Global trends in vegetation fractional cover: Hotspots for change in bare soil and non-photosynthetic vegetation

Abstract

Vegetation Fractional Cover (VFC) is an important indicator of the condition of the terrestrial surface of the Earth. The dynamics of bare soil (BS), non-photosynthetic vegetation (NPV) and photosynthetic vegetation (PV) fractions reveal patterns of growth, senescence, dormancy and regeneration. The dynamics also reveal trends in land cover and land use management across the globe. Analysis of satellite data has indicated increasing greenness of the land surface however the dynamics of the NPV and BS fractions are equally important indicators of many elements of ecosystem function and sustainability. In this study, we provide a comprehensive assessment of trends in VFC for the global terrestrial land surface based on a trend analysis over the period 2001–2018 using the Global Vegetation Fractional Cover Product. Trends were analysed by Mann-Kendell regressions over 18 years for each month of the year. Monthly maps of significant trend areas were aggregated to provide seasonal assessments. Areas and percentage areas of positive and negative trends were compiled for United Nations Subregions, Countries and Ecoregions. Significant negative trends in BS occurred across 20 M km2 of the terrestrial land surface. Although the areas of negative trends in BS were largest in China and India, the top 24 countries by area were widely distributed across the world. However, BS increased significantly across more than 11 M km2. Eleven of the top 24 ranked countries for area of increasing BS were in Africa. Substantial areas exhibited negative trends in NPV split between China and India where PV was increasing, and drylands in Asia and Africa where BS was increasing. Analysis of covariate factors at country scale inferred an association between significant increase in BS and long-term drought and increases in livestock populations. The positive trends in BS and negative trends in NPV are indicators of current and future risk for soil erosion, habitat change, and reduction in ecosystem health across heavily populated regions of the developing world.