Impacts of Climate Variations and Human Activities on the Net Primary Productivity of Different Grassland Types in the Three-River Headwaters Region

Climate variations and human activities, as two major driving forces, have profound impacts on alpine ecosystems. The Three-River Headwaters Region (TRHR) is located in the alpine region and is the source of three major rivers flowing to eastern China and Southeast Asia. Grassland is the dominant vegetation type in the TRHR and is fragile and sensitive to climate variations and human activities due to the alpine environment. Different types of grassland may have varying coping mechanisms with disturbances due to their unique environments and physiological functions. However, there is limited quantitative research on the response of different grassland types to climate variations and human activities in the TRHR. Therefore, the Carnegie–Ames–Stanford approach (CASA) was selected to simulate the net primary productivity (NPP) affected by climate (NPP_C) and the actual NPP (NPP_A) of steppes and meadows in the TRHR from 2001 to 2022, and the NPP affected by human activities (NPP_H) was calculated by subtracting the NPP_A from the NPP_C. Results showed that the NPP_A increased by 0.53 gC/m²/a during the study period, with the NPP_A of steppes and meadows increasing by 0.55 gC/m²/a and 0.51 gC/m²/a, respectively. The regions dominated by climate variations, human activities, and the combined impact of the two accounted for 22.01%, 29.42%, and 48.57% of the NPP_A changes. In terms of climate change, the impact of temperature and soil moisture on the NPP is equally important. It is worth noting that the alpine meadows (67.60%) contributed more to the increases in the NPP_A than the steppes (32.40%). In addition, climate variations and human activities contributed more to the increased total NPP_A of the meadows (20.54 GgC and 36.41 GgC) than that of the steppes (14.35 GgC and 10.20 GgC). The results clarify the quantitative evaluation system for the impact of human activities and climate change on different types of grasslands in the TRHR, providing guidance for the protection and management of these grasslands.