Vegetation variations and driving mechanisms in northern China based on kNDVI

Northern China is an ecologically fragile region, making it highly significant for studying the temporal and spatial variations of vegetation cover and their driving factors. In this study, we analyze the temporal and spatial variations of vegetation cover in northern China from 2001 to 2022 by using a kernel normalized difference vegetation index (kNDVI) dataset, and quantify the contributions of influencing factors by random forest. The results indicate that the spatial distribution of kNDVI in northern China follows a pattern of "low in the west and high in the east," with a gradual increase from west to east. The average kNDVI value for the study area is 0.144. Vegetation cover in northern China exhibits a fluctuating upward trend, with the northern Loess Plateau showing the highest rate of increase at 0.0032 per year and Southern Xinjiang and the Hexi Corridor showing the lowest rate of increase at 0.0003 per year. The average lag period of vegetation response to precipitation across different climate regions ranges from 0.5 to 2.2 months, while the average lag period for vegetation response to temperature ranges from 0.3 to 1.5 months. The response time of vegetation to precipitation is faster than that to temperature in the western arid zone, while in the transition zone influenced by the summer monsoon, vegetation responds more quickly to temperature than to precipitation. In the monsoon zone, the response times to precipitation and temperature are relatively similar. Precipitation is the dominant factor driving changes in vegetation cover, with an average contribution of 67.9%, although the contribution of kNDVI driving factors varies across different regions. This study provides valuable insights for guiding the restoration and sustainable development of ecosystems in northern China.