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A vegetation-focused diachronic study to monitor and assess desertification processes in the southern Tunisian steppes was carried out between 1975 and 2000. Climatic variability and land use changes have caused considerable changes in vegetation units and in the structure of ecological systems during the past 25 years. About 10% of the steppe area has been taken over by agriculture, shrinking perennial plant cover has led to the appearance and dominance of a highly degraded vegetation class (perennial plant cover <5%), and floristic composition transformation has occurred as unpalatable species encroached the area. Studies to monitor the ecosystem dynamics in arid southern Tunisia during the past three decades provided reliable indicators of steppe degradation and the need for long-term ecological monitoring to assess desertification processes. (c) 2007 Elsevier Ltd. All rights reserved.