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Considérations biogéographiques sur les steppes arides du nord de l’Afrique Volume 6, issue 2, Juin 1995

Author
Coordinateur du groupe, Changement de climat, sécheresse et désertisation, GIEC/IPCC, 327, rue A.L. De-Jussieu, 34090 Montpellier, France
  • Page(s) : 167-82
  • Published in: 1995

Located between the annual isohyets of 100 and 400 mm, the arid steppes of North Africa run from the Red Sea to the Atlantic Ocean, covering some 630,000 km2. Another 1,000,000 km2 or so of desert steppes between the 50 and 100 mm annual isohyets in the northern Sahara could also be added. From the phytogeographic viewpoint, the arid steppes consist in what we call the Ibero-Maghribian phytochorous component, the western equivalent of the Irano-Turanian phytochorous component, as defined by Eig in 1931. Together, they constitute the Mediterraneo-Steppic Floristic Subregion of the Mediterranean Floristic Region, and correspond to the arid Mediterranean climate. Despite the great homogeneity in climate, land-use and vegetation - for example, the plant cover is always low and sparse -, when considered in greater depth the North African steppes prove highly diverse. A number of evolutionary patterns and physiognomic or floristic types can be distinguished, linked to various climatic, edaphic or anthropozoic traits; the latter taking on increasing importance in the degradation processes. According to criterion, the steppes may be differentiated in various manners. From the physiognomic point of view: into perennial bunchgrass steppes, dwarf shrub, tall shrub, crassulescent, succulent, pulvinate or tragacanthic steppes. From the evolutionary: primary and secondary steppes, natural or anthropised, fallow, etc. From the climatic and geographic, sub-tellian or pre-Saharan, thermophilous or cryophilous, coastal or continental, and lowland or highland. And from the edaphic: psammophilous or pelophilous, glycophytic or halophytic. Many, but not all, are secondary formations derived from xerophytic forest by various processes such as deforestation, steppisation, and large-scale cropping, clearing or abandonment from antiquity to today. Generally, the effect of man over the past 25 centuries has been both tremendous and enduring. Zoogeographic characterisation is less clear-cut. Over the past 100 years, as larger mammals (most of them Afro-tropical) have either become extinct or been pushed towards extinction, the situation is increasingly so. On the other hand, smaller, mainly Mediterranean, mammals such as rodents, do not seem threatened by extinction in the foreseeable future. A detailed analysis of the mammalian, reptilian and bird populations revealed them to be a mixture of Mediterranean and Saharan species, with a small nucleus of tropical species. Not surprisingly, for animals and plants alike, the steppe area of North Africa is thus a transition zone between the Mediterranean and the Sahara. The human population is growing exponentially by 3% per annum, as it has been for the past 50 years, while the livestock population is growing by about 2%. Growth such as this puts a very heavy burden on the ecosystem. Similarly, over the past half century, some 50% of the land has been cleared for opportunistic and subsistence cultivation of staple cereals. For centuries, the use and management of the arid steppes of North Africa have remained almost unchanged. Under today’s demographic pressure end the effects of serious water and wind erosion, flooding and sedimentation, this kind of management results in acute degradation of both vegetation and soil alike.