Natural restoration should be considered when human beings strengthen ecological conservation and transform their production and life styles.