Temporal and spatial variation of vegetation and its response to topography, climate and land use in Guangxi during 2000-2018

A typical karst landform is widely distributed in Guangxi Province of southern China. Therefore, it is very necessary to explore the temporal and spatial variation characteristics of vegetation in this area. Particularly, the influencing factors can greatly contribute to further promote green development and ecological environment protection in recent years. Among them, the normalized difference vegetation index (NDVI) can serve as an important indicator to evaluate crop growth, farmland, land management, and crop production. It is highly urgent to monitor and then predict the changes in vegetation cover in this region. This article aims to investigate the characteristics of temporal and spatial variation in the vegetation NDVI in Guangxi Province of China using the MODIS NDVI time series data from 2000 to 2018. The maximum value synthesis and trend analysis were also utilized to combine DEM data, land use types, temperature, and precipitation data, thereby exploring the NDVI spatiotemporal changes of different vegetation types, as well as the response to climate factors under various terrain conditions and land use. The results showed that: (1) NDVI presented an increasing trend from 2000 to 2018 in the study area, with a linear growth rate of 0.004/a, and an average NDVI value of 0.81. The largest NDVI value was also found in the third quarter. Moreover, the spatial distribution of vegetation NDVI was distributed in the north and edge, but less in the south and middle. The specific areas with high vegetation coverage were mainly concentrated in Hechi, Baise, and Guilin City. There was an overall increasing trend of vegetation NDVI from 2000 to 2018, particularly distributed in southern Qinzhou and Nanning City, accounting for 37.0% of the total study area. (2) The vegetation NDVI first increased and then decreased, and finally stabilized above 0.8, as the elevation increased in the study area. Specifically, the distribution of elevation was higher in the northwest and lower in the southeast, in total between 0-1 000 m. Furthermore, the vegetation NDVI first increased to stable and then decreased, as the slope increased, where the slope was mainly distributed in between 0-34°. Correspondingly, there was little effect of slop difference on the vegetation NDVI, except for the plain. (3) The annual average temperature was between 18 and 21°C in the study area from 2000 to 2018, while the average precipitation was between 1 100 mm and 1 900 mm. Temperature and precipitation posed a positive effect on the vegetation NDVI, where the multiple correlation coefficient reached 0.32. The response of vegetation NDVI to climate varied significantly in the different types of land use. Specifically, the vegetation NDVI greatly responded to air temperature, but less to precipitation in residential and unused land. By contrast, the response of vegetation NDVI to precipitation was greater than that to temperature in cultivated land, woodland, grassland, and water area. Consequently, there was a relatively significant growth trend of vegetation NDVI in Guangxi Province from 2000 to 2018, indicating that excellent progress in the continuous implementation of afforestation, the conversion of farmland to forests, and ecological protection projects to curb rocky desertification.