POPULATION ECOLOGY OF ACACIA-TORTILIS IN THE SEMIARID REGION OF THE SUDAN

Abstract. The population ecology of Acacia tortilis, one of the important woody species in the arid and semi‐arid regions of northern Africa, was studied in the Sudan: Ten sample plots were established and all individuals in the plots were mapped and their basal diameter, stem height, crown height and cover measured annually for three years. Individuals were grouped into different height‐classes, ‘life‐forms’: trees >1.65 m, shrubs > 0.65 m and < 1.65 m, saplings < 0.65 m and seedlings (saplings less than one year old). The population density was 510 ind./ ha in 1985, 432 ind. / ha in 1986 and 218 ind. / ha in 1987. The life‐form distribution of the population showed clear differences between plots and years. Mortality in Acacia tortilis, expressed as loss in individuals per year, amounted to 167 ind. / ha / yr. Drought and lethal cutting were found to be the main mortality factors. The population declined by 15.3 % from 1985 to 1986 and by 49.5 % from 1986 to 1987. Natality (recruitment), expressed as number of newly established saplings, was 13 ind. / ha / yr. Recruitment is apparently discontinuous and bound to years with a relatively high precipitation. 1990 IAVS ‐ the International Association of Vegetation Science