Spatial and temporal variations in vegetation index and its impact factors in the West Liaohe Plain in Inner Mongolia

The West Liaohe Plain is located in the farming-pasturing ecotone in North China, which lies in the semi-arid region, including the Kerqin Sandy Land, where the ecological environment is extremely fragile. It is of great significance to study the spatio-temporal variation in vegetation index and its influencing factors for predicting the risk of land degradation, which can provide technical support for the protection and management of ecological environment and the rational development and utilization of water resources in the watershed. In this paper, based on the MODIS NDVI data from 2000 to 2019, the univariate linear regression trend method and Mann-Kendall test are used to analyze the vegetation growth trend and mutation in this area in the past 20 years. Considering the water and heat conditions affecting vegetation growth, the correlations between NDVI and meteorological factors (precipitation, air temperature), soil moisture, groundwater depth and other factors are analyzed. In addition, combined with human activities, the impact of land use type change on NDVI is analyzed. The results indicate that (1) the NDVI during the vegetation growing season from 2000 to 2019 shows an overall upward trend, with no significant abrupt change points, the highest value is 0.56 and the lowest value is 0.41. (2) NDVI presents a spatial distribution characteristic of “high in the east and low in the west”. The NDVI of different land use types are in the descending order: cultivated land > forest land > swamp land > flood plain > grassland > saline land > sandy land. (3) The 92.5% of the area shows an increasing trend, and the 7.5% of the area shows a decreasing trend. (4) NDVI is positively correlated with precipitation, temperature and soil moisture, with correlation coefficients of 0.86, 0.80 and 0.81, respectively and precipitation has a greater impact on vegetation. (5) The most suitable groundwater level depth for natural vegetation growth is about 3 m, and when the groundwater level depth is more than 10 m, NDVI will decrease sharply with the increasing groundwater levael depth. (6) Human activities such as land reclamation and afforestation are the main reasons for the increasing trend of NDVI in recent 20 years, which improves the ecological environment to a certain extent.