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Résumé :
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The steppes with sparte occupy large portions of the west Algerian lands even if their decrease is obvious since the beginning of the last century.This places seem to have suffered from the Quick and intense changes. As well as the influence unstable climatic, we have the much amplified effect of the human pressure in the process of the vegetal rug decrease.In the bioclimatic field, the region of Hassi-Mellah belongs to the Mediterranean and continantal climate in arid bioclimate characterized by a cold winter. The dryness period reaches 9 months.The soil of this facies of Lygeum spartum represents a texture full with sand, poor in organic matter, the salinity is raised, the Ph is basic.Through a comparative diachronic study of the phytosociological statements, the authors attempt a floristic characterization approach of a steppic ecosystem of Lygeum spartum L.The comparison of the two periods the floristic tables (1991 and 2005) enabled us to notice that the number of inventoried species, the rate of average covering of the vegetation fell. The frequency of Noaea mucronata thorny species and toxic Atractylis serratuloides species increased whereas that of Lygeum spartum and Artemisia herba-alba regressed. In addition we raise the appearance of Peganum harmala and disappearance of Stipa tenacissima.The pastoral pressure and the severe climatic conditions explain this regressive dynamic of Lygeum spartum steppic population.The study reveals the vital role of anthropic action on the damage of the route fround in the steppe of western Algeria. Its regeneration seem to be prevented by graze and the seeds. Some pratices of farwing, as the use of the tractor with plough of several discs its crumbles and sprays the sandy soil on the one hand and the oder hand, the complete vegetal destruction.
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