General Conclusions – Sand Dune Deserts, Desertification, Rehabilitation and Conservation

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).

There are also less arid deserts. These are found along desert margins or in the form of semi-deserts, where fixed sand dune systems start to become mobile for various reasons (Wang et al. 2006), mainly by overgrazing and trampling, together with firewood collection. In geological timescales, climate change can be a trigger for the reactivation of stable sand dunes (Lancaster 1987; Littmann 1988).

The Nizzana dunes are an example of dunes at a desert margin, more or less stable during the last centuries. They are very small in comparison with other sand dune areas but nevertheless a good example of where, on the one hand, the dynamics of desert ecosystems (as shown in previous chapters) and, on the other hand, the vulnerability of these systems and the problem of desertification as well as the necessary measures of rehabilitation can be studied. Thus, some comparisons with other, selected sand dune ecosystems, with their specific dynamics and threats, can be made but also some general conclusions can be drawn.