The desert in the Sahara. Transitions and boundaries.
Résumé
The Sahara is a system of three main landscape types. It comprises the semidesert in its northern part, the desert in the centre and the savanna in the South. The semidesert is a sparse but diffuse shrub vegetation, which is fed by winter rainfall and contains several Mediterranean affiliations in its floristic composition. A sharp boundary separates it from the desert, which is to define by the dominance of linear or contracted permanent vegetation. A savanna covers the southern part of the Sahara. It is also separated from the desert by a sharp boundary. This savanna is alimented by monsoon summer rain. The floristic composition remains Saharan. The boundaries of the desert are climatic dependent because the annual rainfall is no longer sufficient enough to allow a diffuse plant cover. They can be followed over the whole continent. Even in the climatically favoured region of the Oceanic Sahara in Mauritania these boundaries are clearly identifiable. In general the vegetation shows two different strategies to cope with the severe physical conditions of the Sahara. The first is to equip the individual organisms with protections against drought and frost but also to adjust their number to the amount of water available. The second is to exploit aleatoric resources by short time organisms in a great number without special protection mechanisms. A great number of pioneer elements in the vegetation describes its general vulnerability but also a great resilience. A vegetation map is presented as a modern model for the interpretation of former environments in the Sahara and adjacent areas.