Nebkhas origination in arid and semi-arid regions: An overview

Abstract The evolution processes of nebkhas, including origination (from scratch) and growth thereafter (from small to large) are the typical example of how vegetation shapes landforms, and are associated with regional aeolian activity and sandy desertification. Because the evolution processes are complex, we have only reviewed current literature related to the conditions and age of nebkhas origination and how the origination conditions have been met in arid and semi-arid regions in order to understand the origination mechanisms. Our overview suggests that: (1) Moisture levels must be sufficient to support shrub development, but surfaces must be sufficiently dry to ensure an adequate sand source for nebkhas origination. Whether and when nebkhas origination in a region will depend on whether and when regional moisture levels and available sand supplies achieve a dynamic balance; (2) Most extant nebkhas originated within the last 1000 years, and older nebkhas are fully active due to the death of shrub, therefore nebkhas are more ephemeral than non-phytogenic dunes; (3) The origination of nebkhas is triggered by deterioration in regional moisture levels, which may result from climate change, anthropogenic activities, or a combination of these factors; (4) Nebkhas studies have mainly focused on the extremely arid regions, and have largely ignored farming-pastoral areas affected by anthropogenic activity; (5) Researchers have only sampled a small number of nebkhas in a given region (often only one), even though nebkhas consisting of different shrub species, different evolution stages, and different forms can coexist in that region; (6) To accurately understand the origination mechanisms of nebkhas, we must base our research on extensive and comprehensive regional data on origination age of the massive nebkhas.