SITE-SPECIFIC PROPERTIES AND IRREVERSIBLE VEGETATION CHANGES IN SEMIARID GRAZING SYSTEMS

There is an urgent need to develop a mechanistic understanding of how site-specific properties can lead to irreversible vegetation changes. We show, by means of a bifurcation analysis of two mathematical models , how site-specific properties determine the resilience of vegetation changes in semi-arid grazing systems. The models predict that if avail able soil water limits plant growth, the vegetation supported by sandy soils is generally resilient to herbivore impact and rainfall fluctua tions, unlike the vegetation on clayey soils. This depends on the capa city of vegetation communities to improve the structural and water-hol ding capacities of the soil. In contrast, if plant growth is nutrient limited, vegetation on sandy soils is generally not resilient to herbi vore impact and fluctuations in external nutrient input, unlike the ve getation supported by clayey soils. This is affected by the nutrient r etention capacity of vegetation communities. We stress the applicabili ty of the general theory provided by this model to the Sahel environme nt. The model predictions are consistent with field observations docum ented in the literature.