Moving sand dunes represent a natural phenomenon in most arid and hyper-arid sand deserts, such as the Sahara, Namib, Taklamakan and Rub'al Khali. The preconditions for large sand dunes or even extensive "sand seas" are, on the one hand, the geological situation with a large source of sand provided by the weathering of parent rocks and, on the other hand, the climate, which is normally very arid and exhibiting typically strong wind systems. These dune systems - e.g. in the Gobi, the Rub-al-Khali and the Namib - are typical sand deserts. The water regime of these sand deserts is rather favourable in comparison with that of adjacent rock, gravel or clay deserts. The biomass resulting from 1 mm of rainfall on sandy soils is 2.5 times higher than that produced on fine-texture soils (Le Houérou 1986). This can always be seen in some specific stands of plants, mostly in the stable dune valleys where eventually sometimes even water can be found. The mobility of these dune systems is controlled by the specific wind regime, which may cause different types of dune morphology and dune types (Bagnold 1941; Besler 1980; Lancaster 1982; Tsoar 1984; Tsoar and Møller 1986; Cooke et al. 1993).