Analyse de l'évolution des formations végétales forestières et préforestières du Sahel : cas des régions du Ouaddaï et du Wadi-Fira au Tchad

Abstract

After the drought that began in the mid-1960s, rainfall in Ouaddaï and Wadi-Fira regions, in eastern Chad, improved from 1990s. At the same time, the human impact on vegetal formations rose with the massive arrival of refugees and displaced persons. The effects of these factors are so far little known in this area. The present paper studies the evolution of forest and preforest formations over a period of 23 years (1987-2010). This analysis was carried out using the Landsat TM satellite images of November 4, 1987 and ETM+ of October 22, 2000 and October, 2, 2010. Five classes of land use have been discriminated against (Pure Balanites, Pure Acacias, Mixtures, Bare soil and Rock outcrop). The proportions occupied by the three first classes were respectively 0.1 %, 5.2 % and 54.3 % in 1987, 0.2 %, 2.4 % and 48.7 % in 2000 and 1.0 %, 14.3 % and 61.7 % in 2010. Between 1987 and 2000, a sharp decline was observed for Pure Acacias (-53.6 %) and Mixtures (-10.4 %). However, a significant increase was recorded from 2000 to 2010 for the same classes: +500 % for Pure Acacias and +15.5 % for Mixtures. The class of Pure Balanites never reached 1 % of the total area. In situ observations were also carried out in autumn 2016 on 240 plots. The forest flora richness is of 30 species. The average density is 53 stems.ha-1 for Pure Balanites, 35 stems.ha-1 for Pure Acacias and 24 stems.ha-1 for Mixtures. Improved rainfall has favored species of lower calorific and forage values such as Calotropis procera and Leptadenia pyrotechnica. Despite the return of rainfall, the wood resources of Ouaddaï and Wadi-Fira are declining because they are subject to anthropogenic pressure amplified by increase of the population.

Keywords

• drought
• Landsat image
• forest formation
• preforest formation
• evolution of vegetation
• Ouaddaï