Long Time Series Changes of Vegetation EVI and the Natural Influencing Factors in Qinling-Daba Mountains Area

The Qinling-Daba Mountains Range is an important geographical transition zone between northern and southern China. This paper aims to fully understand the ecological changes and the influencing factors in this area in the past 20 years. To achieve this, MODIS-EVI data were applied, combined with temperature, precipitation, and DEM data, and the maximum synthesis method, trend analysis method, and partial correlation analysis were used on the basis of altitude. Besides, temporal and spatial changes in the enhanced vegetation index (EVI) for different vegetation types at various altitudes and their correlation with temperature and precipitation from 2000 to 2020 were analyzed. The main conclusions are: (1) From 2000 to 2020, the average annual EVI of vegetation in the Qinling-Daba Mountains area presented a fluctuating upward trend. Overall, 80.67% of the area showed a significant improvement on EVI and 13.29% of the area showed a slight improvemont on EVI. The EVI was relatively stable at an altitude of 1 600-1 800 m, which is more suitable for vegetation growth. (2) Cultivated vegetation and broad-leaved forests are the main types of vegetation in the Qinling-Daba Mountains region. The EVI values of the various vegetation types range in the order of broad-leaved forest > shrub > coniferous forest > grassland > cultivated vegetation > others. The inter-vegetation area accounts for the majority of vegetation in the altitude range of 1 000-2 000 m, while the altitude range of 1 600-1 800 m is the most suitable for natural vegetation growth. (3) From 2000 to 2020, the climate in the Qinling-Daba Mountains area became warmer and more humid. The correlation between EVI and temperature was relatively uniform across the region, and the impact of precipitation on EVI was greater than that of temperature. However, at higher altitudes, the correlation between EVI and temperature is generally greater than precipitation.