Reconstitution de la végétation à la suite de mesures de protection des sols en terrains montagneux de Tunisie centrale

Abstract

Recover of vegetation after soil conservation measures in mountain areas of central Tunisia. Sampling sites with a similar soil type, slope and degree of exposure, within areas were selected along the mountain ridge Djebel Kbar in central Tunisia, gouvemorate Sidi Bouzid. Five of the sites had been subjected to various types of soil conservation treatments (e.g. construction of long banks, terraces and pits ; grazing protection, pine plantation establishment) between 5 and 35 years earlier, and another five sites had not been conservateci except grazing protection for the same time période, i.e. from 5 to 35 years. All species of vascular plants were identified, and the number of individuals of each species was counted in each of 9 plots (1 m²) at each sampling site (5 soil conservation areas and four control areas). Biomass of the 25 most frequent species was estimated. Ground cover in % of alfa grass (Stipa tenacissima) was estimated in large plots (10 m x 10 m). The recovery of the plant cover over time was described in terms of character species representing four important stages of succession. About 50 species were identified in the small plots. About 60 plant individuals were present in each small plot. However, plants were much more abundant in the more recently treated areas (about 140) owing to the high frequency of annuals (50-70 % vs 20-40 % in the older ones). The mean biomass per plant was 6,3 g and the range was 0,2-28 g. Approximately 10 species accounted for 80 % of the total biomass, which was 250 g/m². The percent of the ground covered with the alfa grass was considerably higher in the older areas (30 %) than in the more recently established area (5 -10 %). Consequently herbs and grasses were less abundant in the older areas. As the sampling areas increased in age as concerns conservation and protection, plant succession resulted in gradual shift from ruderal, overgrazed vegetation with a high proportion of annuals over to a dwarf shrub -and herb -dominated vegetation type, toward a stage dominated by shrub vegetation. Apparently, these changes were more the result of grazing protection than of the technical conservation measures.