Impacts of Climate Change and Anthropogenic Activities on Vegetation Dynamics Considering Time Lag and Accumulation Effects: A Case Study in the Three Rivers Source Region, China

Examining the effects of climate change (CC) and anthropogenic activities (AAs) on vegetation dynamics is essential for ecosystem management. However, the time lag and accumulation effects of climate change on plant growth are often overlooked, resulting in an underestimation of CC impacts. Combined with the kernel normalized difference vegetation index (kNDVI), climate data during the growing season from 2000 to 2023 in the Three Rivers Source Region (TRSR) and trend and correlation analyses were employed to assess kNDVI dynamics. Furthermore, time lag and accumulation effect analyses and an upgraded residual analysis were applied to explore how climatic and human drivers jointly influence vegetation. The results show the following: (1) The kNDVI showed a fluctuating but overall increasing trend, indicating an overall improvement in vegetation growth. Although future vegetation is likely to continue improving, certain areas—such as the east of the western Yangtze River basin, south of the Yellow River basin, and parts of the Lancang River basin—will remain at risk of deterioration. (2) Overall, both precipitation and temperature were positively correlated with the kNDVI, with temperature acting as the dominant factor affecting plant growth. The predominant temporal effects of precipitation on the kNDVI were a 0-month lag and a 1-month accumulation, while temperature primarily showed a 2–3-month lag and a 0–1-month accumulation. The main category of the overall climatic temporal effects were precipitation accumulation and temperature time lag effects (PA_TL), which accounted for 70.93% of the TRSR. (3) Together, CC and AA drove vegetation dynamics, with contributions of 35.73% and 64.27%, respectively, indicating that AA played a dominant role. Furthermore, incorporating combined time lag and accumulation effects enhanced the explanatory ability of climatic factors for vegetation growth.