Spatiotemporal Characteristics and Influencing Factors of Grassland NDVI in Xinjiang

Abstract

Long-term grassland monitoring in Xinjiang is crucial for understanding the spatiotemporal dynamics of grassland growth and projecting future trends. It contributes to evaluating environmental pressures on grasslands and predicting their ecological health. This study, using Landsat-NDVI as the primary data source, employs univariate regression analysis, multivariate residual analysis, and the Hurst index to investigate changes in the normalised difference vegetation index (NDVI) of Xinjiang's grasslands from 1990 to 2022. The research explores annual-scale variations across 11 grassland types, evaluates the effects of climate and human activities, and predicts future trends in grassland dynamics. (1) From 1990 to 2022, the NDVI of Xinjiang's grasslands exhibited a fluctuating yet overall increasing trend. Spatially, NDVI values showed a decreasing pattern from mountainous regions to basins. Areas with significant NDVI increases accounted for 71.21% of the total area, whilst those with significant decreases represented only 0.33%. Amongst the grassland types, marshlands had the highest NDVI growth rate at 1.26 × 10⁻²a⁻¹, whilst temperate desertified grasslands had the lowest growth rate at 0.18 × 10⁻²a⁻¹. (2) Between 1990 and 2022, the increase in NDVI across most grasslands in Xinjiang was primarily driven by the combined effects of climate change and human activities, which accounted for 87.48% of the total influence. Specifically, human activities have contributed more significantly to changes in NDVI than climate change, exceeding 70% in most grassland types. However, in the Alpine grassland and Alpine desert, the contribution of human activities was relatively lower compared to other types. The NDVI of grasslands in Xinjiang is projected to increase in most areas, covering 88.64% of the region. The overall outlook is positive, with grasslands expected to remain largely stable or show signs of restoration. (3) In the future, the NDVI of grasslands in Xinjiang is anticipated to increase in most areas, accounting for 88.64%, with an overall positive outlook characterised by stability and recovery. Temperate desert grasslands, alpine deserts, and marshlands are expected to show NDVI increases in more than 90% of their areas. However, for the Mountain meadow and Alpine meadow, over 20% of the areas are projected to experience either a decline or unpredictable changes in NDVI. These regions should be prioritised in future grassland management and conservation strategies.