Spatial-temporal dynamic characteristics of NDVI in Shenmu city

Normalized difference vegetation index (NDVI) can indicate the growth and coverage of vegetation, and explore the spatio-temporal dynamic change characteristics of long-time series, which is of great significance for clarifying the ecological change of regional vegetation.

Google Earth Engine (GEE) cloud platform was used to obtain the long-term sequence Landsat images of vegetation growing seasons (July-September) and construct NDVI indexes. Through the variation coefficient, Sen+Mann-Kendall trend analysis, future trend change analysis and spatial autocorrelation calculation, the temporal and spatial variation characteristics and spatial pattern of NDVI in Shenmu city from 2000 to 2020 were analyzed, and the driving force analysis was carried out by extracting different aggregation areas combined with the data of cumulative precipitation and mean temperature in vegetation growing seasons.

The NDVI in Shenmu increased significantly from 2000 to 2020, with a growth rate of 1.25%·a^-1. The areas with increased, stable and degraded NDVI accounted for 97.8%, 0.4% and 1.8% of the total area, respectively. The degraded areas were mainly distributed in the urban agglomeration areas. The NDVI variation coefficient was mainly concentrated in the range of 0.3-0.5, and the overall fluctuation was intense. The variation degree showed a spatial pattern of “high in the southeast and low in the northwest”. The average Hurst index of NDVI in the whole region was 0.69, and 93.7% of the area continuously increased significantly, indicating that the vegetation change in the future was good. The global Moran' I index decreased in volatility, and the global spatial autocorrelation of NDVI decreased gradually. The fluctuation of the aggregation area with low-low NDVI decreased, which was significantly negatively correlated with the cumulative precipitation in vegetation growing seasons (P < 0.05). The area of low-high type aggregation area was significantly negatively correlated with the average temperature in vegetation growing seasons (P < 0.01). The fluctuation of high-high type aggregation area increased, which was significantly positively correlated with the cumulative precipitation in vegetation growing seasons (P < 0.01).

In the past 21 years, the overall change of NDVI in Shenmu city was more dramatic, showing an increasing trend, and its spatial distribution tended to be fragmented. Precipitation and warming had a certain role in promoting vegetation restoration, and precipitation had a greater impact on vegetation restoration. In the future, the change of vegetation shows a significant increase in sustainability.