Spatiotemporal multi-index analysis of desertification in dry Afromontane forests of northern Ethiopia

Abstract

Land degradation and desertification is a great concern of arid and semiarid areas. Desertification trends in northern Ethiopia were quantified using the Normalized Difference Vegetation Index (NDVI), Topsoil Grain Size Index (TGSI), and Precipitation Concentration Index (PCI) calculated from 1990 to 2018 Landsat imagery and meteorological data from fourteen stations. A decision tree approach was used to assess the rate of desertification in northern Ethiopia using 1990, 1995, 2000, 2008, 2014, and 2018 Landsat 5 TM+ and Landsat 8 OLI data. A significant decline (P < 0.005) in mean NDVI was detected between 1990 and 2018 (mu = - 0.012 + 0.072), indicating that desertification has expanded over this time. The analysis showed a significant and positive spatial correlation (R-2 = 0.47; P < 0.0001) between the distribution of NDVI and the independent variables of precipitation and TGSI. Precipitation had the predominant impact on the NDVI. Monthly rainfall was characterized by a significant spatiotemporal variability (PCI > 20%) from 2001 to 2016 due to frequent droughts, particularly in northeastern Ethiopia. This desertification hot spot was confirmed through semi-variogram analyses. The overall findings from the multi-index assessment indicate decreasing vegetation cover, increasing ratio of coarse soil particles, as well as decline and high seasonal variability in rainfall. The desertification also appears linked to the rapid reduction in dense forests and shrublands in favor of bare and desert land as well as agricultural land. Therefore, major causes of land degradation in northern Ethiopia are both anthropogenic and natural climatic factors, requiring mitigating and adaptation actions primarily in northeastern Ethiopia.