Spatiotemporal changes of forest vegetation after the implementation of a natural forest protection project and underlying driving factors: Case study of a typical natural secondary forest area in the Loess Plateau

The Natural Forest Protection Project (NFPP) in China is the first large-scale ecological restoration project aimed at protecting natural forest resources in the world. However, since its implementation in 1998, there was a lack of accurate quantitative evaluation of the ecological governance and restoration effects. This study investigated the core forest area of Ziwuling on the Loess Plateau (LP). Using the Google Earth Engine (GEE) and the Landsat remote sensing dataset, a multidimensional classification feature set comprising spectrum + vegetation index + texture + terrain was constructed by extracting multitemporal remote sensing information. With the help of land use and vegetation type data, a sample database was established, after which the random forest algorithm was used to realize the automatic classification of the five periods of the study area in 2000, 2005, 2010, 2015 and 2020. Combined with geostatistical methods such as Theil-Sen (Sen) trend analysis, Mann-Kendall (MK) trend test and geographic detectors, we realized the trend analysis of the driving factors of vegetation growth and the contribution analysis of the spatial differentiation of different vegetation types. The results showed that: (1) From 2000 to 2020, the vegetation area increased by 141.92 km2. In terms of structural composition, the vegetation was dominated by hardwood forest, followed by sagebrush, and low coverage of coniferous forest. (2) Over time, the expansion of vegetation showed a positive slowing trend, and the growth rate in 2000-2005 was significantly higher than in 2005-2020. (3) The main driving force affecting vegetation restoration is precipitation, and the temperature plays a regulating role. (4) The main driving factors affecting the spatial differentiation of vegetation were annual maximum temperature, annual average temperature, and aspect. These findings not only help us understand the intuitive contribution of the NFPP to the vegetation restoration and ecological management of the LP, but also provide an important reference for future studies using remote sensing technology in ecological restoration monitoring.