Le passage de la végétation méditerranéenne à la végétation saharienne sur les revers méridional du Haut Atlas oriental (Maroc)
Quézel, P.; Barbero, M.; Benabid, A.; Rivas-Martínez, S.
In this study, the authors analyze the syntaxonomic structures of vegetation which colonize the south-eastern face of the High Atlas, including the eastern Anti-Atlas and the valleys between these two mountains. They thus particularly define nine new associations and one new alliance, respectively related to the mediterranean biome and to the saharian biome, the former characterized by pre-steppic forests and structures of matorrals and steppes, the latter by desert steppes or tree and shrub steppes with Acacia raddiana. This analysis makes it possible to investigate the stability of the boundaries between the mediterranean region and the Sahara, which very likely have shifted during the last decades and nearly exactly fit with the 150 mm isohyet and not with the 100 mm one. The effects of man action on the vegetation are everywhere very obvious. In the mediterranean region, they are dramatic for the presteppic forests or the matorrals and, on the contrary, responsible for the spreading of the Artemisia steppes maintained by an intensive, but not yet excessive, grazing. The desertification process due to the action of man and his cattle must be taken into consideration, at least since the Holocene. The extremely unstable climatic and biological equilibriums which are presently setting in the Sahara and on its boundaries, make of this region an outstanding observatory for the study of possible global climatic changes on the surface of our planet.